The 2013 Equity Incentive Plan (2013 Plan) provides the Board of Directors the discretion to grant stock options and other equity-based awards to employees, directors, and consultants of the Company. The Board of Directors administers the 2013 Plan and has discretion to delegate some or all of the administration of the 2013 Plan to a committee or committees or an officer. To date, the Company has only granted Incentive Stock Options (ISOs) and Non-statutory Stock Options (NSOs) to employees, consultants, and directors. Following the completion of the Company’s IPO no additional shares have been granted under the 2013 Plan. However, the 2013 Plan will continue to govern outstanding equity awards granted thereunder. To the extent outstanding options granted under the 2013 Plan are cancelled, forfeited or otherwise terminated without being exercised and would otherwise have been returned to the share reserve under the 2013 Plan, the number of shares underlying such awards will be available for future grant under the 2021 Equity Incentive Plan.

In February 2021, the Board of Directors adopted the 2021 Equity Incentive Plan (2021 Plan), and our stockholders approved the 2021 Plan. The 2021 Plan is a successor to and continuation of the 2013 Plan. To date, the Company has only granted ISOs, NSOs and Restricted Stock Units (RSUs) to employees and directors. Therefore, the below discussion is limited to the terms applicable to ISOs and NSOs (collectively, stock options or options), and RSUs.

In February 2021, the Company’s Board of Directors adopted the ESPP, and our stockholders approved the ESPP. The price at which stock is purchased under the ESPP is equal to 85% of the fair market value of the Company's common stock on the first or the last day of the offering period, whichever is lower. Generally, each offering under the ESPP will be for a period of six months as determined by the Company's Board of Directors. Employees may invest up to 15% of their qualifying gross compensation through payroll deductions. In no event may an employee purchase more than 4,750 shares of common stock during any six-month offering period.

The ESPP is a compensatory plan as defined by the authoritative guidance for stock compensation; therefore, stock-based compensation expense of $0.3 million and $0.4 million related to the ESPP has been recorded for the years ended December 31, 2022 and 2021, respectively.

In November 2021, the Company's Board of Directors adopted the 2021 Inducement Plan (Inducement Plan). The Inducement Plan was adopted without stockholder approval pursuant to Nasdaq Listing Rule 5635(c)(4). Under the Inducement Plan, the Company may grant nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance awards and other awards to individuals not previously employees or directors of the Company, as an inducement toward entering into employment with the Company. The maximum number of shares of common stock that may be issued under the Inducement Plan is 3,000,000 shares.

A summary of option activity during the year ended December 31, 2022 is as follows:


	Number of Units Outstanding			Weighted Average Exercise Price per Unit		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2021		8,590,411		$	5.48		8.8	$	6,196
Granted		4,367,446		$	1.08
Exercised		(65,862	)	$	1.50
Forfeited		(3,030,143	)	$	4.86
Expired		(1,459,226	)	$	5.03
Outstanding at December 31, 2022		8,402,626		$	3.52		8.6	$	—
Options vested and expected to vest at December 31, 2022		8,402,626		$	3.52		8.6	$	—
Options vested and exercisable at December 31, 2022		2,831,700		$	4.97		8.0	$	—

As of December 31, 2022, the total unrecognized stock-based compensation related to stock options was $9.9 million, which is expected be recognized over a weighted-average period of approximately 3 years. Total options vested during the year were 1,812,221 with a total fair value of $6.0 million.

As of December 31, 2022, the Company has granted service-based stock option awards which may accelerate vesting upon performance-based or market-based conditions.

The weighted-average assumptions that the Company used in Black-Scholes option pricing model to determine the grant date fair value of stock options granted to employees and non-employee directors were as follows:


	Year ended December 31,
	2022	2021
Expected term (in years)	8.3	5.8
Expected Volatility	71.3%	73.1%
Risk-free interest rate	2.9%	1.1%
Expected Dividend yield	—%	—%

The weighted-average grant date fair value per share was $0.79 and $4.51 for options granted during the years ended December 31, 2022 and 2021, respectively.

The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of guideline companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The simplified method deems the term to be the average of the time-to-vesting and the contractual life of the stock-based awards. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

A summary of RSU activity during the year ended December 31, 2022 is as follows:


	Number of Units Outstanding			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2021		407,720		$	5.72
Granted		351,900		$	1.20
Vested		(61,760	)	$	5.70
Forfeited		(371,496	)	$	4.06
Outstanding at December 31, 2022		326,364		$	2.75

As of December 31, 2022, the total unrecognized stock-based compensation related to RSUs was $0.8 million, which is expected to be recognized over a weighted average period of approximately 3 years. Outstanding RSUs as of December 31, 2022 includes 5,075 RSUs that were vested, but not yet delivered.

The following table summarizes the components of stock-based compensation expense recorded in the Company’s statement of operations and comprehensive loss (in thousands):


	Year ended December 31,
	2022		2021
Research and development	$	1,313	$	2,388
Selling, general and administrative		4,055		6,819
Total stock-based compensation	$	5,368	$	9,207