In June 2018, stockholders of the Company approved the Company’s 2018 Omnibus Incentive Plan (the “2018 Plan”). The 2018 Plan had previously been approved by the Company’s Board of Directors (the “Board”) and the Compensation Committee (the “Committee”) of the Board, subject to stockholder approval. The 2018 Plan became effective on June 13, 2018. On June 9, 2021, the stockholders of the Company approved an amendment previously approved by the Board to increase the number of shares of common stock reserved for issuance under the 2018 Plan by 2,500,000 to a total of 5,300,000 shares. The amendment became effective immediately upon stockholder approval. After the option modification (as discussed below), the number of shares of common stock reserved for issuance under the 2018 Plan increased by 14,860,784 to a total of 20,160,784. As of December 31, 2022, there were 12,990,839 shares of common stock available for future grant.

In June 2022, the Committee approved the issuance of an option grant to purchase 400,000 shares (1,521,568 shares after the option modification discussed below) of common stock to the Chief Executive Officer pursuant to the 2018 Plan, which will vest upon (a) the achievement of a specified performance goal and (b) the grantee’s continued employment during the service period. For the year ended December 31, 2022, no expense has been recognized related to this award and no options have vested as of December 31, 2022.

On September 20, 2022, the Company paid a special, one-time cash dividend of $45.0 million (or $1.43 per share) to the Company’s common stockholders of record as of the close of business on September 6, 2022. The Company determined, in accordance with the adjustment provision of the 2018 Plan, that the special cash dividend was unusual and non-recurring and that appropriate adjustment to the stock options to purchase shares of the Company’s common stock outstanding under the 2018 Plan was required. The Company treated this adjustment as a modification to the original stock option grants because the terms of the agreements were modified in order to preserve the value of the option awards after a large non-recurring cash dividend. These options were amended to decrease the exercise price and increase the number of shares subject to the stock option on a proportionate basis. No incremental value was provided to the option holders as a result of the modification and no additional compensation cost was recorded by the Company.

The following table summarizes stock option activity under the Company’s 2018 Plan and related information:

	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (thousands)
Outstanding — December 31, 2021		2,603,630		$	7.70		7.46	$	—
Options granted ⁽¹⁾		10,270,911			1.34
Options forfeited and cancelled ⁽¹⁾		(4,148,455	)		4.96
Options expired		(47,319	)		13.27
Outstanding — December 31, 2022		8,678,767		$	1.42		7.47	$	1,051
Exercisable — December 31, 2022		4,317,076		$	2.38		5.94	$	—

The weighted-average grant date fair value of options granted during the years ended December 31, 2022 and 2021 was $0.96 and $4.00, respectively.

No options were exercised during the year ended December 31, 2022. The aggregate intrinsic value of options exercised during the year ended December 31, 2021 was $3,000.

The fair value of options vested during the years ended December 31, 2022 and 2021 was $1.9 million and $2.6 million, respectively.

The Company estimated the fair value of stock options granted using the Black-Scholes option-pricing formula and a single option award approach. Due to its limited relevant historical data, the Company estimated its volatility considering a number of factors including the use of the volatility of comparable public companies. The expected term of options granted under the Plan, all of which qualify as “plain vanilla” per SEC Staff Accounting Bulletin 107, is determined based on the simplified method due to the Company’s limited relevant history. The risk-free rate is based on the yield of a U.S. Treasury security with a term consistent with the option. This fair value is being amortized ratably over the requisite service periods of the awards, which is generally the vesting period.

The fair value of employee stock options was estimated using the following weighted-average assumptions:

	Year Ended December 31,
	2022			2021
Employee Stock Options:
Expected term (in years)		6.02			6.00
Risk-free interest rate		3.00	%		0.84	%
Dividend yield		—			—
Volatility		92.58	%		93.25	%

	Year Ended December 31,
	2022		2021
Research and development	$	330	$	1,202
General and administrative⁽¹⁾		1,012		2,203
Total stock-based compensation	$	1,342	$	3,405

As of December 31, 2022, the Company had unrecognized employee stock-based compensation expense of $1.1 million, related to unvested stock option awards, which is expected to be recognized over an estimated weighted-average period of 1.93 years.

In June 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”). The ESPP had previously been approved by the Board and the Compensation Committee of the Board, subject to stockholder approval which became effective as of June 13, 2018. Under the ESPP, employees meeting certain specific employment qualifications are eligible to participate and can purchase shares of common stock semi-annually on February 9^th and August 9^th of each year, through payroll deductions. The purchase price is 85% of the lower of the fair market value of the stock at the commencement or end of the offering period. The ESPP permits eligible employees to purchase shares of common stock through payroll deductions for up to 15% of qualified compensation.

The Company’s ESPP is subject to an Evergreen provision which shares may be added to the pool as needed. As of December 31, 2022, a total of 359,545 shares of common stock may be granted in accordance with the terms of the ESPP.

For the year ended December 31, 2022, a total of 47,662 shares of common stock for $20,000 have been issued to employees participating in the two ESPP purchases during 2022 and 187,807 shares are available for issuance under the ESPP as of December 31, 2022.

Stock-based compensation expense for the ESPP was not significant and $0.1 million for the years ended December 31, 2022 and 2021, respectively, and is included in total stock-based compensation recognized.