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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_MoneyMarketFundsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Money Market [Member]</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock option vesting period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Period1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel1Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Level 1 [Member]</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Grant revenue</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenue from Contract with Customer, Excluding Assessed Tax</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GrantsReceivableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Grants Receivable, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Total unamortized compensation cost related to outstanding unvested stock options and restricted stock awards</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClosingBidPrice_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Closing bid price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClosingBidPrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Closing Bid Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClosingBidPrice_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Closing bid price.</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2022 Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand Twenty Two Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand twenty two distribution agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DebtPrincipalRepaymentStartDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt principal repayment start date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DebtPrincipalRepaymentStartDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Principal Repayment Start Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DebtPrincipalRepaymentStartDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Debt principal repayment start date.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Noncurrent operating lease liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Liability Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockRepurchasedDuringPeriodShares_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of stock repurchased</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LoremVascularPteLtdMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lorem Vascular Pte Ltd [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LoremVascularPteLtdMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lorem Vascular Pte Ltd [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LoremVascularPteLtdMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Lorem Vascular, Pte. Ltd.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockByClassTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Stock By Class [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockByClassTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Stock By Class [Table]</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockCommonValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Treasury stock (at cost, 258,425 and 78,559 shares as of March 31, 2024 and December 31, 2023, respectively)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockCommonValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Treasury Stock, Common, Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PiramalPharmaSolutionsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Piramal Pharma Solutions, Inc. [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PiramalPharmaSolutionsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Piramal Pharma Solutions Inc [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PiramalPharmaSolutionsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Piramal pharma solutions inc.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfStockLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Class of Stock [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfStockLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Stock [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaidDuringPeriodForAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash paid during period for:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaidDuringPeriodForAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash Paid During Period For [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaidDuringPeriodForAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Cash paid during period for:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total current assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForRepurchaseOfEquity_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Purchase of treasury stock</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForRepurchaseOfEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Payments for Repurchase of Equity, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total liabilities and stockholders' deficit</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities And Stockholders Equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressStateOrProvince_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, State or Province</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressStateOrProvince_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address State Or Province</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_LitigationCaseTypeDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Litigation Case</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_LitigationCaseTypeDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Litigation Case Type [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandEighteenRightsOfferingMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2018 Rights Offering [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandEighteenRightsOfferingMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand Eighteen Rights Offering [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandEighteenRightsOfferingMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand eighteen rights offering.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders' equity requirement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale of common stock, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Issue and sale of stock, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandAndTwentyTwoPurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2022 Purchase Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandAndTwentyTwoPurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand and Twenty Two Purchase Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwoThousandAndTwentyTwoPurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand and twenty two purchase agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders' Equity Note, Stock Split, Conversion Ratio</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock split conversion ratio</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Balance as of March 31, 2024 (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Balance as of December 31, 2023 (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PreferredStockDividendRatePercentageEliminatedDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Series A 3.6% Convertible preferred stock eliminated date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PreferredStockDividendRatePercentageEliminatedDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock Dividend Rate Percentage Eliminated Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PreferredStockDividendRatePercentageEliminatedDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Preferred stock dividend rate percentage eliminated date.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_TradingSymbol_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Trading Symbol</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_TradingSymbol_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trading Symbol</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, shares issued (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock Shares Issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockVotingRights_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock, Voting Rights</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockVotingRights_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, voting rights</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestRateFloorMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest Rate Floor [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestRateFloorMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Interest Rate Floor [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RightsRelatedToDistributionOfAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Rights related to distribution of assets.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RightsRelatedToDistributionOfAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Rights Related To Distribution Of Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RightsRelatedToDistributionOfAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Rights related to distribution of assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfGrantProceedsWithInterestNotExceedAnnually_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of grant proceeds with interest not exceed annually</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfGrantProceedsWithInterestNotExceedAnnually_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage Of Grant Proceeds With Interest Not Exceed Annually</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfGrantProceedsWithInterestNotExceedAnnually_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of grant proceeds with interest not exceed annually.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseExpirationDate1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating leases, expiration date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseExpirationDate1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lease Expiration Date1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Balance (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Balance (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Liability Payments Due Year Three</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2026</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of shares reserved for issuance and sale</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock, Capital Shares Reserved for Future Issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermDebtCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Long-Term Debt, Current Maturities, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermDebtCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Term loan obligation, current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermDebtCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Long Term Debt Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressCityOrTown_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, City or Town</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressCityOrTown_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address City Or Town</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumSharesToBeIssuedUnderAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Maximum shares to be issued under agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumSharesToBeIssuedUnderAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Maximum Shares to be Issued Under Agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumSharesToBeIssuedUnderAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Maximum shares to be issued under agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating lease, weighted average discount rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Weighted Average Discount Rate Percent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Class of warrant outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Warrant Or Right Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseRenewalTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lessee, operating lease, renewal term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseRenewalTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Renewal Term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Term Loan Obligations</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsequentEventMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Subsequent Event</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsequentEventMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subsequent Event [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsequentEventTypeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subsequent Event Type [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CommonStockSalesAvailableForIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Remaining availability under financing facility</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CommonStockSalesAvailableForIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock Sales Available For Issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CommonStockSalesAvailableForIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Common stock, sales, available for issuance.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapitalMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Additional Paid-in Capital [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapitalMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Additional Paid In Capital [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Current liabilities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities Current [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Current assets:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets Current [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, par value (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock Par Or Stated Value Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementOfStockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement Of Stockholders Equity [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LicenseAgreementAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">License Agreement [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LicenseAgreementAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">License agreement abstract.