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 and restricted stock awards under the 2021 Plan generally vest over four years. Restricted stock units granted under the 2021 Plan generally vest in one year. As of September 30, 2023, there were 3.3 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 September 30, 2023, there were 0.5 million shares of common stock available for issuance under the ESPP. During the nine months ended September 30, 2023, 0.5 million 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		1,805			0.86
Exercised		(6	)		0.56
Forfeited		(646	)		2.40
Expired		(297	)		2.20
Outstanding – September 30, 2023		4,752			1.85		8.14	$	—
Exercisable – September 30, 2023		2,213			1.93		7.33		—

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 September 30, 2023.

During the nine months ended September 30, 2023 and 2022, the Company granted options with a weighted-average grant-date fair value of $0.63 per share and $2.31 per share, respectively.

During the nine months ended September 30, 2023, the intrinsic value of options exercised was de minimis. No options were exercised during the nine months ended September 30, 2022.

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 September 30,						Nine Months Ended September 30,
	2023			2022			2023			2022
Expected term (in years)		6.02			6.05			5.85			6.00
Expected volatility		84.66	%		83.46	%		85.86	%		80.94	%
Risk-free rate		4.19	%		2.95	%		3.89	%		1.94	%
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		(36	)		9.68
RSAs, unvested at September 30, 2023		69			9.78

The fair value of restricted stock awards vested during the nine months ended September 30, 2023 and 2022 was de minimis and $0.1 million, respectively.

Restricted Stock Units

The following table summarizes the Company’s restricted stock unit, or RSU, activity (shares in thousands):


			Weighted
			Average
	Number of		Grant Date
	Shares		Fair Value
RSU, unvested at December 31, 2022		—	$	—
Granted		1,345		0.51
RSU, unvested at September 30, 2023		1,345		0.51

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 September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Research and development	$	215	$	359	$	882	$	1,047
General and administrative		698		718		2,318		1,925
Total stock-based compensation expense	$	913	$	1,077	$	3,200	$	2,972

As of September 30, 2023, there was approximately $6.7 million of stock-based compensation expense to be recognized over a weighted-average period of approximately 1.86 years.