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, or RSAs, under the 2021 Plan generally vest over four years. Restricted stock units, or RSUs, granted under the 2021 Plan generally vest in one year. As of June 30, 2024, there were 0.1 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 June 30, 2024, there were 28,000 shares of common stock available for issuance under the ESPP. During the six months ended June 30, 2024, approximately 7,000 shares of common stock 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, 2023		311		$	27.70		7.99
Granted		258			9.64
Forfeited		(9	)		24.31
Expired		(1	)		31.77
Outstanding – June 30, 2024		559			19.41		8.48	$	440
Exercisable – June 30, 2024		216			26.66		7.12		110

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 June 30, 2024.

During the six months ended June 30, 2024 and 2023, the Company granted options with a weighted-average grant-date fair value of $7.45 and $9.47 per share.

No options were exercised during the three and six months ended June 30, 2024 and 2023.

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,
	2024			2023			2024			2023
Expected term (in years)		6.00			5.56			6.00			5.85
Expected volatility		91.60	%		86.16	%		91.59	%		85.86	%
Risk-free rate		4.63	%		3.62	%		4.63	%		3.89	%
Dividend yield		—			—			—			—

The following table summarizes the Company’s RSAs activity (shares in thousands):


				Weighted
				Average
	Number of			Grant Date
	Shares			Fair Value
RSAs, unvested at December 31, 2023		4		$	146.92
Vested		(2	)		145.23
RSAs, unvested at June 30, 2024		2			148.18

The fair value of RSAs vested during the six months ended June 30, 2024 and 2023 was de minimis in both periods respectively.

The following table summarizes the Company’s RSUs activity (shares in thousands):


				Weighted
				Average
	Number of			Grant Date
	Shares			Fair Value
RSUs, unvested at December 31, 2023		90		$	7.73
Granted		40			9.17
Vested		(44	)		7.73
Cancelled		(2	)		7.73
RSUs, unvested at June 30, 2024		84			8.41

The fair value of RSUs vested during the six months ended June 30, 2024 was $0.4 million.

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 June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Research and development	$	350	$	324	$	649	$	667
General and administrative		795		834		1,526		1,620
Total stock-based compensation expense	$	1,145	$	1,158	$	2,175	$	2,287

As of June 30, 2024, there was approximately $5.3 million of stock-based compensation expense to be recognized over a weighted-average period of approximately 2.12 years.