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateImmaterialEffect_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateImmaterialEffect_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Change in accounting principle, accounting standards update, immaterial effect</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Less: current portion</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating lease liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Liability Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TermLoanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Term Loan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TermLoanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Term Loan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TermLoanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Second amendment to the amended and restated loan and security agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentMaturityDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Maturity date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentMaturityDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Maturity Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeContractTypeDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Derivative Contract</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeContractTypeDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Derivative Contract Type [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Property, Plant and Equipment, Net, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Property and equipment, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Property Plant And Equipment Net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_EquityIncentivePlansMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Equity Incentive Plans [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_EquityIncentivePlansMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity Incentive Plans [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_EquityIncentivePlansMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Equity incentive plans.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfStockDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Class of Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfStockDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Stock [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesAConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Series A Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesAConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Series A Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesAConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Series A convertible preferred stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentIncomeInterest_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest income</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentIncomeInterest_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Investment Income Interest</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DeferredIncomeCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Deferred grant liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DeferredIncomeCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Deferred Income, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Vested and expected to vest as of March 31, 2024</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Exercisable at March 31, 2024</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TypeOfArrangementAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaborative Arrangement and Arrangement Other than Collaborative</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TypeOfArrangementAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Type Of Arrangement [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockCommonShares_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Treasury stock, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockCommonShares_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Treasury Stock, Common, Shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCentralIndexKey_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Central Index Key</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCentralIndexKey_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Central Index Key</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Number of securities called by warrant or right</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of shares callable by warrants (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Warrant Or Right Number Of Securities Called By Warrants Or Rights</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Plan Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Plan Name [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdopted_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Change in Accounting Principle, Accounting Standards Update, Adopted [true false]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdopted_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Change in accounting principle, accounting standards update, adopted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOtherCurrentAssets_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Other current assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOtherCurrentAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Other Current Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RisksAndUncertaintiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Risks And Uncertainties [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GainLossOnDispositionOfAssets_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Loss on disposal of property and equipment</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GainLossOnDispositionOfAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GainLossOnDispositionOfAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Loss on disposal of property and equipment</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets, Fair Value Disclosure</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Assets, Fair Value Disclosure, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseOptionToExtend_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lessee, operating lease, option to extend</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseOptionToExtend_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Option To Extend</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Preferred Stock, Shares Outstanding, Ending Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Preferred Stock, Shares Outstanding, Beginning Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, shares outstanding (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UnusualOrInfrequentItemsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">COVID-19 Pandemic and CARES Act</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UnusualOrInfrequentItemsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Unusual Or Infrequent Items Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SeriesFPreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Series F Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SeriesFPreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Series F Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total current liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityTaxIdentificationNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Tax Identification Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityTaxIdentificationNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Tax Identification Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RegainComplianceExpiryDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Regain compliance expiry date.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RegainComplianceExpiryDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Regain Compliance Expiry Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RegainComplianceExpiryDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Regain compliance expiry date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Increases (decreases) in cash caused by changes in operating assets and liabilities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Operating Capital [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">New Accounting Pronouncements or Change in Accounting Principle [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeptemberTwoThousandAndTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">September 2022 Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeptemberTwoThousandAndTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">September Two Thousand and Twenty Two Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeptemberTwoThousandAndTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">September two thousand and twenty two distribution agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Financial Instruments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Transfers And Servicing Of Financial Instruments Types Of Financial Instruments [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Remaining Contractual Life (years) [Roll Forward]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Grant [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Grant [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumValueOfSharesToBeIssuedUnderAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Maximum value of shares to be issued under agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumValueOfSharesToBeIssuedUnderAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Maximum Value of Shares to be Issued Under Agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumValueOfSharesToBeIssuedUnderAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Maximum value of shares to be issued under agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ServicesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Services Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ServicesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Services Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ServicesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Services agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Plan Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Plan Name [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeInstrumentRiskAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Derivative Instrument</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeInstrumentRiskAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Derivative Instrument Risk [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseTermOfContract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lessee, operating lease, term of contract</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseTermOfContract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Term Of Contract</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Assets_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Assets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating lease right-use-of assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Right Of Use Asset</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonClassAMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common Class A [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonClassAMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Class A [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityRegistrantName_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Entity Registrant Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityRegistrantName_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Registrant Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockBasedCompensation2015EquityIncentivePlan1Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2015 Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockBasedCompensation2015EquityIncentivePlan1Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Based Compensation2015 Equity Incentive Plan1 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockBasedCompensation2015EquityIncentivePlan1Member_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Stock based compensation 2015 equity incentive plan 1 member.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseExistenceOfOptionToExtend_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lessee, operating lease, existence of option to extend [true false]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseExistenceOfOptionToExtend_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Existence Of Option To Extend</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Related Party</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Related Party, Type [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RecordedUnconditionalPurchaseObligationLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recorded Unconditional Purchase Obligation [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RecordedUnconditionalPurchaseObligationLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Recorded Unconditional Purchase Obligation [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Retained Earnings (Accumulated Deficit), Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accumulated deficit</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Retained Earnings Accumulated Deficit</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Accumulated deficit</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CanaccordGenuityLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Canaccord Genuity L L C Member</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CanaccordGenuityLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Canaccord Genuity LLC [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CanaccordGenuityLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Canaccord Genuity L L C [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Supplemental disclosure of cash flows information:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Supplemental Cash Flow Information [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CharlottesvilleMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Charlottesville [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CharlottesvilleMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Charlottesville [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CharlottesvilleMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Charlottesville</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accumulated Deficit [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Retained Earnings [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesFairValueAdjustment_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Change in fair value of liability