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Stock option plans | 12 Months Ended
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Dec. 31, 2022
Stock option plan |
Stock option plans | 10. Stock option plans ​ In September 2019, the Company’s Board of Directors approved the 2019 Equity Incentive Plan (the “2019 Plan) and each outstanding option to purchase Aprea AB ordinary shares pursuant to a previous plan was cancelled and the Company issued to each holder of such Aprea AB option, a substitute option to purchase, on the same terms and conditions as were applicable to such Aprea AB option, shares of the Company’s common stock pursuant to the 2019 Plan. The Board of Directors has the discretion to provide for accelerated vesting under the 2019 Plan. At December 31, 2022, there were 87,856 shares available for future grant under the 2019 Plan. The Company recorded stock-based compensation expense of $16,981,266, $7,813,743 and $4,983,023 during the years ended December 31, 2022, 2021 and 2020, respectively. As of December 31, 2022, there was $487,730 of unrecognized compensation cost related to nonvested share-based compensation arrangements granted under the 2019 Plan, which is expected to be recognized over a weighted-average period of approximately 3.6 years. The fair value of each option award is estimated on the date of grant using Black-Scholes, with the assumptions noted in the table below. Expected volatility for the Company’s common stock was determined based on an average of the historical volatility of a peer group of similar public companies. The expected term of options granted to employees was calculated using the simplified method, which represents the average of the contractual term of the option and the weighted-average vesting period of the option. The Company uses the simplified method because it does not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate expected term. The contractual life of the option was used for the expected life of options granted to non-employee. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk-free rate for periods within the expected life of the option is based upon the Swedish Government Bond Rate in effect at the time of grant. The assumptions used in Black-Scholes are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year ended December 31,
​ |  | 2022 |  | 2021 |  | 2020
Expected volatility |  | 84.4%-89.6% |  | 88.1%-90.2% | ​ | 78.9% ‑83.8%
Risk‑free rate |  | 1.95%-2.69% | ​ | 0.96%-1.04% | ​ | 0.36% ‑1.43%
Expected dividend yield |  | 0% |  | 0% | ​ | 0%
Expected term in years |  | 6.08-10.00 |  | 5.50 - 6.08 |  | 5.50 - 6.08
​ A summary of option activity under the Plan during the years ended December 31, 2022, 2021 and 2020 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  |  |  | ​ | Weighted‑ |  | Weighted |  |  | ​
​ | ​ | ​ | ​ | ​ | average | ​ | average | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | exercise | ​ | remaining | ​ | Aggregate
​ | ​ | Number of | ​ | price per | ​ | contractual | ​ | intrinsic
​ | ​ | options | ​ | share | ​ | term (in years) | ​ | value
Outstanding at January 1, 2020 |  |  | 174,975 | ​ | $ | 109.80 |  | 7.5 | ​ | ​ | ​
Granted |  |  | 22,340 | ​ |  | 710.13 |  |  | ​ |  |
Exercised |  |  | (8,203) | ​ |  | 18.33 |  |  | ​ |  |
Cancelled/Forfeited |  |  | (560) | ​ |  | 775.60 |  |  | ​ |  |
Outstanding at December 31, 2020 |  |  | 188,552 | ​ | $ | 182.00 |  | 6.8 | ​ | ​ | ​
Granted |  |  | 58,980 | ​ |  | 117.40 |  |  | ​ |  |
Exercised |  |  | (11,319) | ​ |  | 18.40 |  |  | ​ |  |
Cancelled/Forfeited |  |  | (6,905) | ​ |  | 330.60 |  |  | ​ |  |
Outstanding at December 31, 2021 |  |  | 229,308 | ​ | $ | 169.00 |  | 6.6 | ​ | ​ | ​
Granted |  |  | 96,798 | ​ |  | 31.40 |  |  | ​ |  |
Options assumed in Atrin merger | ​ | ​ | 163,733 | ​ | ​ | 14.21 | ​ | ​ | ​ | ​ | ​
Exercised |  |  | — | ​ |  | — |  |  | ​ |  |
Cancelled/Forfeited |  |  | (9,565) | ​ |  | 118.60 |  |  | ​ |  |
Outstanding at December 31, 2022 | ​ | ​ | 480,274 | ​ | $ | 89.49 |  | 6.8 | ​ | $ | ​
Exercisable at December 31, 2022 | ​ | ​ | 447,721 | ​ | $ | 94.41 |  | 6.1 | ​ | $ | ​
Vested or expected to vest at December 31, 2022 | ​ | ​ | 480,274 | ​ | $ | 89.49 |  | 6.8 | ​ | $ | ​
​ The weighted-average grant date fair value of options granted during the years ended December 31, 2022, 2021 and 2020, was $23.68, $85.82 and $484.02, per share, respectively. Restricted Stock Units During the year ended December 31, 2022, the Company granted a total of 9,320 RSUs to executive officers of the Company which vest ratably on the 1st, 2nd and 3rd anniversaries of the grant date. During the year ended December 31, 2022, the Company granted a total of 3,418 RSUs to non-employee directors of the Company. These RSUs vest on the one-year anniversary of the grant date. During the year ended December 31, 2022, the Company granted a total of 11,330 RSUs to executive officers and certain other employees of the Company which vest ratably on the 1st, 2nd and 3rd anniversaries of the grant date. These RSUs vested on May 16, 2022 in connection with the acquisition of Atrin. During the year ended December 31, 2021, the Company granted a total of 25,000 RSUs to executive officers and certain other employees of the Company, 800 of which were canceled prior to vesting. The remaining RSUs vested on May 16, 2022 in connection with the acquisition of Atrin. During the year ended December 31, 2021, the Company granted a total of 1,152 RSUs to non-employee directors of the Company. These RSUs vested on May 16, 2022 in connection with the acquisition of Atrin. As of December 31, 2022, there was $243,709 of unrecognized compensation cost related to RSUs granted under the 2019 Plan, which is expected to be recognized over a weighted-average period of approximately 2.6 years. The following table shows restricted stock unit activity during the year ended December 31, 2022: ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​
​ | ​ | ​ | ​ | Weighted‑
​ | ​ | ​ | ​ | average
​ | ​ | ​ | ​ | grant date
​ | ​ | Shares | ​ | fair value
Restricted stock units outstanding at December 31, 2020 | ​ | — | ​ | $ | —
Granted | ​ | 26,150 | ​ | ​ | 118.20
Vested | ​ | (5,365) | ​ | ​ | 118.80
Cancelled/Forfeited | ​ | (800) | ​ | ​ | 120.00
Outstanding at December 31, 2021 |  | 19,985 | ​ | $ | 118.20
Granted | ​ | 24,064 | ​ | ​ | 28.68
Vested | ​ | (30,981) | ​ | ​ | 88.21
Cancelled/Forfeited | ​ | (334) | ​ | ​ | 114.60
Outstanding at December 31, 2022 | ​ | 12,734 | ​ | $ | 21.80
​

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