The Company maintains the 2021 Equity Incentive Plan, or the 2021 Plan, which provides for the granting of stock awards to employees, directors and consultants. Options granted under the 2021 Plan may be either incentive stock options or nonqualified stock options. Options granted under the 2021 Plan expire no later than 10 years from the date of grant. Options under the 2021 Plan generally vest over four years. As of March 31, 2023, there were 4.4 million shares of common stock available for issuance under the 2021 Plan.

The Company maintains the 2021 Employee Stock Purchase Plan, or the ESPP, which allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to plan limitations. An offering period under the ESPP consists of four six-month purchase periods, unless otherwise determined by the Company. The eligible employees are able to purchase shares 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 day. As of March 31, 2023, there were 1.0 million shares of common stock available for issuance under the ESPP. During the three months ended March 31, 2023, no shares were issued under the ESPP.

A summary of stock option activity is set forth below (shares in thousands):


	Options Outstanding
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
	Number of			Exercise		Contractual Life		Value
	Options			Price		(In years)		(In thousands)
Outstanding – December 31, 2022		3,896		$	2.43		8.44
Granted		1,603			0.89
Forfeited		(366	)		2.58
Expired		(1	)		1.87
Outstanding – March 31, 2023		5,132			1.94		8.32	$	11
Exercisable – March 31, 2023		1,728			1.95		6.49		2

The aggregate intrinsic value of options outstanding and exercisable are the difference between the exercise price of the options and the fair value of the Company’s common stock at March 31, 2023.

During the three months ended March 31, 2023 and 2022, the Company granted options with a weighted-average grant-date fair value of $0.66 per share and $2.39 per share, respectively.

The fair value of options is estimated at the grant date using the Black-Scholes option-pricing model with the following weighted-average assumptions:


	Three Months Ended March 31,
	2023			2022
Expected term (in years)		5.89			6.03
Expected volatility		85.82	%		80.15	%
Risk-free rate		3.93	%		1.59	%
Dividend yield		—			—

The following table summarizes the Company’s restricted stock award activity (shares in thousands):


				Weighted-
				Average
	Number of			Grant-Date
	Shares			Fair Value
RSAs, unvested at December 31, 2022		105		$	9.74
Vested		(12	)		9.68
RSAs, unvested at March 31, 2023		93			9.75

The fair value of restricted stock awards vested during the three months ended March 31, 2023 and 2022 was de minimis and $0.1 million, respectively.

The total stock-based compensation expense recorded in the unaudited condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):


	Three Months Ended March 31,
	2023		2022
Research and development	$	343	$	333
General and administrative		786		583
Total stock-based compensation expense	$	1,129	$	916

As of March 31, 2023, there was approximately $8.9 million of stock-based compensation expense to be recognized over a weighted-average period of approximately 2.0 years.