instruments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesFairValueAdjustment_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities Fair Value Adjustment</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementClassOfStockAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Class of Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementClassOfStockAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement Class Of Stock [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MinimumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Minimum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MinimumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Minimum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Revenues_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Revenue recognized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Revenues_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Revenue</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Revenues_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenues</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Revenues_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Revenues, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Present value of obligations under leases</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lease liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EquityComponentDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Equity Component [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EquityComponentDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Equity Component</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EquityComponentDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity Component [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdoptionDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Change in Accounting Principle, Accounting Standards Update, Adoption Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdoptionDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Change in accounting principle, accounting standards update, adoption date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LicenseAgreementDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">License Agreements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LicenseAgreementDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">License Agreement Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LicenseAgreementDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Disclosure of accounting policy for license agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AmortizationOfFinancingCostsAndDiscounts_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Amortization of Debt Issuance Costs and Discounts, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AmortizationOfFinancingCostsAndDiscounts_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Amortization of deferred financing costs and debt discount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AmortizationOfFinancingCostsAndDiscounts_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Amortization Of Financing Costs And Discounts</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Adjustments to reconcile net loss to net cash used in operating activities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Adjustments To Reconcile Net Income Loss To Cash Provided By Used In Operating Activities [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Measurement Frequency [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OptionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Outstanding Stock Options [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OptionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Options [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OptionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Options.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Exercise Price [Roll Forward]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price Rollforward</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Basis variable rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Basis Spread On Variable Rate1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCurrentReportingStatus_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Current Reporting Status</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCurrentReportingStatus_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Current Reporting Status</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, shares authorized (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock Shares Authorized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Research and development expense</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Research and development</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Research And Development Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByBalanceSheetGroupingTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value By Balance Sheet Grouping [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByBalanceSheetGroupingTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value By Balance Sheet Grouping [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockRepurchaseProgramAuthorizedAmount1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Authorized amount of repurchase of stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockRepurchaseProgramAuthorizedAmount1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Repurchase Program, Authorized Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationActivityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock Option Activity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationActivityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Share Based Compensation Activity Table [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from sale of common stock, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds From Issuance Of Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Grant Revenue Recognition</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenue from Contract with Customer [Policy Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Securities remaining and available for future issuances (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, $0.001 par value; 100,000,000 shares authorized; 4,522,656 and 4,264,231 issued and outstanding at March 31, 2024, and 4,522,656 issued and 4,444,097 outstanding as of December 31, 2023, respectively</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockValue_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Common stock, $0.001 par value; 100,000,000 shares authorized; 36,123,833 and 33,601,373 issued and outstanding at March 31, 2023 and December 31, 2022, respectively</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Voting stock, value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentOfferingDate1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Origination date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentOfferingDate1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Offering Date1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RepaymentsOfLongTermDebt_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Repayments of Long-Term Debt, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RepaymentsOfLongTermDebt_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Repayments Of Long Term Debt</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RepaymentsOfLongTermDebt_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Principal payments of term loan obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PaymentForTechTransferAndValidationFee_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Payment for tech transfer and validation fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PaymentForTechTransferAndValidationFee_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payment For Tech Transfer And Validation Fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PaymentForTechTransferAndValidationFee_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Payment for tech transfer and validation fee.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CurrentFiscalYearEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Current Fiscal Year End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CurrentFiscalYearEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Current Fiscal Year End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LincolnParkCapitalFundLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lincoln Park Capital Fund, LLC [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LincolnParkCapitalFundLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lincoln Park Capital Fund L L C [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LincolnParkCapitalFundLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Lincoln park capital fund, LLC.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share-based compensation expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensation_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Share-Based Payment Arrangement, Noncash Expense, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Operating loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Income Loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ConsiderationPaidToPreferredStockHolders_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Consideration paid to preferred stock holders.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ConsiderationPaidToPreferredStockHolders_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Consideration Paid To Preferred Stock Holders</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ConsiderationPaidToPreferredStockHolders_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Consideration paid to preferred stock holders</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UseOfEstimatesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Use Of Estimates [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UseOfEstimatesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Use of Estimates [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Loan and Security Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Loan And Security Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Loan and security agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PeriodExercisableFromTheDateOfIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Period exercisable from the date of issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PeriodExercisableFromTheDateOfIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Period Exercisable From The Date Of Issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PeriodExercisableFromTheDateOfIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Refers to the period exercisable from the date of issuance.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisaggregationOfRevenueTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disaggregation of Revenue [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock option vesting percentage</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sharebased Compensation Arrangement By Sharebased Payment Award Award Vesting Rights Percentage</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ConsecutiveBusinessDays_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Consecutive business days</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ConsecutiveBusinessDays_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Consecutive Business Days</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ConsecutiveBusinessDays_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Consecutive business days.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOperatingLeaseLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase (Decrease) in Operating Lease Liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOperatingLeaseLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Change in operating lease liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ContractEffectiveDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Contract effective date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ContractEffectiveDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Contract Effective Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ContractEffectiveDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Contract effective date.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Grant Revenue</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenue from Contract with Customer [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Balance as of March 31, 2024</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Intrinsic Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentFaceAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Original loan amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentFaceAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Face Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UTHSCSALicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">UTHSCSA license agreement member.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UTHSCSALicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">U T H S C S A License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UTHSCSALicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">UTHSCSA License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockPricePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale of Stock, Price Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockPricePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale Of Stock Price Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockIssuedDuringPeriodSharesWarrantsExerciseShares_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Issuance of common stock for exercise of warrants (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockIssuedDuringPeriodSharesWarrantsExerciseShares_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period Shares Warrants Exercise Shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockIssuedDuringPeriodSharesWarrantsExerciseShares_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Stock issued during period shares warrants exercise(shares).