The Company’s 2018 Stock Incentive Plan (the “2018 Plan”) provided for the Company to grant qualified incentive options, nonqualified options, stock grants and other stock-based awards to employees and non-employees to purchase the Company’s common stock. Upon the effectiveness of the 2021 Plan (as defined below), the Company ceased issuing new awards under the 2018 Plan.

The 2021 Equity Incentive Plan (the “2021 Plan”) was approved by the board of directors on January 27, 2021 and the Company’s stockholders on January 28, 2021, and became effective on the execution of the underwriting agreement related to the Company's initial public offering. The 2021 Plan provides for the grant of incentive stock options to employees, including employees of any parent or subsidiary corporations, and for 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, including employees and consultants of the Company’s affiliates. The number of shares initially reserved for issuance under the 2021 Plan was 5,000,000, which began automatically increasing on January 1 of each calendar year, starting on January 1, 2022 through January 1, 2031, in an amount equal to 4.0% of the total number of shares of the Company’s capital 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. In December 2022, the Company's board of directors determined that the automatic increase of available shares for calendar year 2023

would be reduced from 4.0% to 1.0% of the Company's capital stock. As a result, on January 1, 2023 the number of shares available for issuance pursuant to the 2021 Plan increased to 2,900,096 shares. As of March 31, 2023, 2,169,496 shares remained available for issuance pursuant to the 2021 Plan.

The 2021 Employee Stock Purchase Plan (the “2021 ESPP”) was approved by the Company’s board of directors on January 27, 2021 and became effective on the execution of the underwriting agreement related to the initial public offering. A total of 333,333 shares of common stock were initially reserved for issuance under the 2021 ESPP, which will automatically increase on January 1 of each calendar year, beginning on January 1, 2022 through January 1, 2031, by an amount equal to 1.0% of the total shares of common stock outstanding on December 31st of the preceding calendar year. The purchase price of the shares under the 2021 ESPP are at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the purchase date. As of March 31, 2023, the Company had issued 93,047 shares under the 2021 ESPP. As of March 31, 2023, 854,017 shares were available to be issued under the 2021 ESPP. The Company recognized no share-based compensation expense related to the ESPP for the three months ended March 31, 2023.

During 2023, the Company has granted options to purchase shares of common stock to employees and non-employee directors pursuant to the 2021 Plan at a weighted average fair value of $1.10 per share. The Company uses the Black-Scholes option-pricing model to estimate the fair value of the stock options on the applicable grant dates.

The following is a summary of the stock option award activity during the three months ended March 31, 2023:


	Number of Stock Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2022		3,333,434		$	8.80		8.09	$	1
Granted		503,971		$	1.43
Forfeited		(29,723	)	$	8.74
Expired		(64,980	)	$	9.84
Outstanding at March 31, 2023		3,742,702		$	7.79		8.22	$	39
Options expected to vest as of March 31, 2023		1,944,833		$	4.79		8.78	$	39
Exercisable at March 31, 2023		1,797,869		$	11.03		7.61	$	—

The aggregate intrinsic value of the outstanding stock option awards is calculated as the difference between the exercise price and the market price of the Company’s common stock at March 31, 2023. There were no stock options exercised in the three months ended March 31, 2023.

The grant date fair value of options vested during the three months ended March 31, 2023 and 2022 was $1.4 million and $4.1 million, respectively.

At March 31, 2023, there was $7.6 million of unrecognized stock-based compensation expense associated with the stock options, which is expected to be recognized over a weighted-average period of 2.07 years.

The Company has granted restricted stock units with service-based vesting conditions.

The following is a summary of the restricted stock unit activity during the three months ended March 31, 2023:


	Restricted Stock Units			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2022		215,854		$	3.69
Granted		325,369		$	1.47
Vested		(267,024	)	$	1.99
Forfeited		(4,037	)	$	3.04
Unvested at March 31, 2023		270,162		$	2.71

Pursuant to the 2021 Plan, the Company granted restricted stock units which vest annually over a period of one, two, three or four years.

At March 31, 2023, there was approximately $0.6 million of unrecognized stock-based compensation expense associated with the restricted stock units which is expected to be recognized over a weighted-average period of 3 years.

The following is a summary of the employee-issued common stock warrant activity during the three months ended March 31, 2023:


	Number of Common Stock Warrants			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding and exercisable at December 31, 2022		57,004		$	6.94		1.91	$	—
Granted		—			—		—		—
Exercised		—			—		—		—
Expired		(312	)		144
Outstanding and exercisable at March 31, 2023		56,692		$	6.19		1.91	$	—

As of March 31, 2023 there was no unrecognized stock-based compensation expense associated with the common stock warrants.

For the three months ended March 31, 2023, the Company utilized the Black-Scholes option-pricing model for estimating the fair value of the stock options granted. The following table presents the assumptions and the Company’s methodology for developing each of the assumptions used:


	Three Months Ended March 31,
	2023	2022
Volatility	93%	94%-96%
Expected term (years)	6.0-7.0	5.8-7
Risk-free interest rate	3.6%–4.0%	1.7%-2.4%
Dividend rate	—%	—%

Stock-based compensation expense was recorded in the following line items in the condensed consolidated statements of operations for the three months ended March 31, 2023 and 2022 (in thousands):


	Three Months Ended March 31,
	2023		2022
Research and development	$	292	$	555
General and administrative		921		960
Total stock-based compensation expense	$	1,213	$	1,515