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STOCK-BASED COMPENSATION
 9 Months Ended
Sep. 30, 2024
Share-Based Payment Arrangement [Abstract]  
STOCK-BASED COMPENSATION STOCK-BASED COMPENSATION
2021 Equity Incentive Plan
The 2021 Equity Incentive Plan (the “2021 Plan”), which provides for the grant of incentive stock options to employees and the grant of nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance awards and other forms of stock awards to employees, directors and consultants, became effective on June 30, 2021. The 2021 Plan is a successor to the Company’s Amended and Restated Stock Performance Plan that was adopted by the Company’s Board of Directors (the “Board”) and stockholders on April 8, 2013 (as amended from time to time, most recently on November 20, 2020, the “2013 Plan”). The maximum number of shares of common stock that may be issued upon the exercise of incentive stock options under the 2021 Plan is 12,000,000 shares. Following the effectiveness of the 2021 Plan, no further grants may be made under the 2013 Plan; however, any outstanding equity awards granted under the 2013 Plan continue to be governed by the terms of the 2013 Plan. As of September 30, 2024, there were 3,227,248 options outstanding under the 2013 Plan.
The number of shares of common stock reserved for issuance under the 2021 Plan automatically increases on January 1 of each calendar year through January 1, 2031, in an amount equal to 5% of the total number of shares of common stock outstanding on December 31 of the fiscal year before the date of each automatic increase, or a lesser number of shares determined by the Board prior to the applicable January 1. On January 1, 2024, the number of shares of common stock reserved for issuance under the 2021 Plan automatically increased by 2,895,523 shares.
As of September 30, 2024, a total of 14,668,721 shares were authorized for issuance under the 2021 Plan and 2,535,928 shares remained available for issuance under the 2021 Plan.
The Company recorded stock-based compensation expense in the following expense categories of its condensed statements of operations for the periods shown (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2024202320242023
General and administrative$1,436 $1,127 $4,678 $3,143 
Research and development902 473 2,614 1,368 
Total stock-based compensation$2,338 $1,600 $7,292 $4,511 
Stock Options
The Black-Scholes option-pricing model was used to estimate the fair value of stock options granted during the nine months ended September 30, 2024 and 2023 with the following weighted average assumptions:
Nine Months Ended September 30,
20242023
Risk-free interest rate
3.81% - 5.10%
3.47% - 4.43%
Expected term (in years)
0.9 - 6.1
5.5 - 6.1
Expected volatility
92% - 103%
 90% - 98%
Expected dividend yield0%0%
The weighted average grant date fair value of options granted during the nine months ended September 30, 2024 and 2023 was $2.93 per share and $4.53 per share, respectively.
Stock options granted after December 31, 2017 generally vest monthly over a range of 12 to 48 months or vest monthly over a total of 48 months following a one-year cliff and all have a 10-year contractual term. Beginning in 2022, the Company has also issued annual option awards to its Board that vest in full on the first anniversary of the grant date. Stock options granted prior to December 31, 2017 were either fully vested upon grant or generally vested monthly over a range of three to 24 months and also have a 10-year term. The Company’s common stock became publicly traded in July 2021 and lacks sufficient company-specific historical and implied volatility information. Therefore, the Company estimates expected stock volatility using a weighted average blend of historical volatility of a publicly traded set of peer companies, as well as its own historical volatility. Due to the lack of historical exercise history, the expected term of the Company’s stock options has been determined using the “simplified” method for awards. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is zero based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.
The following table reflects summarized stock option activity:
Stock OptionsWeighted Average Exercise PriceWeighted Average Remaining Contractual Life (in years)Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 20237,523,947$4.01 
Granted2,961,850$3.71 
Forfeited(57,871)$7.36 
Expired(53,555)$9.10 
Outstanding as of September 30, 202410,374,371$3.88 7.7$4,461 
Vested and exercisable as of September 30, 20245,664,870$3.48 6.8$4,186 
As of September 30, 2024, total unrecognized compensation costs related to unvested stock option awards were approximately $15.1 million, which the Company expects to recognize over a weighted-average period of approximately 2.5 years.
Restricted Stock Units
In each of January 2024 and 2023, the Company granted a restricted stock unit (“RSU”) award to each of its then-current employees. These RSU awards vest in equal annual installments on the first three anniversaries of the grant date.
Number of SharesWeighted Average Grant Date Fair Value
Unvested as of December 31, 2023328,500$6.11 
Granted1,204,640$4.15 
Vested(109,493)$6.11 
Forfeited(12,317)$4.59 
Unvested as of September 30, 20241,411,330$4.45 
As of September 30, 2024, total unrecognized compensation costs related to unvested RSUs were approximately $4.6 million, which the Company expects to recognize over a weighted-average period of approximately 2.1 years.
Employee Stock Purchase Plan
The 2021 Employee Stock Purchase Plan (the “ESPP”), which permits employees to purchase shares of common stock, became effective on June 30, 2021. The number of shares of common stock reserved for issuance automatically increases on January 1 of each calendar year through January 1, 2031, by the lesser of (1) 1% of the total number of shares of common stock outstanding on the last day of the fiscal year before the date of the automatic increase, and (2) 800,000 shares; provided, however, that before the date of any such increase, the Board may determine that such increase will be less than the amount set forth in clauses (1) and (2). On January 1, 2024, the number of shares of common stock reserved for issuance under the ESPP automatically increased by 579,105 shares. As of September 30, 2024, there are a total of 1,769,088 shares authorized for issuance under the ESPP and there have been no purchases of shares under the ESPP, as the ESPP has not yet been implemented.