</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating Leases</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Liabilities Payments Due [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating expenses:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Expenses [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsequentEventsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subsequent Events [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commitments And Contingencies Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Assets Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Granted (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock shares to be issued (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stockholders' Equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders Equity Note Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestPaidNet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestPaidNet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Interest Paid Net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LiquidityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liquidity [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LiquidityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Liquidity Abstract</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_InitialCommitmentShares_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Initial commitment shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_InitialCommitmentShares_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Initial Commitment Shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_InitialCommitmentShares_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Initial commitment shares.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SaleOfJapaneseSubsidiaryAndCertainAssetsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale of the Japanese Subsidiary and Certain Assets [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SaleOfJapaneseSubsidiaryAndCertainAssetsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale Of Japanese Subsidiary And Certain Assets [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SaleOfJapaneseSubsidiaryAndCertainAssetsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Sale of the japanese subsidiary and certain assets.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapital_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Additional Paid in Capital, Total</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapital_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Additional Paid In Capital</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonClassBMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common Class B [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonClassBMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Class B [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Vested and expected to vest at March 31, 2024 (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Vested And Expected To Vest Exercisable Weighted Average Exercise Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stockholders' deficit:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders Equity [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseCostTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Lease Costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseCostTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lease Cost Table [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityMaturityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Future Minimum Annual Lease Payments under Operating Leases</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityMaturityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Liability Maturity Table [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_LitigationCaseAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Litigation Case</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_LitigationCaseAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Litigation Case [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ContractTerminationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Contract termination date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ContractTerminationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Contract Termination Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ContractTerminationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Contract termination date.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockConsiderationReceivedOnTransaction_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale of Stock, Consideration Received on Transaction</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockConsiderationReceivedOnTransaction_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Consideration to purchaser in cash</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share Based Compensation Arrangement By Share Based Payment Award [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Earnings Per Share, Basic, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss per share, basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share Basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingencies_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Commitments and contingencies (Note 8)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingencies_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commitments And Contingencies</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeStatementAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Income Statement [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Related Party</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Related Party Transactions By Related Party [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Increase (Decrease) in Accounts Payable and Accrued Liabilities, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accounts payable and accrued expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Accounts Payable And Accrued Liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_GrantFundingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Grant funding term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_GrantFundingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Grant Funding Term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_GrantFundingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Grant funding term.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Vested and expected to vest at March 31, 2024 (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Vested And Expected To Vest Outstanding Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Statistical Measurement [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statistical Measurement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Range [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net cash used in operating activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net cash used in operating activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided By Used In Operating Activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeLossFromContinuingOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net income from continuing operations</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeLossFromContinuingOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeLossFromContinuingOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentPeriodEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Period End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentPeriodEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Period End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Statistical Measurement [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statistical Measurement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Range [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Commitments and Contingencies</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commitments And Contingencies Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityNoteAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders Equity Note [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeasePayments_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating cash used for operating leases</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeasePayments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Payments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Earnings Per Share, Diluted, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss per share, diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share Diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_ProductsAndServicesDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Product and Service [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MinimumLiquidityCovenantAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Minimum liquidity covenant</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MinimumLiquidityCovenantAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Minimum Liquidity Covenant Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MinimumLiquidityCovenantAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Minimum liquidity covenant amount.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseCostAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lease expense:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseCostAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lease Cost [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenue from Contract with Customer [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument, Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Name [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Hierarchy and NAV [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Exercisable at March 31, 2024 (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementsRecurringMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recurring Basis [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementsRecurringMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Recurring [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LiborMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">LIBOR [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LiborMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">LIBOR [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LiborMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">LIBOR.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Options [Roll Forward]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_USTreasurySecuritiesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Treasury Bills and Government Agency Bonds [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_USTreasurySecuritiesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">US Treasury Securities [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RepresentativeSeriesUWarrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Representative Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RepresentativeSeriesUWarrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Representative Series U Warrant [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_RepresentativeSeriesUWarrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Representative Series U Warrant.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_InitialCommitmentFee_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Initial commitment fee paid in cash</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_InitialCommitmentFee_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Initial Commitment Fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_InitialCommitmentFee_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Initial commitment fee.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AggregateExpenseIncludingFeeAndDisbursementExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Reimburse of fee and disbursements expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AggregateExpenseIncludingFeeAndDisbursementExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Aggregate Expense Including Fee and Disbursement Expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AggregateExpenseIncludingFeeAndDisbursementExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Aggregate expense including fee and disbursement expenses.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesFConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Series F Convertible Preferred Stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesFConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Series F Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Liabilities and Stockholders&#8217; Equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities And Stockholders Equity [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractWithCustomerAssetGross_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Grant amount awarded</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractWithCustomerAssetGross_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Contract with Customer, Asset, before Allowance for Credit Loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClassOfWarrantOrRightExpirationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrant expiration date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClassOfWarrantOrRightExpirationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Warrant Or Right Expiration Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClassOfWarrantOrRightExpirationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">The date that the warrant or right expires.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressPostalZipCode_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, Postal Zip Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressPostalZipCode_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address Postal Zip Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityInteractiveDataCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Interactive Data Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityInteractiveDataCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Interactive Data Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockDividendRatePercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Convertible preferred stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockDividendRatePercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock Dividend Rate Percentage</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Less: amount representing interest</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Liability Undiscounted Excess Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CompensatoryDamagesAndOperational_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Compensatory damages and operational</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CompensatoryDamagesAndOperational_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Compensatory Damages And Operational</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CompensatoryDamagesAndOperational_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Compensatory damages and operational.