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Stock-Based Compensation | 9 Months Ended
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Sep. 30, 2021
Stock-Based Compensation |
Stock-Based Compensation | (8) Stock-Based Compensation ​ A summary of stock option activity for the nine months ended September 30, 2021 is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Weighted
​ | ​ | ​ | ​ | Average | ​ | Average
​ | ​ | ​ | ​ | Exercise | ​ | Remaining
​ | ​ | ​ | ​ | Price | ​ | Contractual
​ |  | Shares |  | Per Share |  | Term (In Years)
Options outstanding at December 31, 2020 |  | 3,042,229 | ​ | $ | 28.93 | ​ | 8.2
Granted |  | 1,350,210 | ​ | $ | 28.16 | ​ | ​
Exercised |  | (192,464) | ​ | $ | 9.53 | ​ | ​
Canceled |  | (70,349) | ​ | $ | 35.62 | ​ | ​
Options outstanding at September 30, 2021 |  | 4,129,626 | ​ | $ | 29.46 | ​ | 8.2
Options vested and expected to vest at September 30, 2021 |  | 3,982,965 | ​ | $ | 29.81 | ​ | 8.2
Options exercisable at September 30, 2021 |  | 1,387,147 | ​ | $ | 51.75 | ​ | 6.7
Shares available for grant under the 2021 Plan |  | 3,319,116 | ​ | ​ | ​ | ​ | ​
​ The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2021 was $41.34 and $21.86, respectively. ​ The aggregate intrinsic value of stock options vested and expected to vest at September 30, 2021 was $140.6 million. The aggregate intrinsic value of stock options exercisable at September 30, 2021 was $47.3 million. As of September 30, 2021, total compensation cost related to non-vested employee, consultant and non-employee director stock options not yet recognized was approximately $33.7 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 nine months ended September 30, 2021 and 2020 was recorded as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended September 30, | ​ | Nine months ended September 30,
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020
​ | ​ | (In thousands) | ​ | (In thousands)
Research and development | ​ | $ | 1,504 | ​ | $ | 642 | ​ | $ | 2,927 | ​ | $ | 1,294
General and administrative | ​ |  | 1,525 | ​ |  | 608 | ​ |  | 2,886 | ​ |  | 1,364
Total stock-based compensation expense | ​ | $ | 3,029 | ​ | $ | 1,250 | ​ | $ | 5,813 | ​ | $ | 2,658
​ The fair values of employee, consultant and non-employee director stock options granted during the three and nine months ended September 30, 2021 and 2020 were valued using the Black-Scholes option pricing model with the following assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended September 30, | ​ | Nine months ended September 30,
|  | 2021 | ​ | 2020 |  | 2021 |  | 2020
Expected stock price volatility |  | 98% | ​ | 98% | ​ | 97 – 98% | ​ | 91 – 98%
Expected option term |  | 6.0 Years | ​ | 6.0 Years | ​ | 6.0 Years | ​ | 6.0 Years
Risk-free interest rate |  | 1.1 – 1.2% | ​ | 0.5% | ​ | 0.8 – 1.3% | ​ | 0.5 – 0.6%
Expected dividend yield |  | None | ​ | None | ​ | None | ​ | None
​

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