The Company’s 2020 Equity Incentive Plan (the “2020 Plan”) provides for the Company to grant incentive stock options, non-qualified stock options, restricted stock awards, restricted stock units and other stock-based awards to employees, members of the board of directors and consultants. The 2020 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The board of directors may also delegate to one or more officers of the Company the power to grant awards to employees and certain officers of the Company. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or its committee or any such officer if so delegated.

The exercise price for stock options granted may not be less than the fair market value of the Company’s common stock on the date of grant, as determined by the board of directors, or at least 110% of the fair market value of the Company’s common stock on the date of grant in the case of an incentive stock option granted to an employee who owns stock representing more than 10% of the voting power of all classes of stock as determined by the board of directors as of the date of grant. Prior to the IPO, the Company’s board of directors determined the fair value of the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant. Stock options granted under the 2020 Plan expire after ten years and typically vest over a four-year period with the first 25% vesting upon the first anniversary of a specified vesting commencement date and the remainder vesting in 36 equal monthly installments over the succeeding three years, contingent on the recipient’s continued employment or service. Certain awards of stock options permit the holders to exercise the option in whole or in part prior to the full vesting of the option in exchange for unvested shares of restricted common stock with respect to any unvested portion of the option so exercised.

As of March 31, 2024, there were 4,298,068 shares authorized to be issued upon the exercise of outstanding stock option grants and no shares reserved for future issuance under the 2020 Plan.

In July 2021, the Company’s board of directors adopted, and its stockholders approved, the 2021 Equity Incentive Plan (the “2021 Plan”), which became effective immediately prior to and contingent upon the execution of the underwriting agreement related to the Company’s IPO. The 2021 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares reserved for issuance under the 2021 Plan was equal to 35,075,122, which is the sum of 11,413,572 new shares; plus the number of shares (not to exceed 23,661,550 shares), which represents (i) the number of shares that remained available for issuance under the 2020 Plan, at the time the 2021 Plan became effective, and (ii) any shares subject to outstanding stock options or other stock awards that were granted under the 2020 Plan that are forfeited, terminate, expire or are otherwise not issued. In addition, the number of shares of the Company’s common stock reserved for issuance under the 2021 Plan will automatically increase on the first day of each calendar year pursuant to the evergreen provision thereof, beginning on January 1, 2022 and continuing through January 1, 2031, in an amount equal to 5% of the shares of common stock outstanding on the last day of the calendar month before the date of each automatic increase, or a lesser number of shares determined by the board of directors. On January 1, 2022, 5,539,145 shares of common stock were automatically added to the shares authorized for issuance under the 2021 Plan pursuant to the evergreen provision thereof. The number of shares to be issued under the 2021 Plan did not increase on January 1, 2023 as determined by the Company’s board of directors. On January 1, 2024, 3,304,820 shares of common stock were added to the shares authorized for issuance under the 2021 Plan, pursuant to the evergreen provision thereof, as determined by the Company’s board of directors. The shares of common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2021 Plan will be added back to the shares of common stock available for issuance under the 2021 Plan.

As of March 31, 2024, there were an aggregate of 45,301,059 shares authorized to be issued under the 2020 Plan and the 2021 Plan, which included 4,298,068 and 22,065,965 shares authorized to be issued upon the exercise of outstanding stock option grants from the 2020 Plan and 2021 Plan, respectively, and 0 and 18,937,026 shares reserved for future issuance under the 2020 Plan and 2021 Plan, respectively.

The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. Prior to its IPO in August 2021, the Company had been a private company. Due to the proximity to the IPO, the Company continues to lack sufficient company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method. 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 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 presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant date fair value of stock options granted:


	Three Months Ended March 31,			Three Months Ended March 31,
	2024			2023
Expected term (in years)		5.8			5.8
Expected volatility		62.8	%		69.3	%
Risk-free interest rate		4.0	%		3.5	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity since December 31, 2023:


	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding at December 31, 2023		23,065,514		$	5.08		8.7	$	24,745
Granted		3,647,000		$	4.22
Exercised		—		$	—
Forfeited		(348,481	)	$	6.05
Outstanding at March 31, 2024		26,364,033		$	4.95		8.4	$	33,249
Vested and expected to vest at March 31, 2024		26,364,033		$	4.95		8.4	$	33,249
Options exercisable at March 31, 2024		9,143,369		$	6.79		7.4	$	9,256

The weighted-average grant date fair value of stock options granted during the three months ended March 31, 2024 and 2023 was $2.53 and $1.34, respectively, per share.

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair market value of the common stock for the options that had exercise prices lower than the estimated fair value of the Company’s common stock at March 31, 2024 and 2023.

The total intrinsic value of stock options exercised was $0 and $0.3 million for the three months ended March 31, 2024 and 2023, respectively.

The Company recorded stock-based compensation expense (service-based stock options and employee stock purchase plan) in the following expense categories of its condensed consolidated statements of operations and comprehensive loss (in thousands):


	Three Months Ended March 31,		Three Months Ended March 31,
	2024		2023
Research and development	$	1,677	$	2,263
Selling, general and administrative		3,702		3,137
	$	5,379	$	5,400

As of March 31, 2024, total unrecognized stock-based compensation expense related to unvested stock-based awards was $40.2 million, which is expected to be recognized over a weighted-average period of 2.4 years.

In July 2021, the Company’s board of directors adopted, and its stockholders approved, the 2021 Employee Stock Purchase Plan (the “2021 ESPP”), which became effective immediately prior to and contingent upon the execution of the underwriting agreement related to the Company’s IPO. A total of 1,342,773 shares of common stock were initially reserved for issuance under the 2021 ESPP. There were 351,542 shares issued under the 2021 ESPP as of March 31, 2024. The number of shares of common stock that may be issued under the 2021 ESPP will automatically increase on the first day of each calendar year, pursuant to the evergreen provision thereof, beginning on January 1, 2022 and continuing through January 1, 2031, by an amount equal to the lesser of (i) 1% of the shares of common stock outstanding on the last day of the calendar month before the date of each automatic increase, (ii) 2,685,546 shares and (iii) an amount determined by the Company’s board of directors. The number of shares to be issued under the 2021 ESPP did not increase on January 1, 2024 or January 1, 2023, pursuant to the evergreen provision thereof, as determined by the Company’s board of directors. The first offering under the 2021 ESPP was June 6, 2022. As of March 31, 2024, 991,231 shares remained available for issuance under the 2021 ESPP. During both the three months ended March 31, 2024 and 2023, the Company recognized less than $0.1 million in related stock-based compensation expense.

In November 2022, the Company entered into the PHP MSA, the PHP Work Order and a warrant agreement with respect to the PHP Warrant. To compensate for the services and deliverables provided by PHP, the Company issued 6,824,712 equity-classified warrants to PHP. Each warrant shall give the right to acquire common stock of the Company at a purchase price of $3.48 per share. Per the agreement, the PHP Warrant is exercisable upon either the achievement of corresponding market capitalization targets or a consummation of a fundamental transaction (as defined in the PHP Warrant); as such, there are no other requirements, including any continuous service requirements, in order for PHP to be entitled to the PHP Warrant, if and when any portion of it vests.

The aggregate grant date fair value of the PHP Warrant was $17.4 million, which was recognized as warrant expense on the grant date in November 2022.

There were no warrants issued during the three months ended March 31, 2024. As of March 31, 2024, there were 6,824,712 warrants outstanding at a weighted-average exercise price of $3.48, with a weighted-average remaining contractual term of 8.63 years.