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityIncorporationStateCountryCode_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Incorporation, State or Country Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityIncorporationStateCountryCode_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Incorporation State Country Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Aggregated Intrinsic Value [Roll Forward]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement by Share Based Payment Award Options Aggregate Intrinsic Value [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Share based compensation arrangement by share based payment award options aggregate intrinsic value.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Equity Components [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Equity Components</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement Equity Components [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NanoTxLicensesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Nano Tx Licenses Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NanoTxLicensesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Nano Tx Licenses Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NanoTxLicensesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Nano Tx license agreement Member</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recently Issued Accounting Pronouncements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">New Accounting Pronouncements And Changes In Accounting Principles [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DisclosureOfLiquidityAndGoingConcernTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Disclosure of liquidity and going concern text block</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DisclosureOfLiquidityAndGoingConcernTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disclosure Of Liquidity And Going Concern Text Block</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DisclosureOfLiquidityAndGoingConcernTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Liquidity and Going Concern</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage issuance of common stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage Issuance Of Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage issuance of common stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LocalPhoneNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Local Phone Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LocalPhoneNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Local Phone Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale of Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subsidiary Sale Of Stock [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Exercisable at March 31, 2024 (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OperatingLeasesExpiryYear_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating leases expiry year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OperatingLeasesExpiryYear_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Leases Expiry Year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OperatingLeasesExpiryYear_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Operating leases, expiry year.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Goodwill_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Goodwill, Ending Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Goodwill_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Goodwill, Beginning Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Goodwill_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Goodwill, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Goodwill_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Goodwill</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Goodwill_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Goodwill</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementOfCashFlowsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement Of Cash Flows [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfNonDilutiveMonetaryAwardsRequiredToPay_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of non-dilutive monetary awards required to pay.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfNonDilutiveMonetaryAwardsRequiredToPay_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage of Non-dilutive Monetary awards Required to Pay</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfNonDilutiveMonetaryAwardsRequiredToPay_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of non-dilutive monetary awards required to pay</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementInputsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Measurements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementInputsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurement Inputs Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesCConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Series C Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesCConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Series C Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesCConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Series C convertible preferred stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Organization Consolidation And Presentation Of Financial Statements [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, par value (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock Par Or Stated Value Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PreClinicalResearchStudyObligationsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Pre-clinical Research Study Obligations [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PreClinicalResearchStudyObligationsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Pre Clinical Research Study Obligations [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PreClinicalResearchStudyObligationsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Contractual obligation to provide funding of research and development costs over periods that initially exceed one year or the normal operating cycle, if longer.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument, Interest Rate, Stated Percentage</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Weighted average period for recognition of cost</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Vested and expected to vest at March 31, 2024</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sharebased Compensation Arrangement By Sharebased Payment Award Options Vested And Expected To Vest Outstanding Weighted Average Remaining Contractual Term1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Share Based Compensation Arrangements By Share Based Payment Award [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Share Based Compensation Arrangements By Share Based Payment Award [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MaximumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Maximum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MaximumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Maximum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OptionForExclusiveWorldwideLicense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Option for exclusive worldwide license</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OptionForExclusiveWorldwideLicense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Option For Exclusive Worldwide License</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OptionForExclusiveWorldwideLicense_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Option for exclusive worldwide license.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OutstandingWarrantsAndOptionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Outstanding Warrants and Options [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OutstandingWarrantsAndOptionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Outstanding Warrants and Options [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_OutstandingWarrantsAndOptionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Outstanding warrants and options.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesBConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Series B Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesBConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Series B Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesBConvertiblePreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Series B convertible preferred stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel3Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Level 3 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel3Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Inputs, Level 3 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">APIC, Share-Based Payment Arrangement, Increase for Cost Recognition, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock-based compensation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Recurring and Nonrecurring [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAdjustmentOfWarrants_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Change in fair value of liability instruments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAdjustmentOfWarrants_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Adjustment Of Warrants</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net decrease in cash and cash equivalents</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_JanuaryTwoThousandAndTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">January 2022 Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_JanuaryTwoThousandAndTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">January Two Thousand and Twenty Two Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_JanuaryTwoThousandAndTwentyTwoDistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">January two thousand and twenty two distribution agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfAggregateAmountOfProceedsAwardedUnderCpritGrant_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of aggregate amount of proceeds awarded under CPRIT grant</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfAggregateAmountOfProceedsAwardedUnderCpritGrant_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage Of Aggregate Amount Of Proceeds Awarded Under CPRIT Grant</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfAggregateAmountOfProceedsAwardedUnderCpritGrant_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of aggregate amount of proceeds awarded under CPRIT grant.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Cancelled/forfeited (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures In Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfVotesPerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Number of votes per share.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfVotesPerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Number Of Votes Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleSecuritiesPurchasedOptionsPricePolicy_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Available-for-Sale Securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleSecuritiesPurchasedOptionsPricePolicy_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale, Premium on Purchased Options [Policy Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule of Antidilutive Securities Excluded from Diluted Income (Loss) per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClosingBidPriceForCommonStockHadFallenBelow_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Closing bid price for common stock had fallen below</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClosingBidPriceForCommonStockHadFallenBelow_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Closing Bid Price For Common Stock Had Fallen Below</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ClosingBidPriceForCommonStockHadFallenBelow_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Closing bid price for common stock had fallen.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForRoyalties_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">License fee payment in cash</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForRoyalties_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payments For Royalties</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CityAreaCode_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">City Area Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CityAreaCode_lbl" xlink:role="http://www.xbrl.org/2003/role/label">City Area Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Research and Development Arrangement, Contract to Perform for Others, Type</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Research And Development Arrangement Contract To Perform For Others By Type [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash flows used in operating activities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided By Used In Operating Activities [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockTransactionDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale of Stock, Transaction Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockTransactionDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale closed date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaintainMinimumMarketValueOfListedSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Maintain minimum market value of listed securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaintainMinimumMarketValueOfListedSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Maintain Minimum Market Value of Listed Securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaintainMinimumMarketValueOfListedSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Maintain minimum market value of listed securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">General and Administrative Expense, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">General and administrative</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">General And Administrative Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Weighted Average Number of Shares Outstanding, Basic, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Basic weighted average shares used in calculating net loss per share attributable to common stockholders</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Weighted Average Number Of Shares Outstanding Basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CpritGrantFundingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">CPRIT grant funding term, description</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CpritGrantFundingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/label">CPRIT Grant Funding Term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CpritGrantFundingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">CPRIT grant funding term.