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Stock Based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2023
Share-Based Payment Arrangement [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 exercised |  |  | (215,067 | ) |  | $ | 0.10 |  |  |  |
Options forfeited and cancelled (1) |  |  | (14,210,119 | ) |  | $ | 0.91 |  |  |  |
Options expired |  |  | (14,729 | ) |  | $ | 34.33 |  |  |  |
Outstanding — June 30, 2023 |  |  | 8,246,945 |  |  | $ | 1.32 |  |  |  | 5.68 |
Exercisable — June 30, 2023 |  |  | 7,412,252 |  |  | $ | 1.39 |  |  |  |
(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 June 30, |  |  | Six Months Ended June 30, |
|  | 2023 |  | 2022 |  |  | 2023 |  | 2022 |
Employee Stock Options: |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  | n/a |  |  | 2.93 | % |  | n/a |  |  | 2.22 | %
Expected term (in years) |  | n/a |  |  | 6.0 |  |  | n/a |  |  | 6.0 |
Dividend yield |  | n/a |  |  | — |  |  | n/a |  |  | — |
Volatility |  | n/a |  |  | 91.82 | % |  | n/a |  |  | 91.63 | %
Weighted-average fair value of stock options granted |  | n/a |  | $ | 0.87 |  |  | n/a |  | $ | 0.55 |
Summary of Stock-Based Compensation Expense Recognized | Total stock-based compensation expense recognized was as follows (in thousands):
|  | Three Months Ended June 30, |  |  | Six Months Ended June 30, |
|  | 2023 |  |  | 2022 |  |  | 2023 |  |  | 2022 |
Research and development |  | $ | 1 |  |  | $ | 83 |  |  | $ | 67 |  |  | $ | 211 |
General and administrative(1) |  |  | 88 |  |  |  | 263 |  |  |  | 232 |  |  |  | 650 |
Total stock-based compensation expense |  | $ | 89 |  |  | $ | 346 |  |  | $ | 299 |  |  | $ | 861 |
(1) No shares of common stock were issued to Board members for the three and six months ended June 30, 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 -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + 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 -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -SubTopic 10 -Topic 718 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (e) -SubTopic 10 -Topic 718 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + 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 -Topic 718 -SubTopic 10 -Subparagraph (f)(2) -Name Accounting Standards Codification -Paragraph 2 -Section 50 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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