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Stock-Based Compensation Plans (Tables) | 6 Months Ended
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Jun. 30, 2023
Share-Based Payment Arrangement [Abstract] |
Summary of Stock Option Activity | The following table summarizes stock option activity for the six months ended June 30, 2023 and 2022, respectively, for 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 | 2,059,314 |  |  | $ | 1.45 |  |  |  |  |
Exercised | — |  |  | $ | — |  |  | — |  |  |
Forfeited or expired | (80,926) |  |  | $ | 8.25 |  |  |  |  |
Outstanding as of June 30, 2023 | 5,770,076 |  |  | $ | 7.85 |  |  | 8.42 |  | $ | 2,170,958 |
Options vested and expected to vest as of June 30, 2023 | 5,770,076 |  |  | $ | 7.85 |  |  | 8.42 |  | $ | 2,170,958 |
Options exercisable as of June 30, 2023 | 2,184,848 |  |  | $ | 12.05 |  |  | 7.43 |  | $ | 62,614 |
| 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 | 1,702,513 |  |  | $ | 9.29 |  |  |  |  |
Exercised | (852) |  |  | $ | 5.67 |  |  |  |  |
Forfeited or expired | (145,873) |  |  | $ | 11.88 |  |  |  |  |
Outstanding as of June 30, 2022 | 3,713,248 |  |  | $ | 11.66 |  |  | 8.86 |  | $ | 23,870 |
Options vested and expected to vest as of June 30, 2022 | 3,713,248 |  |  | $ | 11.66 |  |  | 8.86 |  | $ | 23,870 |
Options exercisable as of June 30, 2022 | 1,127,085 |  |  | $ | 13.83 |  |  | 8.00 |  | $ | — |
Schedule of Valuation Assumptions Used | The following are the underlying assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options granted to employees and to non-employees under this stock plan:
| Six Months Ended June 30,
| 2023 | 2022
Risk-free interest rate | 3.93% | 1.95%
Expected dividend yield | 0% | 0%
Expected volatility | 101.6% | 97.9%
Expected term of options (years) | 5.99 | 6.00
Schedule of Stock-based Compensation Expense for Stock Awards Recognized | Total stock-based compensation expense for all stock awards recognized in the accompanying unaudited condensed consolidated statements of operations is as follows:
|  |  | Three Months Ended June 30, |  | Six Months Ended June 30,
|  |  |  |  | 2023 |  | 2022 |  | 2023 |  | 2022
Research and development |  |  |  |  | $ | 889,000 |  |  | $ | 765,000 |  |  | $ | 1,792,000 |  |  | $ | 1,561,000 |
General and administrative |  |  |  |  | 909,000 |  |  | 1,297,000 |  |  | 1,985,000 |  |  | 2,570,000 |
Total |  |  |  |  | $ | 1,798,000 |  |  | $ | 2,062,000 |  |  | $ | 3,777,000 |  |  | $ | 4,131,000 |

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