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Research and Development Arrangement, Contract to Perform for Others, Type</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Research And Development Arrangement Contract To Perform For Others Type [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash flows used in/provided by financing activities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided By Used In Financing Activities [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statement [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalPeriodFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Fiscal Period Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalPeriodFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Fiscal Period Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total minimum lease payments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Liability Payments Due</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LongTermDebtOutstandingThresholdAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Long term debt outstanding threshold amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LongTermDebtOutstandingThresholdAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Long Term Debt Outstanding Threshold Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_LongTermDebtOutstandingThresholdAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Long term debt outstanding threshold amount.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Liability Payments Due Year Two</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2025</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statement [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementTermsDescription_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Master services agreement terms, Description</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementTermsDescription_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Master Services Agreement Terms Description</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementTermsDescription_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Master services agreement Terms description.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Dilutive common shares excluded from the calculations of diluted loss per share (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseCost_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total lease expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LeaseCost_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lease Cost</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwentyTwentyPurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2020 Purchase Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwentyTwentyPurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Twenty Twenty Purchase Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_TwentyTwentyPurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Twenty twenty purchase agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Outstanding Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Warrant [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfWarrantsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of warrants exercised</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfWarrantsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Number Of Warrants Exercised</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfWarrantsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Number of warrants exercised</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_IncreaseDecreaseInDeferredGrantLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Deferred grant liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_IncreaseDecreaseInDeferredGrantLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Deferred Grant Liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_IncreaseDecreaseInDeferredGrantLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Increase (decrease) in deferred grant liability.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2020 Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Distribution Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DistributionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Distribution agreement member.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Cash and Cash Equivalents, at Carrying Value, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash and cash equivalents</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash And Cash Equivalents At Carrying Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other current assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Assets Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Exercisable at March 31, 2024</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Intrinsic Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Accretion of discount on short-term investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accretion (Amortization) of Discounts and Premiums, Investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Common Stock, Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Common Stock Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountingStandardsUpdate201613Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounting Standards Update 2016-13 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountingStandardsUpdate201613Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">ASU 2016-13</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Conversion of Series B Convertible Preferred Stock into common stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period Value Conversion Of Convertible Securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Treasury stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Treasury Stock, Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Treasury Stock, Value, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CoverAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cover [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalYearFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Fiscal Year Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalYearFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Fiscal Year Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfMaximumCommonSharesCanBeResale_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of maximum common shares can be resale</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfMaximumCommonSharesCanBeResale_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Number of Maximum Common Shares Can Be Resale</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfMaximumCommonSharesCanBeResale_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Number of maximum common shares can be resale.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LitigationSettlementExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Litigation settlement, expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LitigationSettlementExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Litigation Settlement Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale of Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale Of Stock Name Of Transaction [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Convertible Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Preferred Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MedidataSolutionsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Medidata Solutions, Inc. [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MedidataSolutionsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Medidata Solutions Inc [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MedidataSolutionsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Medidata solutions, inc.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_SecurityExchangeName_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Security Exchange Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_SecurityExchangeName_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Security Exchange Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock option contractual term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sharebased Compensation Arrangement By Sharebased Payment Award Expiration Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounting Standards Update and Change in Accounting Principle [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recently Issued Accounting Pronouncements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">New Accounting Pronouncements, Policy [Policy Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, shares issued (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock Shares Issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, shares authorized (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock Shares Authorized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaymentInformationAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash payment information:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaymentInformationAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash Payment Information [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaymentInformationAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Cash payment information.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockIssuedDuringPeriodSharesWarrantsExercise_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Issuance of common stock for exercise of warrants</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockIssuedDuringPeriodSharesWarrantsExercise_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period Shares Warrants Exercise</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockIssuedDuringPeriodSharesWarrantsExercise_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Stock issued during period shares warrants exercise.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockSharesAcquired_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Purchase of treasury stock (in Shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockSharesAcquired_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Purchase of treasury stock (in Shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockSharesAcquired_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Treasury Stock, Shares, Acquired</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cancelled/forfeited (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangements By Share Based Payment Award Options Forfeitures In Period Weighted Average Exercise Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AtTheMarketOfferingProgramMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">At The Market Offering Program [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AtTheMarketOfferingProgramMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">At The Market Offering Program [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AtTheMarketOfferingProgramMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">At the market offering program.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LegalEntityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Legal Entity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LegalEntityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Legal Entity [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FinancialInstrumentAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Financial Instrument</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FinancialInstrumentAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Financial Instrument [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantsAndRightsOutstandingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrants expected term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantsAndRightsOutstandingTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Warrants And Rights Outstanding Term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForLoans_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Payments for principal, interest and fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForLoans_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payments For Loans</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityEmergingGrowthCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Emerging Growth Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityEmergingGrowthCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Emerging Growth Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_AmendmentFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Amendment Flag</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_AmendmentFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Amendment Flag</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquireIntangibleAssets_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Purchase of intangible assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquireIntangibleAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Purchases of intangible assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquireIntangibleAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payments to Acquire Intangible Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ReverseStockSplitEffectiveDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Reverse stock split effective date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ReverseStockSplitEffectiveDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Reverse Stock Split Effective Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_ReverseStockSplitEffectiveDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Reverse stock split effective date.