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 June 2022, the Committee approved the issuance of an option grant to purchase 400,000 shares 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 grantees’ continued employment during the service period. For the six months ended June 30, 2022, no expense has been recognized related to this award.

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)
Outstanding — December 31, 2021		2,603,630		$	7.70		7.46
Options granted		1,314,200		$	0.72
Options forfeited		(1,173,227	)	$	4.19
Options expired		(47,192	)	$	12.84
Outstanding — June 30, 2022		2,697,411		$	5.64		7.85
Exercisable — June 30, 2022		1,375,625		$	9.01

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:

	Three Months Ended June 30,						Six Months Ended June 30,
	2022			2021			2022			2021
Employee Stock Options:
Risk-free rate		2.93	%		0.96	%		2.22	%		0.74	%
Expected term (in years)		6.0			5.7			6.0			6.0
Dividend yield	—			—			—			—
Volatility		91.82	%		92.79	%		91.63	%		93.64	%
Weighted-average fair value of stock options granted	$	0.87		$	3.37		$	0.55		$	4.45

Total stock-based compensation expense recognized was as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Research and development	$	83	$	394	$	211	$	763
General and administrative⁽¹⁾		263		589		650		1,246
Total stock-based compensation expense	$	346	$	983	$	861	$	2,009

As of June 30, 2022, 3,013,716 shares of common stock were available for future grant.