In October 2017, the board of directors approved the 2017 Stock Option and Incentive Plan (the “2017 Stock Plan”) which became effective in November 2017, upon the closing of the Company’s IPO. The 2017 Stock Plan will expire in October 2027. Under the 2017 Stock Plan, the Company may grant incentive stock options, non-statutory stock options, restricted stock awards, RSUs and other stock-based awards. The Company’s 2014 Stock Incentive Plan (the “2014 Stock Plan”) terminated in November 2017 effective upon the completion of the Company’s IPO. No additional options will be granted under the 2014 Stock Plan. At December 31, 2022, there were 598,411 options outstanding under the 2014 Stock Plan.

At December 31, 2022, there were 6,105,147 shares of the Company’s common stock authorized for issuance under the 2017 Stock Plan, including 359,860 options that have been forfeited from the 2014 Stock Plan.

At December 31, 2022, there were 2,655,289 options available for issuance and 3,409,766 options outstanding under the 2017 Stock Plan. Awards granted under the 2017 Stock Plan have a term of ten years. Vesting of awards under the 2017 Stock Plan is determined by the compensation committee of the board of directors but is generally over one to four-year terms.

The fair value of options at date of grant was estimated using the Black-Scholes option-pricing model with the following assumptions:


	2021		2022
Risk-free interest rate	0.50%—1.25%		1.67%—4.18%
Expected dividend yield	0%		0%
Expected life	5.5—6.7 years		5.5—6.7 years
Expected volatility	72%—74%		70%—84%
Weighted-average grant date fair value	$	4.18	$	3.18

Due to the lack of a public market for the trading of the Company’s common stock prior to its initial public offering and the lack of company-specific historical volatility, volatility was estimated using historical volatilities of similar companies. The expected life of the awards is estimated based on the simplified method, which calculates the expected life based upon the midpoint of the term of the award and the vesting period. The Company uses the simplified method because it does not have sufficient option exercise data to provide a reasonable basis upon which to estimate the expected term. The Company has no history of paying dividends nor does management expect to pay dividends over the contractual terms of these options. The risk-free interest rates are based on the United States Treasury yield curve in effect at the time of grant, with maturities approximating the expected life of the stock options.

The following table summarizes information about stock option activity during 2021 and 2022 (in thousands, except share and per share data):


	NUMBER OF SHARES			WEIGHTED- AVERAGE EXERCISE PRICE		WEIGHTED- AVERAGE REMAINING CONTRACTUAL TERM		AGGREGATE INTRINSIC VALUE
Outstanding, December 31, 2020		2,224,913		$	6.26
Granted		953,506			6.57
Exercised		(2,501	)		3.81
Forfeited		(513,166	)		6.72
Outstanding, December 31, 2021		2,662,752		$	6.28
Granted		1,455,594			4.86
Exercised		(11,756	)		3.71
Forfeited		(98,413	)		6.73
Outstanding, December 31, 2022		4,008,177		$	5.76		7.47	$	7,147
Vested and exercisable, December 31, 2022		1,982,329		$	6.15		6.01	$	3,436
Vested and expected to vest, December 31, 2022		3,489,134		$	5.81		7.25	$	6,226

The following table summarizes information about RSU activity during 2021 and 2022:


	RSUs			AVERAGE GRANT DATE FAIR VALUE (IN DOLLARS PER SHARE)
RSUs outstanding, December 31, 2020		80,450		$	3.25
Granted		—			—
Vested		(38,200	)		3.25
Forfeited		—			—
RSUs outstanding, December 31, 2021		42,250			3.25
Granted		—			—
Vested		(42,250	)		3.25
Forfeited		—			—
RSUs outstanding, December 31, 2022		—		$	—

The number of RSUs vested includes shares of common stock withheld on behalf of employees to satisfy the minimum statutory tax withholding requirements.

During 2021 and 2022, the Company received $9,000 and $44,000, respectively, upon exercise of stock options. The intrinsic value of the options exercised in 2021 and 2022 was $7,000 and $18,000, respectively.

Unrecognized compensation expense related to unvested options as of December 31, 2022 was $4.2 million and will be recognized over the remaining vesting periods of the underlying awards. The weighted-average period over which such compensation is expected to be recognized is 2.5 years.

In October 2017, the board of directors approved the 2017 Employee Stock Purchase Plan (“the ESPP”) which became effective in November 2017, upon the closing of the Company’s IPO. As part of the ESPP, eligible employees may acquire an ownership interest in the Company by purchasing common stock, at a discount, through payroll deductions. Eligible employees who elected to participate were able to participate in the ESPP beginning September 1, 2021.

During 2021 and 2022, 11,253 and 45,938 shares of common stock were issued under the ESPP, respectively. As of December 31, 2022, there were 1,142,691 shares of common stock available for issuance under the ESPP.

The Company recorded stock-based compensation expense in the following expense categories of its accompanying condensed consolidated statements of operations and comprehensive loss for employees, directors and non-employees during the years ended December 31, 2021 and 2022 as follows (in thousands):


	2021		2022
Research and development	$	947	$	1,050
General and administrative		1,419		1,788
Total	$	2,366	$	2,838