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TypeOfAdoptionMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounting Standards Update [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Basis of Presentation and New Accounting Standards</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Basis Of Accounting Policy Policy [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Shares issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Remaining common stock issued and sold (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Shares Issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Variable Rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Variable Rate [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFileNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity File Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFileNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity File Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee Operating Lease Liability Payments Remainder Of Fiscal Year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Remainder of 2023</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Date from which warrants are exercisable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Warrant Or Righst Date From Which Warrants Or Rights Exercisable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Vested and expected to vest as of March 31, 2024</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermDebtNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Long-Term Debt, Excluding Current Maturities, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermDebtNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Term loan obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermDebtNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Long Term Debt Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_BioceptAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Biocept Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_BioceptAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Biocept Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_BioceptAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Biocept Agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RecordedUnconditionalPurchaseObligationTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recorded Unconditional Purchase Obligation [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RecordedUnconditionalPurchaseObligationTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Recorded Unconditional Purchase Obligation [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DeferredIncomeNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Deferred Income, Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DeferredIncomeNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Deferred grant liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisposalGroupClassificationDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Disposal Group Classification</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisposalGroupClassificationDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disposal Group Classification [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AccruedLicensePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Accrued license payable.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AccruedLicensePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accrued License Payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_AccruedLicensePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued payments due to NanoTx</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total operating expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherIntangibleAssetsNet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Intangible assets, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherIntangibleAssetsNet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Intangible Assets Net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityReverseStockSplit_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Reverse stock split</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityReverseStockSplit_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders' Equity, Reverse Stock Split</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine2_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, Address Line Two</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine2_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address Address Line2</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, $0.001 par value; 5,000,000 shares authorized; 1,952 shares issued and outstanding at March 31, 2024 and December 31, 2023, respectively</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntitySmallBusiness_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Small Business</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntitySmallBusiness_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Small Business</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityShellCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Shell Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityShellCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Shell Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentTypeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Investment Type</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentTypeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Investment Type [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrant exercise price (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Warrant Or Right Exercise Price Of Warrants Or Rights1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_GrossProceedsFromIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Aggregate gross proceeds from sales of common stock</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Class of Warrant or Right</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Warrant Or Right [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Loss per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesUWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Series U Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesUWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Series U Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SeriesUWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Series U warrants member</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, Address Line One</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address Address Line1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Contractual Obligation, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Contractual obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Contractual Obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsequentEventTypeDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subsequent Event Type [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumGrantAmountToBeReceivedAsPerGrantContract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Maximum grant amount to be received as per grant contract</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumGrantAmountToBeReceivedAsPerGrantContract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Maximum Grant Amount to be Received as per Grant Contract</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MaximumGrantAmountToBeReceivedAsPerGrantContract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Maximum grant amount to be received as per grant contract.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AmortizationOfDebtDiscountPremium_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Non-cash amortization</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SalesOrderTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sales order term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SalesOrderTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sales Order Term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SalesOrderTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Sales order term.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel2Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Level 2 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel2Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Inputs, Level 2 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Closing price per share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Arrangement and Arrangement Other than Collaborative [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Right Of Use Asset Obtained In Exchange For Finance Lease Liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Right-of-use assets obtained in exchange for operating lease liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Master service agreement date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Master Services Agreement Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Master services agreement date.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DepreciationDepletionAndAmortization_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Depreciation, Depletion and Amortization, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DepreciationDepletionAndAmortization_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Depreciation and amortization</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DepreciationDepletionAndAmortization_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Depreciation Depletion And Amortization</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_ProductOrServiceAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Product and Service [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockValueAcquiredCostMethod_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Purchase of treasury stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockValueAcquiredCostMethod_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Purchase of treasury stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockValueAcquiredCostMethod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Treasury Stock, Value, Acquired, Cost Method</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementInitialTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Master services agreement initial term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementInitialTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Master Services Agreement Initial Term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_MasterServicesAgreementInitialTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Master services agreement initial term.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockSplits_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, reverse stock split, fractional shares issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockSplits_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period, Shares, Stock Splits</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfCommissionOnGrossProceedsFromSaleOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of commission on gross proceeds from sale of common stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfCommissionOnGrossProceedsFromSaleOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage of Commission on Gross Proceeds from Sale of Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_PercentageOfCommissionOnGrossProceedsFromSaleOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of commission on gross proceeds from sale of common stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_Security12bTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Title of 12(b) Security</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_Security12bTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Security12b Title</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_GrantFundingPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Grant funding period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_GrantFundingPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Grant Funding Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_GrantFundingPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Grant funding period.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaidForIPRDAssetAcquiredInAnAssetAcquisition_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">In process research and development acquired</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaidForIPRDAssetAcquiredInAnAssetAcquisition_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash Paid For I P R D Asset Acquired In An Asset Acquisition</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CashPaidForIPRDAssetAcquiredInAnAssetAcquisition_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Cash paid for IPRD asset acquired in an asset acquisition.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Financial Assets Measured at Fair Value on Recurring Basis</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Assets Measured on Recurring Basis [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentTypeCategorizationMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentTypeCategorizationMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Investment Type Categorization [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounting Standards Update [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockCommonMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Treasury Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TreasuryStockCommonMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Treasury Stock, Common [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SubscriptionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Subscription Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SubscriptionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subscription Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SubscriptionAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Subscription and Investment Representation Agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UseOfEstimatesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Use of Estimates</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UseOfEstimatesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Use Of Estimates [Text Block]</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash flows used in investing activities:</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UnpaidOfferingCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Unpaid offering cost</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UnpaidOfferingCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Unpaid Offering Costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_UnpaidOfferingCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Unpaid Offering Costs.