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 of the Board, subject to stockholder approval. The 2018 Plan became effective on June 13, 2018. On June 11, 2020, 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 1,300,000 to a total of 2,800,000 shares. The amendment became effective immediately upon stockholder approval.

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, 2019		1,577,541		$	10.85		8.15
Options granted		971,250		$	6.25
Options exercised		(44,605	)	$	5.04
Options forfeited		(223,708	)	$	9.61
Options expired		(16,337	)	$	49.67
Outstanding — September 30, 2020		2,264,141		$	8.84		8.09
Exercisable — September 30, 2020		1,032,133		$	10.99

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:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Employee Stock Options:
Risk-free interest rate	0.39%		1.65%		0.96%		2.39%
Expected term (in years)		6.06		5.90		5.78		5.98
Dividend yield	—		—		—		—
Volatility	115.62%		109.47%		112.87%		89.76%
Weighted-average fair value of stock options granted	$	4.61	$	5.60	$	5.17	$	5.93

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	493	$	284	$	1,151	$	805
General and administrative⁽¹⁾		575		517		1,556		1,728
Total stock-based compensation	$	1,068	$	801	$	2,707	$	2,533

As of September 30, 2020, 1,373,834 shares of common stock were available for future grant and 2,264,141 options to purchase shares of common stock were outstanding. As of September 30, 2020, the Company had unrecognized employee stock-based compensation expense of $6.7 million, related to unvested stock awards, which is expected to be recognized over an estimated weighted-average period of 2.67 years.