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Stock Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2023
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Option Activity Under Company's Equity Incentive Plans | The following table summarizes stock option activity under the Company’s 2018 Plan and related information:
|  | Number of Shares Underlying Outstanding Options |  |  | Weighted- Average Exercise Price |  |  | Weighted- Average Remaining Contractual Term (Years) |
Outstanding — December 31, 2022 |  |  | 8,678,767 |  |  | $ | 1.42 |  |  |  | 7.47 |
Options granted (1) |  |  | 14,008,093 |  |  | $ | 0.86 |  |  |  |  |
Options forfeited and cancelled (1) |  |  | (14,210,119 | ) |  | $ | 0.91 |  |  |  |  |
Options expired |  |  | (12,174 | ) |  | $ | 36.16 |  |  |  |  |
Outstanding — March 31, 2023 |  |  | 8,464,567 |  |  | $ | 1.30 |  |  |  | 6.01 |
Exercisable — March 31, 2023 |  |  | 7,306,692 |  |  | $ | 1.40 |  |  |  |  |
| (1) | Includes options that were cancelled and re-granted as part of the option modification from the special cash dividend, as further discussed above.
Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options | The fair value of employee stock options was estimated using the following weighted-average assumptions:
|  | Three Months Ended March 31, |
|  | 2023 |  |  | 2022 |
Employee Stock Options: |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | — |  |  |  | 1.87 | %
Expected term (in years) |  |  | — |  |  |  | 6.1 |
Dividend yield |  |  | — |  |  |  | — |
Volatility |  |  | — |  |  |  | 91.54 | %
Weighted-average fair value of stock options granted |  | $ | — |  |  | $ | 0.39 |
Summary of Stock-Based Compensation Expense Recognized | Total stock-based compensation expense recognized was as follows (in thousands):
|  | Three Months Ended March 31, |
|  | 2023 |  |  | 2022 |
Research and development |  | $ | 66 |  |  | $ | 128 |
General and administrative(1) |  |  | 144 |  |  |  | 387 |
Total stock-based compensation expense |  | $ | 210 |  |  | $ | 515 |
| (1) | No shares of common stock were issued to board members for the three months ended March 31, 2023.

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
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