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Stock-based Compensation Plans (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Schedule of Stock Option Activity | The following table summarizes stock option activity for the twelve months ended December 31, 2023 and 2022 under the 2019 Plan:
| Options |  | Weighted- Average Exercise Price |  | Weighted- Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value
Outstanding as of January 1, 2023 | 3,791,688 |  |  | $ | 11.33 |  |  |  |  |
Granted | 2,810,564 |  |  | $ | 1.70 |  |  |  |  |
Exercised | — |  |  | $ | — |  |  |  |  |
Forfeited or expired | (406,112) |  |  | $ | 8.52 |  |  |  |  |
Outstanding as of December 31, 2023 | 6,196,140 |  |  | $ | 7.15 |  |  | 8.14 |  | $ | 185,169 |
Options vested and expected to vest as of December 31, 2023 | 6,196,140 |  |  | $ | 7.15 |  |  | 8.14 |  | $ | 185,169 |
Options exercisable as of December 31, 2023 | 2,534,017 |  |  | $ | 11.33 |  |  | 7.19 |  | $ | 12,381 |
| Options |  | Weighted- Average Exercise Price |  | Weighted- Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value
Outstanding as of January 1, 2022 | 2,157,460 |  |  | $ | 13.54 |  |  |  |  |
Granted | 1,886,263 |  |  | $ | 8.73 |  |  |  |  |
Exercised | (852) |  |  | $ | 5.67 |  |  |  |  |
Forfeited or expired | (251,183) |  |  | $ | 10.79 |  |  |  |  |
Outstanding as of December 31, 2022 | 3,791,688 |  |  | $ | 11.33 |  |  | 8.35 |  | $ | 375 |
Options vested and expected to vest as of December 31, 2022 | 3,791,688 |  |  | $ | 11.33 |  |  | 8.35 |  | $ | 375 |
Options exercisable as of December 31, 2022 | 1,398,490 |  |  | $ | 13.33 |  |  | 7.50 |  | $ | — |
Schedule of Valuation Assumptions Used | The following are the underlying assumptions used in the Black-Scholes-Merton option pricing model to determine the fair value of stock options granted to employees and to non-employees under this stock plan:
| 2023 | 2022
Risk-free interest rate | 3.98% | 2.09%
Expected dividend yield | 0% | 0%
Expected volatility | 96.1% | 97.8%
Expected term of options (years) | 6.0 | 6.0
Schedule of Stock-based Compensation Expense for Stock Awards Recognized | Total stock-based compensation expense for all stock awards recognized in the accompanying audited consolidated statements of operations is as follows (in thousands):
| Year Ended December 31,
| 2023 |  | 2022
Research and development | $ | 3,398 |  |  | $ | 3,219 |
General and administrative | 3,704 |  |  | 4,710 |
Total | $ | 7,102 |  |  | $ | 7,929 |

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