Stock-based compensation expense was as follows:


	Years Ended December 31,
(in thousands)	2022		2021
Research and development	$	14,014	$	20,632
General and administrative		27,301		23,343
Total stock-based compensation expense	$	41,315	$	43,975

The Company’s 2018 Plan provided for the Company to issue restricted stock, restricted stock units, incentive stock options, and non-statutory stock options and other stock-based awards to employees, officers, members of the Board, consultants and advisors of the Company. The 2018 Plan was most recently amended in July 2020. The awards granted under this plan generally vest over a four-year period and have a 10-year contractual term.

At December 31, 2022, the Company had granted 6,715,415 shares of common stock under the 2018 Plan. At December 31, 2022, 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.

On July 2, 2020, the Company’s Board of Directors adopted and in July 2020 the stockholders approved the 2020 Stock Option and Grant Plan (the “2020 Plan”) which became effective on July 28, 2020, the date immediately prior to the date on which the registration statement related to the IPO was declared effective, and as a result no further awards were made under the 2018 Plan thereafter. Initially, the aggregate number of shares of our common stock that may be issued pursuant to stock awards under the 2020

Plan was 8,008,734 shares. The number of shares of our common stock reserved for issuance under the 2020 Plan shall be cumulatively increased on January 1, 2021 and each January 1 thereafter by 5% of the total number of shares of our common stock outstanding on December 31 of the preceding calendar year or a lesser number of shares determined by our board of directors. Unless our board of directors elects not to increase the number of shares available for future grant each year, our stockholders may experience additional dilution, which could cause our stock price to fall. Accordingly, on January 1, 2022, 3,528,817 shares were added to the number of available shares under the 2020 Plan. The awards granted under this plan generally vest over a four-year period and have a 10-year contractual term.

At December 31, 2022, there were an aggregate of 7,858,755 shares of common stock issuable upon the exercise of outstanding options under the 2020 Plan and 3,076,516 shares of restricted common stock granted under the 2020 Plan. There is an aggregate of 4,253,680 shares reserved for future issuance under the 2020 Plan.

The following table summarizes restricted common stock activity for the year ended December 31, 2022:


	Shares			Weighted Average Grant Date Fair Value
Unvested at January 1, 2022		2,866,909		$	15.45
Granted		1,632,996			7.70
Forfeited		(348,589	)		17.74
Vested		(1,912,210	)		10.40
Unvested at December 31, 2022		2,239,106		$	13.75

At December 31, 2022, there was $27.4 million of unrecognized stock-based compensation cost related to the restricted stock, which is expected to be recognized over a weighted average period of 2.54 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		Aggregate Intrinsic Value
Options outstanding at January 1, 2022		6,155,055		$	23.42	8.8		$	480
Granted		3,051,906			7.48		—		—
Exercised		—			—		—		—
Forfeited		(1,284,164	)		20.13		—		—
Options outstanding at December 31, 2022		7,922,797		$	17.81		8.3	$	786
Options vested and exercisable at December 31, 2022		2,945,832		$	22.10		7.8	$	88

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 aggregate intrinsic value of options exercised were $0 and $0.3 million during the year ended December 31, 2022 and 2021, respectively.

The weighted average grant-date fair value of stock options granted during the year ended December 31, 2022 and 2021 was $5.61 per share and $24.76 per share, respectively. The weighted average grant date fair value of options vested was $19.16 and $13.61 per share for the year ended December 31, 2022 and 2021, respectively. At December 31, 2022, there was $50.5 million of unrecognized stock-based compensation expense related to unvested stock options, which is being recognized over a period of 2.20 years.

The fair value was estimated on the date of grant using the Black-Scholes option-pricing model, with the following weighted-average assumptions:


	Years Ended December 31,
	2022			2021
Expected term (in years)		6.07			6.06
Expected volatility		90	%		95	%
Risk-free interest rate		2.00	%		0.87	%
Expected dividend yield		—			—
Fair value of common stock	$	7.48		$	32.41

In July 2020, the 2020 Employee Stock Purchase Plan (the “2020 ESPP”) was adopted by the Board of Directors and approved by the stockholders. The purpose of the 2020 ESPP is to provide eligible employees of the Company and other designated companies, with opportunities to purchase shares of the Company’s common stock, par value $0.0001 per share.

Initially, 611,354 shares of common stock in the aggregate were approved and reserved for this purpose. The number of shares of common stock reserved and available for issuance under the 2020 ESPP shall be cumulatively increased on January 1, 2021 and each January 1 thereafter by the least of (i) 1,222,707 shares of common stock, (ii) 1% of the number of shares of common stock issued and outstanding on the immediately preceding December 31, and (iii) such number of shares of common stock as determined by the Administrator. The Board of Directors elected not to increase the number of available shares as of January 1, 2021 and 2022. At December 31, 2022, there was an aggregate of 454,302 shares reserved for future issuance under the ESPP.

The ESPP allows eligible employees to authorize payroll deductions of up to 15% of their base salary or wages up to $25,000 annually to be applied toward the purchase of shares of the Company's common stock on the last trading day of the offering period. Participating employees will purchase shares of the Company's common stock at a discount of up to 15% on the lesser of the closing price of the Company's common stock on the NASDAQ Global Market (i) on the first trading day of the offering period or (ii) the last day of any offering period. The Company utilizes the Black Scholes option pricing model to compute the fair market value of the shares and compensation expense is recognized over the offering period. The first ESPP offering period was shortened in duration to three months and commenced on April 1, 2022 and ended on June 30, 2022. New six-month offering periods will commence each January 1 and July 1 thereafter during the term of the plan, with the administrator having the right to establish different offering periods.

Participation in the ESPP is voluntary. Eligible employees become participants in the ESPP by enrolling in the plan and authorizing payroll deductions. At the end of each offering period, accumulated payroll deductions are used to purchase the Company’s shares at the discounted price. The Company makes no contributions to the ESPP. A participant may withdraw from the ESPP or suspend contributions to the ESPP. If the participant elects to withdraw during an offering period, all contributions are refunded as soon as administratively practicable. If a participant elects to withdraw or suspend contributions, they will not be able to re-enroll in the current offering but may elect to participate in future offerings. The ESPP purchases only whole shares of the Company’s common stock.

The Company issued 157,052 common shares under the ESPP during the year ended December 31, 2022, at an average price per share of $3.42. Cash received from purchases under the ESPP for the year ended December 31, 2022 was $0.5 million. The Company recognized $0.2 million of compensation expense for the ESPP during the year ended December 31, 2022.