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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2023
Stock-Based Compensation |
Stock-Based Compensation | (8) Stock-Based Compensation ​ A summary of stock option activity for the six months ended June 30, 2023 is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Weighted
​ | ​ | ​ | ​ | Average | ​ | Average
​ | ​ | ​ | ​ | Exercise | ​ | Remaining
​ | ​ | ​ | ​ | Price | ​ | Contractual
​ |  | Shares |  | Per Share |  | Term (In Years)
Options outstanding at December 31, 2022 |  | 5,085,662 | ​ | $ | 29.26 | ​ | 7.9
Granted |  | 1,534,575 | ​ | $ | 36.92 | ​ | ​
Exercised |  | (46,624) | ​ | $ | 15.63 | ​ | ​
Canceled |  | (127,852) | ​ | $ | 31.62 | ​ | ​
Options outstanding at June 30, 2023 |  | 6,445,761 | ​ | $ | 31.13 | ​ | 8.0
Options vested and expected to vest at June 30, 2023 |  | 6,284,352 | ​ | $ | 31.16 | ​ | 7.9
Options exercisable at June 30, 2023 |  | 2,896,986 | ​ | $ | 33.88 | ​ | 6.7
Shares available for grant under the 2021 Plan |  | 1,031,245 | ​ | ​ | ​ | ​ | ​
​ The weighted average grant-date fair value of stock options granted during the three and six months ended June 30, 2023 was $28.32 and $28.41, respectively. ​ The aggregate intrinsic value of stock options vested and expected to vest at June 30, 2023 was $68.6 million. The aggregate intrinsic value of stock options exercisable at June 30, 2023 was $46.6 million. As of June 30, 2023, total compensation cost related to non-vested employee, consultant and non-employee director stock options not yet recognized was approximately $73.8 million, net of estimated forfeitures, which is expected to be recognized as expense over a weighted average period of 2.9 years. ​ Stock-based compensation expense for the three and six months ended June 30, 2023 and 2022 was recorded as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended June 30, | ​ | Six months ended June 30,
​ |  | 2023 |  | 2022 |  | 2023 |  | 2022
​ | ​ | (In thousands) | ​ | (In thousands)
Research and development | ​ | $ | 2,489 | ​ | $ | 1,798 | ​ | $ | 4,650 | ​ | $ | 3,412
General and administrative | ​ |  | 2,728 | ​ |  | 1,656 | ​ |  | 4,907 | ​ |  | 3,195
Total stock-based compensation expense | ​ | $ | 5,217 | ​ | $ | 3,454 | ​ | $ | 9,557 | ​ | $ | 6,607
​ The fair values of employee, consultant and non-employee director stock options granted during the three and six months ended June 30, 2023 and 2022 were valued using the Black-Scholes option pricing model with the following assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended June 30, | ​ | Six months ended June 30,
|  | 2023 |  | 2022 |  | 2023 |  | 2022
Expected stock price volatility |  | 92% | ​ | 90 - 91% | ​ | 92% | ​ | 90 – 97%
Expected option term |  | 6.0 Years | ​ | 6.0 Years | ​ | 6.0 Years | ​ | 6.0 Years
Risk-free interest rate |  | 3.5 – 3.9% | ​ | 2.7 - 3.6% | ​ | 3.5 – 4.0% | ​ | 1.7 – 3.6%
Expected dividend yield |  | None | ​ | None | ​ | None | ​ | None
​

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