Stock-based compensation expense was as follows:


	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2023		2022		2023		2022
Research and development	$	3,514	$	3,892	$	6,937	$	7,232
General and administrative		6,774		7,059		13,380		14,186
Total stock-based compensation expense	$	10,288	$	10,951	$	20,317	$	21,418

At June 30, 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 June 30, 2023, there was an aggregate of 10,240,361 shares of common stock issuable upon the exercise of outstanding options under the 2020 Plan and 5,150,960 shares of restricted common stock granted under the 2020 Plan. There is an aggregate of 4,476,879 shares reserved for future issuance under the 2020 Plan.

The following table summarizes restricted common stock activity for the six months ended June 30, 2023:


	Shares			Weighted Average Grant Date Fair Value
Unvested at January 1, 2023		2,239,106		$	13.75
Granted		2,074,444			6.31
Forfeited		(296,252	)		11.78
Vested		(513,839	)		12.67
Unvested at June 30, 2023		3,503,459		$	9.67

At June 30, 2023, there was $30.3 million of unrecognized stock-based compensation cost related to the restricted stock, which is expected to be recognized over a weighted average period of 2.57 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		3,400,892			6.60		—		—
Exercised		—			—		—		—
Forfeited		(1,019,286	)		15.83		—		—
Options outstanding at June 30, 2023		10,304,403		$	14.31		8.39	$	19
Options vested and exercisable at June 30, 2023		3,751,655		$	19.66		7.60	$	19

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 six months ended June 30, 2023 and 2022 was $5.16 per share and $6.33 per share, respectively. At June 30, 2023, there was $50.1 million of unrecognized stock-based compensation expense related to unvested stock options, which is being recognized over a period of 2.24 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 June 30, 2023			Six Months Ended June 30, 2023
Expected term (in years)		6.09			6.11
Expected volatility		93.95	%		94.49	%
Risk-free interest rate		3.61	%		3.46	%
Expected dividend yield		—		—
Fair value of common stock	$	3.76		$	6.60

The Company issued 108,936 shares of common stock under the 2020 Employee Stock Purchase Plan (the “ESPP”) during the six months ended June 30, 2023 at an average price per share of $2.89. Cash received from purchases under the ESPP for the six months ended June 30, 2023 and 2022 were $0.3 million and $0.2 million, respectively. The Company recognized $0.1 million and $0.1 million of compensation expense for the ESPP during the three months ended June 30, 2023 and 2022, respectively, and $0.3 million and $0.1 million of compensation expense for the ESPP during the six months ended June 30, 2023 and 2022, respectively.

At June 30, 2023, there was an aggregate of 505,366 shares reserved for future issuance under the ESPP.