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.

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.

The following table summarizes stock option activity under the Company’s equity incentive plans and related information:

	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)
Outstanding — December 31, 2020		2,355,615		$	8.59		7.96
Options granted		939,613		$	6.21
Options exercised		—		$	—
Options forfeited		(87,083	)	$	7.62
Options expired		—		$	—
Outstanding — March 31, 2021		3,208,145		$	7.92		8.37
Exercisable — March 31, 2021		1,213,061		$	10.18

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 for the three months ended March 31, 2021 and 2020:

	Three Months Ended March 31,
	2021			2020
Employee Stock Options:
Risk-free interest rate		0.69	%		1.57	%
Expected term (in years)		6.05			5.53
Dividend yield		—			—
Volatility		93.83	%		110.00	%
Weighted-average fair value of stock options granted	$	4.70		$	5.58

	Three Months Ended March 31,
	2021		2020
Research and development	$	369	$	344
General and administrative⁽¹⁾		657		461
Total stock-based compensation	$	1,026	$	805

As of March 31, 2021, 391,477 shares of common stock were available for future grant and 3,208,145 options to purchase shares of common stock were outstanding. As of March 31, 2021, the Company had unrecognized employee stock-based compensation expense of $8.2 million, related to unvested stock awards, which is expected to be recognized over an estimated weighted-average period of 2.9 years.