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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2022
Stock-Based Compensation |
Summary of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Options Outstanding and Exercisable
​ | ​ | ​ | ​ | ​ | ​ | ​ | Weighted | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Average | ​ | Aggregate
​ | ​ | ​ | ​ | Average | ​ | Remaining | ​ | Intrinsic
​ | ​ | Number of | ​ | Exercise | ​ | Contractual | ​ | Value(1)
​ |  | Shares |  | Price |  | Life (Years) |  | (in thousands)
Outstanding as of January 1, 2021 | ​ | 3,112,376 | ​ | $ | 16.95 | ​ | 8.2 | ​ | $ | 10,810
Granted |  | 2,020,718 | ​ | $ | 10.89 |  | — | ​ |  | —
Exercised | ​ | (105,731) | ​ | $ | 1.62 | ​ | — | ​ | ​ | —
Forfeitures | ​ | (481,569) | ​ | $ | 21.33 | ​ | — | ​ | ​ | —
Outstanding as of December 31, 2021 |  | 4,545,794 | ​ | $ | 14.15 |  | 8.1 | ​ | $ | 2,860
Granted | ​ | 1,739,350 | ​ | $ | 5.34 | ​ | — | ​ | ​ | —
Exercised | ​ | (50,420) | ​ | $ | 1.32 | ​ | — | ​ | ​ | —
Forfeitures | ​ | (972,545) | ​ | $ | 13.77 | ​ | — | ​ | ​ | —
Outstanding as of December 31, 2022 | ​ | 5,262,179 | ​ | $ | 11.44 | ​ | 7.6 | ​ | $ | 115
Exercisable as of December 31, 2022 |  | 2,790,206 | ​ | $ | 12.97 |  | 6.6 | ​ | $ | 115
| (1) | The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2022 and 2021.
Summary of stock based compensation expense recorded | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31,
​ | ​ | ​ | ​ | ​ | ​ | ​
(in thousands) | ​ | 2022 |  | 2021
Research and development | ​ | $ | 3,056 | ​ | $ | 4,081
General and administrative | ​ |  | 6,452 | ​ |  | 6,207
Total stock-based compensation expense | ​ | $ | 9,508 | ​ | $ | 10,288
​
Summary of assumptions used in the Black Scholes option pricing model for stock options granted | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended | ​
​ | ​ | December 31, | ​
​ |  | 2022 |  | 2021 | ​
Expected term |  | 5.5 - 6.1 | years | ​ | 5.3 - 6.1 | years | ​
Expected volatility |  | 79.7 - 81.4 | % | ​ | 79.7 | % | ​
Risk free interest rate |  | 1.8 - 4.2 | % | ​ | 0.8 - 1.4 | % | ​
Expected dividend yield |  | — | % | ​ | — | % | ​

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