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Stock-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |
Stock-based Compensation Expense Recognized | Stock‑based compensation expense recognized was as follows (in thousands):
|  | Year Ended December 31, |
|  | 2023 |  |  | 2022 |
Research and development |  | $ | 4,003 |  |  | $ | 3,217 |
General and administrative |  |  | 6,620 |  |  |  | 7,389 |
Total stock-based compensation expense |  | $ | 10,623 |  |  | $ | 10,606 |
Weighted-Average Assumptions used in Estimating Fair Value of each Stock Options using Black-Scholes Option-Pricing Model | The fair value of each stock option was estimated using the Black‑Scholes option‑pricing model with the following weighted-average assumptions:
|  | Year Ended December 31, |
|  | 2023 |  |  | 2022 |
Risk-free interest rate |  |  | 3.54 | % |  |  | 2.15 | %
Expected volatility |  |  | 99.18 | % |  |  | 92.95 | %
Expected term (in years) |  |  | 5.84 |  |  |  | 6.02 |
Expected dividend yield |  |  | — |  |  |  | — |
Summary of Option Activity Changes in Shares Available for Grant under the 2020 Plan | Changes in shares available for grant under the 2020 Plan during the year ended December 31, 2023 were as follows:
|  | Shares Available for Grant |
Shares available for grant at December 31, 2022 |  |  | 4,253,854 |
2020 Plan reserve increase on January 1, 2023 |  |  | 1,893,869 |
Options and restricted stock units granted |  |  | (3,834,783 | )
Options and restricted stock units forfeited, cancelled, or expired |  |  | 845,154 |
Shares available for grant at December 31, 2023 |  |  | 3,158,094 |
Summary of Option Activity | A summary of option activity for the year ended December 31, 2023 was as follows:
|  | Shares |  |  | Weighted- Average Exercise price per Share |  |  | Weighted- Average Remaining Contractual Term (in years) |  |  | Aggregate Intrinsic Value (in thousands) |
Balance at December 31, 2022 |  |  | 4,033,522 |  |  | $ | 11.75 |  |  | 8.40 |  |  | $ | 827 |
Granted |  |  | 3,696,783 |  |  |  | 3.34 |  |  |  |  |  |  |
Exercised |  |  | (128,534 | ) |  |  | 1.29 |  |  |  |  |  |  |
Forfeited/expired |  |  | (780,902 | ) |  |  | 6.72 |  |  |  |  |  |  |
Balance at December 31, 2023 |  |  | 6,820,869 |  |  | $ | 7.97 |  |  |  | 8.24 |  |  | $ | 387 |
Expected to vest |  |  | 4,000,554 |  |  | $ | 6.41 |  |  |  | 8.72 |  |  | $ | — |
Options exercisable |  |  | 2,820,315 |  |  | $ | 10.18 |  |  |  | 7.56 |  |  | $ | 387 |
Summary of Stock Options Outstanding and Exercisable | Stock options outstanding and exercisable consisted of the following at December 31, 2023:
Exercise Price ($) |  | Shares Outstanding |  |  | Shares Exercisable |
0.16 to 4.22 |  |  | 3,709,593 |  |  |  | 1,005,785 |
8.93 to 19.94 |  |  | 2,759,028 |  |  |  | 1,615,758 |
20.55 to 29.41 |  |  | 352,248 |  |  |  | 198,772 |
Total |  |  | 6,820,869 |  |  |  | 2,820,315 |
Restricted Stock Units (RSUs) |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |
Summary of Restricted Stock Activity | A summary of RSU activity for the year ended December 31, 2023 is as follows:
|  | Share Equivalent |  |  | Weighted- Average Grant Date Fair Value |
Non-vested at December 31, 2022 |  |  | 236,339 |  |  | $ | 15.16 |
Granted |  |  | 138,000 |  |  |  | 3.28 |
Cancelled |  |  | (64,252 | ) |  |  | 7.63 |
Vested |  |  | — |  |  |  | — |
Non-vested at December 31, 2023 |  |  | 310,087 |  |  | $ | 11.44 |

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