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Accounts Payable and Accrued Liabilities, Current, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accounts payable and accrued expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounts Payable And Accrued Liabilities Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash and Cash Equivalents [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Exercisable at March 31, 2024</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Remained authorized amount for repurchase</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Common Stock, Shares, Outstanding, Ending Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Common Stock, Shares, Outstanding, Beginning Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, shares outstanding (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock-based Compensation</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SalesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sales Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SalesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sales Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_SalesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Sales agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LesseeOperatingLeaseDiscountRate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lessee, Operating Lease, Discount Rate</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentType_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Type</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentType_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Type</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Conversion of Series B Convertible Preferred Stock into common stock (share)</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentFeeAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fees amount associated with loan</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentQuarterlyReport_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Quarterly Report</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisposalGroupClassificationAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Disposal Group Classification</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisposalGroupClassificationAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disposal Group Classification [Axis]</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided By Used In Financing Activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFilerCategory_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Filer Category</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFilerCategory_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Filer Category</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_NumberOfPreferredStockSharesExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Number Of Preferred Stock Shares Exercised.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Supplemental schedule of non-cash investing and financing activities:</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recorded Unconditional Purchase Obligation by Category of Item Purchased</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Recorded Unconditional Purchase Obligation By Category Of Item Purchased [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Variable Rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Variable Rate [Axis]</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Short-Term Investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Short-Term Investments, Total</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Liabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Liabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquireShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Purchase of short-term investments</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesGrossUnrealizedGain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Unrealized gain on available-for-sale securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesGrossUnrealizedGain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale, Unrealized Gain</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total stockholders' deficit</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stockholders' deficit</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders Equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">SOFR [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock issued during period reverse stock splits, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period, Shares, Reverse Stock Splits</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Income Loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Interest Expense, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpense_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Interest expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Interest Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash and Cash Equivalents [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementOfFinancialPositionAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement Of Financial Position [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BasisOfPresentationAndSignificantAccountingPoliciesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Basis of Presentation and New Accounting Standards</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BasisOfPresentationAndSignificantAccountingPoliciesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Basis Of Presentation And Significant Accounting Policies [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Weighted Average Number of Shares Outstanding, Diluted, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Diluted weighted average shares used in calculating net loss per share attributable to common stockholders</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Weighted Average Number Of Diluted Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Reduction in the carrying amount of operating lease right-of-use assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease, Right-of-Use Asset, Periodic Reduction</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherIncomeAndExpensesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other income (expense):</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherIncomeAndExpensesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Income And Expenses [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating lease, weighted average remaining term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease Weighted Average Remaining Lease Term1</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UseOfEstimates_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Use of Estimates</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UseOfEstimates_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Use of Estimates, Policy [Policy Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Balance as of March 31, 2024 (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Balance as of December 31, 2023 (in dollars per share)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Unconditional Purchase Obligation, Category of Goods or Services Acquired</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Unconditional Purchase Obligation Category Of Goods Or Services Acquired [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DebtInstrumentFloatingInterestRatePercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DebtInstrumentFloatingInterestRatePercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Floating Interest Rate Percentage</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_DebtInstrumentFloatingInterestRatePercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Floating interest rate for funds borrowed, under the debt agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CancerPreventionAndResearchInstituteOfTexasContractMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">CPRIT Contract [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CancerPreventionAndResearchInstituteOfTexasContractMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cancer Prevention And Research Institute Of Texas Contract [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_CancerPreventionAndResearchInstituteOfTexasContractMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Cancer prevention and research institute of texas contract.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisaggregationOfRevenueLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disaggregation of Revenue [Line Items]</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueDisclosuresAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Disclosures [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Hierarchy and NAV [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Measurement Frequency [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockBasedCompensation2020EquityIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2020 Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockBasedCompensation2020EquityIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Based Compensation2020 Equity Incentive Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_StockBasedCompensation2020EquityIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Stock based compensation 2020 equity incentive plan.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Payments to Acquire Property, Plant, and Equipment, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Purchases of property and equipment</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale of common stock, net</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="pstv_WarrantsIssuedUnderwrittenPublicOfferingPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Warrants issued, underwritten public offering, period.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Rent expense</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Cash and cash equivalents at beginning of period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Cash and cash equivalents at end of period</link:label>
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  <xsd:element id="pstv_NumberOfPreferredStockSharesExercised" name="NumberOfPreferredStockSharesExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
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  <xsd:element id="pstv_ContractTerminationDate" name="ContractTerminationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
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  <xsd:element id="pstv_OptionsMember" name="OptionsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_ServicesAgreementMember" name="ServicesAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_DistributionAgreementMember" name="DistributionAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_TwoThousandTwentyTwoDistributionAgreementMember" name="TwoThousandTwentyTwoDistributionAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_MasterServicesAgreementInitialTerm" name="MasterServicesAgreementInitialTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_CancerPreventionAndResearchInstituteOfTexasContractMember" name="CancerPreventionAndResearchInstituteOfTexasContractMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_PercentageOfNonDilutiveMonetaryAwardsRequiredToPay" name="PercentageOfNonDilutiveMonetaryAwardsRequiredToPay" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_DebtInstrumentFloatingInterestRatePercentage" name="DebtInstrumentFloatingInterestRatePercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false"/>
  <xsd:element id="pstv_LoremVascularPteLtdMember" name="LoremVascularPteLtdMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_LoanAndSecurityAgreementMember" name="LoanAndSecurityAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_ClassOfWarrantOrRightExpirationDate" name="ClassOfWarrantOrRightExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_SeriesBConvertiblePreferredStockMember" name="SeriesBConvertiblePreferredStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_OperatingLeasesExpiryYear" name="OperatingLeasesExpiryYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_CashPaymentInformationAbstract" name="CashPaymentInformationAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_StockIssuedDuringPeriodSharesWarrantsExercise" name="StockIssuedDuringPeriodSharesWarrantsExercise" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="pstv_PreClinicalResearchStudyObligationsMember" name="PreClinicalResearchStudyObligationsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_SaleOfJapaneseSubsidiaryAndCertainAssetsMember" name="SaleOfJapaneseSubsidiaryAndCertainAssetsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_CashPaidDuringPeriodForAbstract" name="CashPaidDuringPeriodForAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_CanaccordGenuityLLCMember" name="CanaccordGenuityLLCMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_SeriesCConvertiblePreferredStockMember" name="SeriesCConvertiblePreferredStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_MaximumSharesToBeIssuedUnderAgreement" name="MaximumSharesToBeIssuedUnderAgreement" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false"/>
  <xsd:element id="pstv_InitialCommitmentFee" name="InitialCommitmentFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="pstv_ConsecutiveBusinessDays" name="ConsecutiveBusinessDays" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_CommonStockSalesAvailableForIssuance" name="CommonStockSalesAvailableForIssuance" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="pstv_InitialCommitmentShares" name="InitialCommitmentShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false"/>
  <xsd:element id="pstv_TermLoanMember" name="TermLoanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_StockBasedCompensation2020EquityIncentivePlanMember" name="StockBasedCompensation2020EquityIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_GrantFundingPeriod" name="GrantFundingPeriod" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_SalesOrderTerm" name="SalesOrderTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_CpritGrantFundingTerm" name="CpritGrantFundingTerm" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_SeriesAConvertiblePreferredStockMember" name="SeriesAConvertiblePreferredStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_PercentageOfCommissionOnGrossProceedsFromSaleOfCommonStock" name="PercentageOfCommissionOnGrossProceedsFromSaleOfCommonStock" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_DisclosureOfLiquidityAndGoingConcernTextBlock" name="DisclosureOfLiquidityAndGoingConcernTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_LicenseAgreementDisclosureTextBlock" name="LicenseAgreementDisclosureTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_ContractEffectiveDate" name="ContractEffectiveDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_EquityIncentivePlansMember" name="EquityIncentivePlansMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_PercentageIssuanceOfCommonStock" name="PercentageIssuanceOfCommonStock" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="pstv_SeriesUWarrantsMember" name="SeriesUWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="pstv_NumberOfMaximumCommonSharesCanBeResale" name="NumberOfMaximumCommonSharesCanBeResale" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false"/>
  <xsd:element id="pstv_PeriodForRegainCompliance" name="PeriodForRegainCompliance" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
</xsd:schema>
