Stock-based compensation expense was as follows:


	Three Months Ended March 31,
(in thousands)	2024			2023
Research and development	$	(274	)	$	3,423
General and administrative		5,308			6,606
Total stock-based compensation expense	$	5,034		$	10,029

In connection with the reduction in the Company’s workforce (“RIF”) (see Note 13), the Company accelerated certain unvested stock options and restricted common stock scheduled to vest in the three month period following the employees’ separation date. The Company determined that the acceleration of the unvested units constituted a Type III modification in accordance with ASC 718, resulting in a new measurement of compensation cost. As of March 31, 2024, 601,115 units were accelerated resulting in the recognition of $0.1 million of stock-based compensation expense using the reassessed fair value on the modification date and a reversal of $3.5 million in stock-based compensation expense for previously recognized expense using the original grant date fair value, of which $1.9

million was related to research and development expense and $1.6 million was related to general and administrative expense for the three months ended March 31, 2024.

At March 31, 2024, there was an aggregate of 64,042 shares of common stock issuable upon the exercise of outstanding options under the Company’s 2018 Equity Incentive Plan (the “2018 Plan”) and 6,616,772 shares of restricted common stock granted under the 2018 plan. No shares remain available for future issuance under the 2018 Plan. Any options or awards outstanding under the 2018 Plan remain outstanding and effective.

At March 31, 2024, there was an aggregate of 8,529,959 shares of common stock issuable upon the exercise of outstanding options under the Company’s 2020 Stock Option and Grant Plan (the “2020 Plan”) and 5,356,510 shares of restricted common stock granted under the 2020 Plan. There is an aggregate of 12,692,809 shares reserved for future issuance under the 2020 Plan.

The following table summarizes restricted common stock activity for the three months ended March 31, 2024:


	Shares			Weighted Average Grant Date Fair Value
Unvested at January 1, 2024		3,254,863		$	8.73
Granted		—			—
Forfeited		(957,148	)		8.13
Vested		(732,774	)		8.12
Unvested at March 31, 2024		1,564,941		$	9.37

At March 31, 2024, there was $12.3 million of unrecognized stock-based compensation expense related to restricted stock, which is expected to be recognized over a weighted average period of 1.90 years. The total fair value of restricted stock vested was $0.5 million and $1.6 million for the three months ended March 31, 2024 and 2023, respectively.

The following table summarizes stock option activity (in thousands, except share and per share data):


	Shares			Weighted Average Exercise Price		Weighted Average Contractual Life (in years)		Aggregate Intrinsic Value
Options outstanding at January 1, 2024		10,439,751		$	13.81		7.95	$	—
Granted		—			—		—		—
Exercised		—			—		—		—
Forfeited		(1,845,750	)		8.61		—		—
Options outstanding at March 31, 2024		8,594,001		$	14.93		7.54	$	—
Options vested and exercisable at March 31, 2024		5,972,960		$	16.44		7.29	$	—

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the common stock as of the end of the period.

There were no options granted during the three months ended March 31, 2024. The weighted-average grant date fair value of stock options granted during the three months ended March 31, 2023 was $5.18 per share. At March 31, 2024, there was $19.4 million of unrecognized stock-based compensation expense related to unvested stock options, which is being recognized over a period of 1.63 years.

There were no options granted during the three months ended March 31, 2024. For the three months ended March 31, 2023, the fair value was estimated on the date of grant using the Black-Scholes option-pricing model, with the following weighted-average assumptions:


	Three Months Ended March 31, 2023
Expected term (in years)		6.11
Expected volatility		94.50	%
Risk-free interest rate		3.46	%
Expected dividend yield	—
Fair value of common stock	$	6.63

The Company did not issue common shares under the Employee Stock Purchase Plan (the “ESPP”) during the three months ended March 31, 2024. The Company recognized $0.0 and $0.1 million of compensation expense for the ESPP during the three months ended March 31, 2024 and 2023, respectively.

At March 31, 2024, there was an aggregate of 1,621,594 shares reserved for future issuance under the ESPP.