The Company has two equity incentive plans: the 2016 Equity Incentive Plan, as amended, and the 2020 Equity Incentive Plan. New awards can only be granted under the 2020 Equity Incentive Plan (the “Plan”) and as of June 30, 2022, 5,708,289 shares were available for future grants. The number of shares of the Company’s common stock that may be issued pursuant to rights granted under the Plan shall automatically increase on January 1st of each year, commencing on January 1, 2021, and continuing for ten years, in an amount equal to five percent of the total number of shares of the Company’s common stock outstanding on December 31st of the preceding calendar year, subject to the discretion of the board of directors or compensation committee to determine a lesser number of shares shall be added for such year. The Plan provides for the granting of common stock, incentive stock options, nonqualified stock options, restricted stock awards, restricted stock units and/or stock appreciation rights to employees, directors, and other persons, as determined by the Company’s board of directors. The Company’s stock options vest based on the terms in each award agreement, generally over four-year periods with 25% of options vesting after 1 year and then monthly thereafter, and have a term of ten years.

The Company measures stock-based awards at their grant-date fair value and records compensation expense on a straight-line basis over the vesting period of the awards. The Company recorded stock-based compensation expense in the following expense categories in its accompanying statements of operations:

	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2022		2021		2022		2021
Research and development	$	2,472	$	2,203	$	5,672	$	4,043
General and administrative		3,556		2,034		7,185		4,080
	$	6,028	$	4,237	$	12,857	$	8,123

	Number of shares			Weighted average exercise price per share		Weighted average remaining contractual term (years)
Outstanding at January 1, 2022		7,179,482		$	15.36		8.66
Granted		2,705,609		$	8.32
Exercised		(124,285	)	$	1.70
Forfeited		(845,517	)	$	18.06
Outstanding at June 30, 2022		8,915,289		$	13.16		8.60
Exercisable at June 30, 2022		2,738,389		$	9.81		7.71

At June 30, 2022, the aggregate intrinsic value of outstanding options and exercisable options was $7.7 million and $4.9 million, respectively.

The following table summarizes information about stock options outstanding at June 30, 2022 under the Plan:

	Options Outstanding						Options Exercisable
Range of Exercise Prices	Number Outstanding		Weighted Average Remaining Contractual Life (in years)		Weighted Average Exercise Price		Number Exercisable		Weighted Average Exercise Price
$0.31 - $3.05		2,109,347		7.26	$	1.80		1,414,546	$	1.76
$3.06 - $10.34		1,068,193		9.88		4.84		4,500		10.09
$10.35 - $11.72		1,447,250		9.63		10.58		—		0.00
$11.73 - $13.04		2,253,228		8.17		12.85		1,031,129		12.85
$13.05 - $88.98		2,037,271		9.05		31.48		288,214		38.39
		8,915,289						2,738,389

The weighted-average grant date fair value of options granted was $6.00 and $38.31 per share for the six months ended June 30, 2022 and 2021, respectively. The aggregate intrinsic value of options exercised was $0.7 million for the six months ended June 30, 2022. The Company recorded stock-based compensation expense of $5.7 million and $4.2 million for the three months ended June 30, 2022 and 2021, respectively, related to stock options. The Company recorded stock-based compensation expense of $12.2 million and $7.7 million for the six months ended June 30, 2022 and 2021, respectively, related to stock options. As of June 30, 2022, the total unrecognized compensation expense related to unvested stock option awards was $61.8 million, which the Company expects to recognize over a weighted-average period of 2.79 years.

The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below:

	Six months ended June 30,
	2022			2021
Expected volatility		84.82	%		115.05	%
Risk-free interest rate		2.39	%		0.93	%
Expected life (in years)		6.01			5.91
Expected dividend yield		—			—

The Company issues restricted stock awards (“RSA”) to employees that generally vest over a four-year period with 25% of awards vesting after one year and then monthly thereafter. Any unvested shares will be forfeited upon termination of services. The fair value of an RSA is equal to the fair market value price of the Company’s common stock on the date of grant. RSA expense is recorded on a straight-line basis over the vesting period.

The following table summarizes activity related to RSA stock-based payment awards:

	Number of shares			Weighted-average grant date fair value
Unvested balance at January 1, 2022		611,608		$	2.29
Vested		(231,760	)	$	1.88
Forfeited		(23,416	)	$	1.89
Unvested balance at June 30, 2022		356,432		$	2.58

The Company recorded stock-based compensation expense of $0.2 million for both the three months ended June 30, 2022 and 2021 related to RSAs. The Company recorded stock-based compensation expense of $0.4 million for both the six months ended June 30, 2022 and 2021, related to RSAs. As of June 30, 2022, the total unrecognized expense related to all RSAs was $0.9 million, which the Company expects to recognize over a weighted-average period of 1.45 years.

The Company granted restricted stock units (“RSU”) to employees that generally vest over a four-year period with 25% of awards vesting after one year and then quarterly thereafter. Any unvested units will be forfeited upon termination of services.

The following table summarizes activity related to RSU stock-based payment awards:

	Number of shares		Weighted-average grant date fair value
Unvested balance at January 1, 2022		20,000	$	18.32
Granted		150,000	$	4.86
Unvested balance at June 30, 2022		170,000	$	6.44

The Company recorded stock-based compensation expense of $58 thousand and $82 thousand for the three and six months ended June 30, 2022, respectively, related to RSUs. At June 30, 2022 the total unrecognized expense related to the RSUs was $1.0 million, which the Company expects to recognize over 3.61 years.

The Company has an Employee Stock Purchase Plan (the “ESPP”), which, as of June 30, 2022, had 1,339,285 shares of common stock reserved for future issuance. The number of shares of the Company’s common stock that may be issued pursuant to rights granted under the ESPP shall automatically increase on January 1st of each year and continuing for ten years beginning in 2021, in an amount equal to one percent of the total number of shares of all classes of the Company’s common stock outstanding on December 31st of the preceding calendar year, subject to the discretion of the board of directors or compensation committee to determine a lesser number of shares shall be added for such year. On January 1, 2022, 476,023 shares were added to the ESPP.

Under the ESPP, eligible employees can purchase the Company’s common stock through accumulated payroll deductions at such times as are established by the compensation committee. Eligible employees may purchase the Company’s common stock at 85% of the lower of the fair market value of the Company’s common stock on the first day of the offering period or on the last day of the offering period. Eligible employees may contribute up to 15% of their eligible compensation. Under the ESPP, a participant may not accrue rights to purchase more than $25,000 worth of the Company’s common stock for each calendar year in which such right is outstanding.

The ESPP is considered compensatory under the FASB stock compensation rules. Accordingly, share-based compensation expense is determined based on the option’s grant-date fair value as estimated by applying the Black Scholes option-pricing model and is recognized over the withholding period. The Company recognized share-based compensation expense of $0.1 million and $0.2 million for the three and six months ended June 30, 2022, respectively, related to the ESPP. There was no share-based compensation expense recognized in 2021 related to the ESPP.