In April 2021, the Company adopted the 2021 Employee Stock Purchase Plan (ESPP) and the 2021 Equity Incentive Plan (2021 EIP), each of which became effective in connection with the IPO. The Company has reserved 707,942 and 4,650,796 shares of Class A common stock for issuance under the ESPP and 2021 EIP, respectively.

The Company may not grant any additional awards under the 2017 Equity Incentive Plan (2017 EIP). The 2017 EIP will continue to govern outstanding equity awards granted thereunder. As of December 31, 2023, there were 288,775 shares available for issuance under the 2021 EIP.

Stock option activity under the 2017 EIP and 2021 EIP was as follows:


	NUMBER OF OPTIONS			WEIGHTED- AVERAGE EXERCISE PRICE PER SHARE		WEIGHTED- AVERAGE REMAINING CONTRACTUAL LIFE (YEARS)		AGGREGATE INTRINSIC VALUE (000s)
Outstanding as of December 31, 2022		3,821,787		$	10.30		8.51	$	35,872
Granted		2,291,000
Exercised		(111,927	)
Cancelled and forfeited		(238,624	)
Outstanding as of December 31, 2023		5,762,236		$	9.42		7.66	$	39,197
Options exercisable as of December 31, 2023		2,611,696		$	8.45		7.10	$	20,112
Vested and expected to vest as of December 31, 2023		5,762,236		$	9.42		7.66	$	39,197

The aggregate intrinsic value of stock options exercised during the year ended December 31, 2023 was $1.0 million. The weighted-average grant date fair value of options granted during the years ended December 31, 2023 and 2022, was $5.92 per share and $13.63 per share, respectively.

The ESPP enables eligible employees to purchase shares of the Company’s Class A common stock at the end of each offering period at a price equal to 85% of the fair market value of the shares on the first trading day or the last trading day of the offering period, whichever is lower. Eligible employees generally include all employees. Share purchases are funded through payroll deductions of at least 1% and up to 15% of an employee’s eligible compensation for each payroll period. The number of shares reserved for issuance under the ESPP increase automatically on the first day of each fiscal year, beginning on January 1, 2022, by a number equal to the lesser of 440,502 shares, 1% of the total number of shares of the Company’s capital stock (including all classes of the Company’s common stock) outstanding on the last day of the calendar month prior to the date of the increase, or such lower number of shares (including no

shares) approved by the Company’s board of directors. As of December 31, 2023, 59,808 shares have been issued pursuant to the ESPP. The ESPP generally provides for six-month consecutive offering periods beginning on September 14, 2021. The ESPP is a compensatory plan as defined by the authoritative guidance for stock compensation. Stock-based compensation expense for the year ended December 31, 2023, was $0.2 million.

The following tables summarize the stock-based compensation expense for stock options, restricted stock awards, and restricted stock units granted to employees and nonemployees and for ESPP stock-based compensation that was recorded in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2023 and 2022 (in thousands).


	Year Ended December 31,
	2023		2022
Research and development	$	4,845	$	4,592
General and administrative		6,648		4,295
Total stock-based compensation expense	$	11,493	$	8,887


	Year Ended December 31,
	2023		2022
Employees	$	10,494	$	5,975
Nonemployees		999		2,912
Total stock-based compensation expense	$	11,493	$	8,887

As of December 31, 2023, the Company had $22.0 million of unrecognized stock-based compensation expense related to unvested stock options, which is expected to be recognized over a weighted-average period of approximately 2.5 years.

The fair value of stock options granted during the years ended December 31, 2023 and 2022, was estimated using the Black-Scholes option pricing model based on the following weighted average assumptions.


	Year Ended December 31,
	2023	2022
Expected term (in years)	5.5 – 6.1	5.5 – 6.1
Expected volatility	79.5% – 80.5%	75.8% – 77.6%
Risk-free rate	3.4% – 4.7%	1.76% – 4.2%
Dividend yield	—	—

The Company grants restricted stock units (RSU) pursuant to the 2021 EIP and satisfies such grants through the issuance of the Company’s Class A common stock. The following table shows RSU activity for the period ending December 31, 2023.


	NUMBER OF SHARES			WEIGHTED- AVERAGE GRANT DATE FAIR VALUE PER SHARE
Unvested balance at December 31, 2022		204,000		$	18.37
Granted		—			—
Vested		(66,321	)		18.36
Cancelled and forfeited		(6,000	)		18.60
Unvested balance at December 31, 2023		131,679		$	18.36

For the year ended December 31, 2023, the Company recognized $1.6 million of stock-based compensation for RSUs.

As of December 31, 2023, the Company had $1.5 million of unrecognized stock-based compensation expense related to unvested RSUs, which is expected to be recognized over a weighted-average period of approximately 0.8 years.