Stock-based compensation expense was as follows:


	Three Months Ended March 31,
(in thousands)	2023		2022
Research and development	$	3,423	$	3,341
General and administrative		6,606		7,126
Total stock-based compensation expense	$	10,029	$	10,467

At March 31, 2023, there was an aggregate of 64,042 shares of common stock issuable upon the exercise of outstanding options under 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, 2023, there was an aggregate of 10,689,300 shares of common stock issuable upon the exercise of outstanding options under the 2020 Plan and 4,883,035 shares of restricted common stock granted under the 2020 Plan. There is an aggregate of 4,037,290 shares reserved for future issuance under the 2020 Plan.

The following table summarizes restricted common stock activity for the three months ended March 31, 2023:


	Shares			Weighted Average Grant Date Fair Value
Unvested at January 1, 2023		2,239,106		$	13.75
Granted		1,806,519			6.63
Forfeited		(35,162	)		14.48
Vested		(334,747	)		11.93
Unvested at March 31, 2023		3,675,716		$	10.41

At March 31, 2023, there was $35.5 million of unrecognized stock-based compensation cost related to the restricted stock, which is expected to be recognized over a weighted average period of 3.12 years.

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, 2023		7,922,797		$	17.81		8.30	$	786
Granted		3,358,317			6.63		—		—
Exercised		—			—		—		—
Forfeited		(527,772	)		19.61		—		—
Options outstanding at March 31, 2023		10,753,342		$	14.23		8.66	$	46
Options vested and exercisable at March 31, 2023		3,231,896		$	20.57		7.76	$	42

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.

The weighted-average grant date fair value of stock options granted during the three months ended March 31, 2023 and 2022 was $5.18 per share and $6.89 per share, respectively. At March 31, 2023, there was $60.5 million of unrecognized stock-based compensation expense related to unvested stock options, which is being recognized over a period of 2.61 years.

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			Three Months Ended March 31, 2022
Expected term (in years)		6.11			6.11
Expected volatility		94.50	%		90.53	%
Risk-free interest rate		3.46	%		1.73	%
Expected dividend yield		—			—
Fair value of common stock	$	6.63		$	9.19

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

At March 31, 2023, there was an aggregate of 614,302 shares reserved for future issuance under the ESPP.