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 ISOs, or nonqualified stock options, or NSOs. Options granted under the 2021 Plan expire no later than 10 years from the date of grant. Options under the 2021 Plan generally vest 25% upon one year of continued service to the Company, with the remainder in monthly increments over three additional years. As of June 30, 2022, there were 4.8 million shares of common stock available for issuance under the 2021 Plan.

The Company adopted the 2021 Employee Stock Purchase Plan, or the ESPP, in August 2021. The ESPP 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 June 30, 2022, there were 0.9 million shares of common stock available for issuance under the ESPP. No shares have been issued under the ESPP as of June 30, 2022.

Stock-based compensation expense under the ESPP is measured at the beginning of the offering period using the Black-Scholes option-pricing model and recognized on a straight-line basis over the offering period.

A summary of stock option activity is set forth below:


	Options outstanding
						Weighted
				Weighted		Average		Aggregate
	Number of			Average		Remaining		Intrinsic
	Options			Exercise		Contractual Life		Value
	(In thousands)			Price		(In years)		(In thousands)
Outstanding – December 31, 2021		1,794		$	6.31		8.43
Granted		1,761			3.38
Forfeited		(43	)		5.17
Expired		(9	)		6.15
Outstanding – June 30, 2022		3,503			4.86		8.80	$	1,209
Exercisable – June 30, 2022		950			3.56		7.40		1,047

The aggregate intrinsic values of options outstanding and exercisable are the differences between the exercise price of the options and the fair value of the Company’s common stock at June 30, 2022.

During the six months ended June 30, 2022, the Company granted options with a weighted-average grant-date fair value of $2.34 per share.

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 June 30,					Six Months Ended June 30,
	2022			2021(1)		2022			2021
Expected term (in years)		5.66			—		5.99			5.95
Expected volatility		82.97	%		—		80.46	%		63.43	%
Risk-free rate		2.96	%		—		1.74	%		0.78	%
Dividend yield		—			—		—			—

The following table summarizes the Company’s restricted stock award activity:


				Weighted
	Number of			Average
	Shares			Grant Date
RSAs, unvested at December 31, 2021		161		$	9.39
Vested		(31	)		8.12
RSAs, unvested at June 30, 2022		130			9.69

The fair value of restricted stock awards vested during the six months ended June 30, 2022 was $0.1 million.

Total stock-based compensation recorded in the unaudited condensed consolidated statements of operations and comprehensive loss related to stock options, restricted stock awards and ESPP was as follows (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Research and development	$	354	$	172	$	687	$	347
General and administrative		625		360		1,208		660
Total stock-based compensation expense	$	979	$	532	$	1,895	$	1,007

As of June 30, 2022, there was approximately $10.6 million of stock-based compensation expense to be recognized over a weighted-average period of approximately 2.81 years.