Stock-Based Compensation | 9. Stock-Based Compensation The Company grants stock options under the 2021 Stock Incentive Plan (the “2021 Plan”). The Company also has stock options outstanding under the 2016 Stock Incentive Plan (“2016 Plan”), but is no longer granting awards under such plan. All shares of common stock underlying any awards that are forfeited, cancelled, expired, repurchased, or otherwise terminated under the 2021 and 2016 Plans are added back to the shares of common stock available for issuance under the 2021 Plan. On January 1, 2023 the number of shares available for issuance under the 2021 Plan increased by 1,450,299 shares as a result of the evergreen provision. As of March 31, 2023 , 3,033,386 shares of common stock remain available for future issuance under the 2021 Plan. The total compensation expense recognized in the statements of operations associated with all the stock-based compensation awards granted by the Company is as follows (in thousands): Three Months Ended March 31, 2023 2022 Research and development $ 1,072 $ 997 General and administrative 928 903 Total $ 2,000 $ 1,900 The weighted-average fair value of the stock options granted during the three months ended March 31, 2023 was $ 1.96 . As of March 31, 2023, the total unrecognized stock-based compensation expense for unvested options was $ 17.8 million which is expected to be recognized over a weighted average period of 2.7 years. The following table summarizes stock option activity under the 2021 Plan for the three months ended March 31, 2023: Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2022 6,589,479 $ 7.29 7.73 $ 362 Granted 1,370,975 2.63 Exercised — — Cancelled or forfeited ( 314,147 ) 5.71 Outstanding as of March 31, 2023 7,646,307 $ 6.52 7.95 $ 2,037 Vested or expected to vest as of March 31, 2023 7,646,307 $ 6.52 7.95 $ 2,037 Options exercisable as of March 31, 2023 3,311,203 $ 6.32 6.65 $ 892 No options were exercised during the three months ended March 31, 2023. The aggregate intrinsic value of options exercised for the three months ended March 31, 2022 was $ 0.9 million . The fair value of each option award granted is estimated on the date of grant using the Black-Scholes option pric ing model and the weighted average assumptions. The underlying assumptions used to value stock options granted to participants using the Black-Scholes option-pricing presented on a weight average basis were as follows: Three Months Ended March 31, 2023 2022 Risk-free interest rate 3.92 % 1.67 % Expected dividend yield 0 % 0 % Expected option term (in years) range 6.08 6.07 Expected stock price volatility 86.45 83.55 Employee Stock Purchase Plan On March 20, 2021, the Company’s stockholders approved the 2021 Employee Stock Purchase Plan (the “ESPP”), which became effective on March 30, 2021 . The ESPP initially provides participating employees with the opportunity to purchase up to an aggregate of 346,613 shares of the Company’s common stock. An annual increase in the number of shares of common stock reserved and available for issuance under the ESPP shall be equal to 1 % of the number of shares of common stock outstanding on the immediately preceding December 31; and such lesser number of shares as determined by the Administrator as provided in the ESPP. As of March 31, 2023 , there has been no increase in the shares reserved and available for issuance and no shares have been purchased by employees under the ESPP. |