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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Summary of All Option Activity Under 2007 Plan, 2014 Plan, 2020 Plan and Inducement Grants | The following table summarizes all option activity under the 2007 Plan, 2014 Plan, 2020 Plan and inducement grants:
|  | Shares Issuable Under Options |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Contractual Term |  |  | Aggregate Intrinsic Value (in thousands) |
|  |  |  |  |  |  |  | (In years) |  |  |  |
Outstanding as of January 1, 2022 |  |  | 4,213,977 |  |  | $ | 10.33 |  |  |  | 8.4 |  |  | $ | — |
Granted |  |  | 1,267,700 |  |  |  | 3.81 |  |  |  | — |  |  |  | — |
Forfeited / Canceled |  |  | (263,644 | ) |  |  | 12.12 |  |  |  | — |  |  |  | — |
Outstanding as of December 31, 2022 |  |  | 5,218,033 |  |  | $ | 8.69 |  |  |  | 8.1 |  |  | $ | — |
Granted |  |  | 11,026,451 |  |  |  | 2.10 |  |  |  | — |  |  |  | — |
Forfeited / Canceled |  |  | (862,631 | ) |  |  | 8.07 |  |  |  | — |  |  |  | — |
Outstanding as of December 31, 2023 |  |  | 15,381,853 |  |  | $ | 4.21 |  |  |  | 8.9 |  |  | $ | 235 |
Options vested and expected to vest as of December 31, 2023 |  |  | 9,476,369 |  |  | $ | 5.40 |  |  |  | 8.2 |  |  | $ | 235 |
Options exercisable as of December 31, 2023 |  |  | 4,065,023 |  |  | $ | 8.54 |  |  |  | 7.0 |  |  | $ | — |
Schedule of Assumptions Used in Black-Scholes Option Pricing Model to Determine the Fair Value of Stock Options Granted | The Company estimates the fair value of stock option awards on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:
|  | Year Ended December 31,
|  | 2023 |  | 2022
Expected stock price volatility |  | 90.5% |  | 83.9%
Risk-free interest rate |  | 3.7% |  | 2.0%
Expected life of options |  | 6.08 years |  | 6.27 years
Estimated dividend yield |  | 0% |  | 0%
Summary of RSU Activity | The following table shows the RSU activity, as follows:
|  | Shares Issuable Under RSUs |  |  | Weighted Average Grant Date Fair Value |
|  |  |  |  | (In years) |
Outstanding as of January 1, 2022 |  |  | 20,000 |  |  | $ | 5.07 |
Granted |  |  | 15,000 |  |  |  | 2.47 |
RSUs Vested |  |  | (20,000 | ) |  |  | 5.07 |
Forfeited / Canceled |  |  | — |  |  |  | — |
Outstanding as of December 31, 2022 |  |  | 15,000 |  |  | $ | 2.47 |
Granted |  |  | — |  |  |  | — |
RSUs Vested |  |  | (15,000 | ) |  |  | 2.47 |
Forfeited / Canceled |  |  | — |  |  |  | — |
Outstanding as of December 31, 2023 |  |  | — |  |  | $ | — |
Schedule of Stock-Based Compensation Expense Related to Stock-Based Awards | Total compensation expense related to all of the Company’s stock-based awards for the years ended December 31, 2023 and 2022 was comprised of the following (in thousands):
|  | Year Ended December 31, |
|  | 2023 |  |  | 2022 |
Stock-based compensation classified as: |  |  |  |  |  |
Research and development expense |  | $ | 1,491 |  |  | $ | 3,230 |
General and administrative expense |  |  | 5,054 |  |  |  | 4,923 |
Total stock-based compensation expense |  | $ | 6,545 |  |  | $ | 8,153 |

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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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