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Stock Based Compensation
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Stock Based Compensation Stock-Based Compensation
2021 Plan
In September 2021 the Company’s board of directors adopted, and in October 2021 the Company’s stockholders approved, the 2021 Plan, which became effective as of the date immediately prior to the date of the effectiveness of the registration statement for the IPO. The 2021 Plan allows the board of directors to grant incentive stock options or non-qualified stock options, restricted stock, restricted stock units and other equity awards to the Company’s officers, employees, directors and other key persons. In addition, the 2021 Plan includes a provision that allows for an automatic annual increase of 4% in the number of shares of common stock available for issuance under the 2021 Plan. Upon the adoption of the 2021 Plan, the Company ceased granting awards under the 2016 Plan. The total number of shares of common stock authorized for issuance under the 2021 Plan as of December 31, 2023 was 6,336,068 shares and was 5,262,917 shares as of December 31, 2022.
As of December 31, 2023, the Company had issued stock options and restricted stock units (RSUs) under the 2021 Plan. Both stock options and RSUs issued are comprised of service-based awards granted to employees. Vesting of stock options is subject to the recipient’s continued employment or service. Stock options and RSUs typically vest over a four-year period.
2016 Plan
Prior to the adoption of the 2021 Plan, the 2016 Plan provided for the Company to grant incentive stock options or non-qualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2016 Plan was administered by the board of directors of the Company or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. The 2016 Plan allows for early exercise of all stock option grants if authorized by the board of directors at the time of grant. The shares of common stock issued from the early exercise of stock options are restricted and continue to vest over the original service based vesting condition of the original stock option award. The Company has the option to repurchase any unvested shares at the original purchase price upon any voluntary or involuntary termination.
The 2016 Plan will continue to govern the outstanding equity awards granted thereunder. The total number of shares of common stock authorized for issuance under the 2016 Plan as of December 31, 2023 and 2022 was 2,044,585 shares and 2,206,655 shares, respectively. As of both dates, all shares of common stock authorized for issuance under the 2016 Plan relate to outstanding stock options.
2021 Employee Stock Purchase Plan
In September 2021, the Company’s board of directors adopted, and in October 2021 the Company’s stockholders approved, the ESPP, which became effective as of the date immediately prior to the date of the effectiveness of the registration statement for the IPO. The ESPP is administered by the person or persons appointed by the Company’s board of directors for such purpose. The ESPP initially provided participating employees with the opportunity to purchase up to an aggregate of 278,762 shares of common stock. The number of shares of common stock reserved for issuance under the ESPP will automatically increase on January 1st of each year beginning in 2022 and continuing through and including 2031 by the lesser of (i) 1% of the outstanding number of shares of our common stock of the immediately preceding December 31, (ii) 557,524 shares or (iii) such number of shares as determined by the ESPP administrator. The total number of shares of common stock authorized for issuance under the 2021 ESPP as of December 31, 2023 and 2022 was 839,539 shares and 563,115 shares, respectively.
Compensation expense for discounted purchases under the 2021 ESPP is measured using the Black-Scholes model to compute the fair value of the lookback provision plus the purchase discount and is recognized as compensation expense over the course of the offering period.
Stock-Based Compensation
The Company recognized stock-based compensation expense in the consolidated statements of operations and comprehensive loss, by award type, as follows (in thousands):
Year ended December 31,
20232022
Stock Options$9,887 $8,648 
Restricted Stock Units3,015 1,106 
ESPP209141
Total$13,111 $9,895 
Stock-based compensation expense recorded as research and development and general and administrative expenses in the consolidated statements of operations and comprehensive loss is as follows (in thousands):
Year ended December 31,
20232022
Research and development expenses$6,169 $4,166 
General and administrative expenses6,942 5,729 
Total$13,111 $9,895 
Stock Option Valuation
The following table presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted for the years then ended:
December 31, 2023December 31, 2022
Risk‑free interest rate4.09%2.20%
Expected volatility72%71%
Expected dividend yield——
Expected term (in years)6.016.04
Early Exercise of Unvested Stock Options
Shares purchased by employees pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be outstanding shares until those shares vest according to their respective vesting schedules. Cash received from employee exercises of unvested options is included in current liabilities on the balance sheet. Amounts recorded are reclassified to common stock and additional paid-in capital as the shares vest. Vesting can occur in the year of exercise and thereafter. There were 14,745 and 53,741 unvested shares related to early exercises of stock options as of December 31, 2023 and December 31, 2022, respectively. In the years ended December 31, 2023 and 2022, the liability associated with the unvested early exercise of stock options was $0.1 million and $0.2 million, respectively.
Stock Options
The following table summarizes the Company’s stock option activity since December 31, 2022:
Number of SharesWeighted‑Average Exercise PriceWeighted‑Average Remaining Contractual Term
Aggregate Intrinsic Value(2)
(in years)(in thousands)
Outstanding as of December 31, 2022
5,028,850$10.95
Granted799,14413.85
Exercised(214,078)5.54
Forfeited(199,556)15.63
Outstanding as of December 31, 2023
5,414,360$11.427.78$25,569
Exercisable as of December 31, 2023(1)
3,297,593$9.49 7.26$21,529
(1)This represents the number of vested and unvested options exercisable as of December 31, 2023.
(2)The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the closing price of the Company's common stock at December 31, 2023 for the options that were in the money as of December 31, 2023.
The aggregate intrinsic value of stock options exercised during the years ended December 31, 2023 and 2022 was $2.1 million and $1.0 million, while the company received $1.2 million and $0.2 million in proceeds for the exercise of these options, respectively.
The weighted-average grant-date fair value of stock options granted during the years ended December 31, 2023 and 2022 was $9.23 per share and $7.51 per share, respectively. As of December 31, 2023, there was $20.7 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of 2.2 years.
Restricted Stock Units
During the year ended December 31, 2023, RSUs were granted to employees with vesting conditions based on continued service over time. Accordingly, stock-based compensation expense for such awards is recognized using a straight-line attribution model over the vesting term of each RSU. The fair value of each RSU is based on the closing price of the Company's common stock on the date of grant. For the majority of RSUs, the restricted stock vests over a four-year period, with 25% of the shares vesting on each anniversary of the grant date.
A summary of restricted stock activity during the year ended December 31, 2023 is as follows:
SharesWeighted‑
Average
Grant‑Date
Fair Value
Unvested as of December 31, 2022
463,964$12.26
Issued990,16714.29
Vested
(138,361)12.30
Forfeited(47,309)12.51
Unvested as of December 31, 2023
1,268,461$13.83
As of December 31, 2023, there was $15.1 million of unrecognized stock-based compensation expense related to unvested RSUs, which is expected to be recognized over a weighted-average remaining vesting period of 3.17 years.