The Company’s 2015 Stock Plan, as amended, (the 2015 Plan) provides for the Company to sell or issue common shares or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2015 Plan is administered by the board of directors and exercise prices, vesting and other restrictions are determined at its discretion. All stock option grants are non-statutory stock options except option grants to employees intended to qualify as incentive stock options under the Internal Revenue Code of 1986, as amended. Incentive stock options may not be granted at less than the fair market value of the Company’s common stock on the date of grant, as determined in good faith by the board of directors at its sole discretion. Nonqualified stock options may be granted at an exercise price established by the board of directors at its sole discretion and the vesting periods may vary. Vesting periods are generally four years and are determined by the board of directors. Stock options become exercisable as they vest. Options granted under the 2015 Plan expire no more than ten years from the date of grant. As of March 31, 2024, there are no shares available for grants under the 2015 Plan and the 2015 Plan continues to govern the terms and conditions of the outstanding awards under the 2015 Plan.

On July 14, 2021, the Company’s 2021 Equity Incentive Plan (the 2021 Plan) was approved by the Company’s stockholders, and became effective upon completion of the IPO and serves as the successor to the 2015 Plan. 6,645,259 shares of common stock are reserved under the 2021 Plan, of which 2,042,216 shares remain available for future grants as of March 31, 2024.

Stock option activity is summarized as follows:


	NUMBER OF STOCK OPTIONS			WEIGHTED- AVERAGE EXERCISE PRICE		WEIGHTED- AVERAGE REMAINING CONTRACTUAL TERM (IN YEARS)		AGGREGATE INTRINSIC VALUE (IN THOUSANDS)
Outstanding as of December 31, 2023		5,666,621		$	2.47		6.61
Granted		67,500			1.59
Exercised		(146,964	)		1.54
Cancelled, forfeited or expired		(474,484	)		3.42
Outstanding as of March 31, 2024		5,112,673		$	2.40		6.66	$	378
Exercisable as of March 31, 2024		3,500,102		$	2.41		5.80	$	133
Unvested as of March 31, 2024		1,612,571		$	2.37		8.53	$	245

The 2015 Plan, permits participants to use common stock they previously acquired to pay for the exercise of stock options based upon the fair value on the date of exercise. In connection with the exercise of a stock options to purchase 306,518 shares of our common stock at an exercise price of $1.46, option holders tendered 122,608 shares of our common stock previously acquired in consideration of the full aggregate exercise price in accordance with the terms of the option and the 2015 Plan. The shares tendered are recorded as treasury stock within the Company’s condensed consolidated financial statements at March 31, 2024.

The fair value of stock options granted was estimated on the grant date using the Black-Scholes option pricing model based on the following assumptions:


	THREE MONTHS ENDED MARCH 31,
	2024			2023
Expected option life (years)		6.08			6.08
Risk-free interest rate	4.21% - 4.27%			3.58% - 4.13%
Expected volatility	99.30% - 99.45%			91.33% - 92.25%
Expected dividend yield		0	%		0	%

The total intrinsic value of stock options vested during each of the three months ended March 31, 2024 and 2023 was zero.

Under the terms of the restricted stock unit agreements covering the common stock, shares of common stock related to restricted stock units are subject to time-based vesting. The restricted stock units will immediately be forfeited to the Company if the relationship between the recipient and the Company ceases.

Restricted stock activity is summarized as follows:


	NUMBER OF SHARES			WEIGHTED- AVERAGE GRANT DATE FAIR VALUE
Unvested at December 31, 2023		2,526,432		$	1.06
Granted		—		$	—
Vested		—		$	—
Forfeited		(219,543	)	$	1.13
Unvested at March 31, 2024		2,306,889		$	1.06

The aggregate fair value of restricted stock units that vested during each of the three months ended March 31, 2024 and 2023 was zero.

Stock-based compensation expense for the three months ended March 31, 2024 and 2023 was classified in the condensed consolidated statements of operations and comprehensive loss as follows (in thousands):


	THREE MONTHS ENDED MARCH 31,
	2024		2023
Research and development	$	553	$	311
General and administrative		490		422
Total stock-based compensation expense	$	1,043	$	733

As of March 31, 2024, total unrecognized compensation cost related to unvested stock-based awards was $4.4 million, which is expected to be recognized over a weighted average period of 2.30 years.