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.

In February 2021, the Committee approved the issuance of option grants to purchase 647,000 shares of common stock for executive officers pursuant to the 2018 Plan, which will vest upon (a) the achievement of specified performance goals and (b) the grantees’ continued employment during the service period specified in each grant. For the year ended December 31, 2021, no expense has been recognized related to these awards. In

addition, the options never vested and expired since the performance conditions were not satisfied as of December 31, 2021.

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, 2019		1,577,541		$	10.85		8.15	$	1,350
Options granted		1,091,250			6.26
Options exercised		(44,605	)		5.04
Options forfeited		(250,641	)		10.32
Options expired		(17,930	)		51.12
Outstanding — December 31, 2020		2,355,615		$	8.59		7.96	$	1,337
Options granted		1,493,238			5.31
Options exercised		(5,000	)		4.63
Options forfeited		(1,239,630	)		6.26
Options expired		(593	)		566.33
Outstanding — December 31, 2021		2,603,630		$	7.70		7.46	$	—
Exercisable — December 31, 2021		1,513,039		$	9.37		6.35	$	—
Shares available to be granted — December 31, 2021		3,333,791

The weighted-average grant date fair value of options granted during the years ended December 31, 2021 and 2020 was $4.00 and $5.20, respectively.

The aggregate intrinsic value of options exercised during the years ended December 31, 2021 and 2020 was $3,000 and $0.1 million, respectively.

The fair value of options vested during the years ended December 31, 2021 and 2020 was $2.6 million and $3.0 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 history as a public company and limited number of sales of its common stock, 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 operating 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,
	2021			2020
Employee Stock Options:
Expected term (in years)		6.00			5.81
Risk-free interest rate		0.84	%		0.91	%
Dividend yield		—			—
Volatility		93.25	%		113.36	%

As of December 31, 2021, the Company had unrecognized employee stock-based compensation expense of $3.9 million, related to unvested stock option awards, which is expected to be recognized over an estimated weighted-average period of 2.49 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, 2021, 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, 2021, a total of 64,975 shares of common stock for $0.3 million have been issued to employees participating in the two ESPP purchases during 2021 and 235,469 shares are available for issuance under the ESPP as of December 31, 2021.

Stock-based compensation expense for the ESPP was $0.1 million and $0.1 million for the years ended December 31, 2021 and 2020, respectively, and is included in total stock-based compensation recognized.