In December 2015, the Company’s board of directors adopted and approved the 2015 Stock Incentive Plan (as amended to date, the “2015 Plan”). The 2015 Plan provided for the granting of incentive stock options, non-statutory stock options, restricted stock awards and other stock-based awards to eligible employees, officers, directors, consultants and advisors as determined by the Company’s board of directors.

In February 2021, the Company’s board of directors adopted and stockholders approved the 2021 Equity Incentive Plan (the “2021 Plan”). The 2021 Plan became effective on February 5, 2021, following which no further grants were or will be made under the 2015 Plan. The 2021 Plan provides for the grant of stock options, non-statutory stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of stock compensation to our employees, consultants and directors.

The number of shares of our common stock reserved for issuance under our 2021 Plan will automatically increase on January 1 of each year through January 1, 2031, by 4.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year. Any grants that expire or are canceled, terminated, forfeited, fail to vest, or are withheld to satisfy a tax withholding obligation are allowed to be reissued under 2021 Plan. As of December 31, 2022, the Company had 570,800 shares of its common stock available for future issuance under the 2021 Plan.

The Company’s stock options generally vest over 48 months with 25% vesting after one year followed by ratable monthly vesting over three years and have a contractual term of 10 years. The weighted-average assumptions used principally in determining the fair value of options granted were as follows:


	Year Ended December 31,
	2022			2021
Fair value of common stock	$	4.78		$	23.63
Expected term (in years)		5.9			6.0
Expected volatility		78.8	%		78.4	%
Risk-free interest rate		2.5	%		0.9	%
Dividend yield		—			—

The following table summarizes the Company’s stock option activity for the year ended December 31, 2022:


	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2021		5,086,792		$	6.17		8.00	$	38,000
Granted		3,045,264			6.99
Vested and exercised		(671,796	)		1.84
Forfeited		(889,678	)		7.53
Expired		(99,058	)		14.81
Outstanding at December 31, 2022,		6,471,524			6.69		8.28	$	14,949
Exercisable at December 31, 2022		2,537,544		$	6.18		7.68	$	8,089

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 respective date.

The weighted-average grant-date fair value of stock options granted during the years ended December 31, 2022 and 2021 was $7.10 and $13.14 per share, respectively. As of December 31, 2022, total unrecognized compensation expense related to stock options was $16.2 million which is expected to be recognized over a weighted-average period of 2.29 years. The intrinsic value of stock options exercised was $2.2 million for the year ended December 31, 2022.

During the year ended December 31, 2020, options for 159,197 shares with a weighted-average exercise price of $2.59 and a weighted-average grant date fair value of $1.90 were exercised but unvested at the time of exercise. As of December 31, 2022 and 2021, options for 83,459 and 200,687 shares with weighted average exercise prices of $2.23 and $1.90 were exercised and unvested, respectively. The underlying proceeds from the unvested exercises of $0.2 million and $0.4 million is recorded in other current liabilities as of December 31, 2022 and 2021, on the consolidated balance sheet.

As of December 31, 2022, there were 1,430,200 restricted stock units outstanding under the 2021 Plan. The following table summarizes the Company's unvested restricted stock unit activity for the year ended December 31, 2022:


	Shares			Weighted-Average Grant Date Fair Value
Unvested at December 31, 2021		—		$	-
Granted		2,009,000			5.64
Vested		(461,700	)		6.00
Forfeited		(117,100	)		6.00
Unvested at December 31, 2022		1,430,200		$	5.49

As of December 31, 2022, total unrecognized compensation expense related to the unvested restricted stock units was $6.9 million, which is expected to be recognized over a weighted average period of 1.16 years. The Company did not grant any restricted stock units during the year ended December 31, 2021.

In February 2021, the Company’s board of directors adopted and stockholders approved the 2021 Employee Stock Purchase Plan (the “ESPP”). The ESPP became effective on February 5, 2021. The number of shares of common stock reserved for issuance under the ESPP will automatically increase on January 1 of each year through January 1, 2031, by the lesser of (i) 1% of the total number of shares of our common stock outstanding on December 31 of the preceding calendar year, and (ii) 1,800,000 shares. If purchase rights granted under the ESPP terminate without having been exercised, the shares of common stock not purchased under such purchase rights will again become available for issuance under the ESPP. As of December 31, 2022, the Company had 711,512 shares of its common stock available for future issuance under the ESPP.

The ESPP permits eligible employees to purchase common stock through accumulated payroll deductions at a purchase price equal to 85% of the lesser of the market value of the common stock at the beginning of the 6-month offering period or on the purchase date. The Company issued 34,242 shares with a weighted average purchase price of $3.41 during fiscal year 2022, which resulted in an immaterial amount of compensation expense. The Company did not issue any shares under the ESPP during the year ended December 31, 2021.

Stock-based compensation expense was allocated as follows:


	Year Ended December 31,
(in thousands)	2022		2021
Research and development	$	6,036	$	2,281
General and administrative		4,659		2,034
Total stock-based compensation expense	$	10,695	$	4,315