In March 2016, the Company’s board of directors and stockholders adopted the 2016 Stock Incentive Plan which was amended and restated in December 2020, (as so amended and restated, the “2016 Plan”) which permits the granting of (1) options to purchase common stock intended to qualify as incentive stock options under Section 422 of the Code, and (2) options that do not so qualify.

In March 2021, the Company’s stockholders approved the 2021 Stock Incentive Plan (the “2021 Plan”), which became effective on March 30, 2021. The 2021 Plan replaced the 2016 Plan as the board of directors had determined it would not to make additional awards under the 2016 Plan following the closing of the initial public offering. However, the 2016 Plan will continue to govern outstanding equity awards granted thereunder. The shares of the Company’s common stock subject to outstanding awards under the 2016 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right will be added back to the shares of common stock available for issuance under the 2021 Plan.

The 2021 Plan allows the Company to make equity-based and cash-based incentive awards to officers, employees, directors and consultants. The number of shares initially reserved under the 2021 Plan was 3,263,664 shares of the Company’s common stock. The 2021 Plan contains an “evergreen” provision, which allows for an annual increase in the number of shares of common stock available for issuance under the 2021 Plan on the first day of each fiscal year during the period beginning in fiscal year 2022. The annual increase in the number of shares shall be equal to 4% of the number of shares of common stock outstanding on the immediately preceding December 31; and such lesser number of shares as determined by the Administrator as provided in the 2021 Plan. On January 1, 2022, the number of shares of common stock available for issuance under the 2021 Plan increased by 1,439,001 shares as a result of the automatic increase provision of the 2021 Plan. As of December 31, 2022, 2,639,915 shares of common stock remain available for future issuance under the 2021 Plan.

The vesting periods for equity awards, which generally is four years, are determined by the Board of Directors. The contractual term for stock option awards is ten years.

The total compensation expense recognized in the statements of operations associated with all the stock-based compensation awards granted by the Company is as follows (in thousands):


	Year Ended December 31,
	2022		2021
Research and development	$	3,974	$	2,427
General and administrative		3,551		2,750
Total share-based compensation expense	$	7,525	$	5,177

The weighted-average fair value of the stock options granted during the year ended December 31, 2022 and 2021 was $5.52 and $6.13 per share, respectively. As of December 31, 2022, the total unrecognized stock-based compensation balance for unvested options was $17.7 million which is expected to be recognized over 2.6 years.

The following table summarizes stock option activity under the Plan for the year ended December 31, 2022:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2021		5,822,373		$	7.17		8.27	$	33,275
Granted		1,927,649			7.71
Exercised		(282,459	)		3.88
Cancelled or forfeited		(878,084	)		8.50
Outstanding as of December 31, 2022		6,589,479		$	7.29	7.73		$	362,112
Vested or expected to vest as of December 31, 2022		6,589,479		$	7.29	7.73		$	362,112
Options exercisable as of December 31, 2022		3,009,095		$	5.76		6.50	$	358,798

The intrinsic value of options exercised for the years ended December 31, 2022 and 2021 was $1.2 million and $1.9 million, respectively.

The fair value of each option award granted during the years ended December 31, 2022 and 2021 is estimated on the date of grant using the Black-Scholes option pricing model. This model incorporates various assumptions, including the expected volatility, expected term, and interest rates. The underlying assumptions used to value stock options granted to participants using the Black-Scholes option-pricing were as follows:


	Year Ended December 31,
	2022			2021
Risk-free interest rate	1.67% to 3.59%			0.68% to 1.33%
Expected dividend yield		0	%		0	%
Expected option term (in years)	5.15 to 6.08			6.00 to 6.08
Expected stock price volatility range	83.60% to 86.00%			68.58% to 73.30%

On March 20, 2021, the Company’s stockholders approved the 2021 Employee Stock Purchase Plan (the “ESPP”), which became effective on March 30, 2021. The ESPP initially provides participating employees with the opportunity to purchase up to an aggregate of 346,613 shares of the Company’s common stock. An annual increase in the number of shares of common stock reserved and available for issuance under the ESPP shall be equal to 1% of the number of shares of common stock outstanding on the immediately preceding December 31; and such lesser number of shares as determined by the Administrator as provided in the ESPP. As of December 31, 2022, no shares have been purchased by employees under the ESPP.