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 and provided an additional 1,500,000 stock options, following receipt of the requisite stockholder approval. The 2018 Plan replaces the Company’s 2015 Stock Incentive Plan, as amended (the “2015 Plan”). All awards outstanding under the 2015 Plan will remain in effect in accordance with their respective terms.

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)		Aggregate Intrinsic Value (thousands)
Outstanding — December 31, 2017		821,741		$	13.69		9.17	$	6,376
Options granted		612,050		$	13.13
Options exercised		(12,716	)	$	4.17			$	213
Options canceled/forfeited		(43,315	)	$	7.76
Options expired		(15,783	)	$	158.81
Outstanding — December 31, 2018		1,361,977		$	12.04		8.71	$	2,294
Options granted		450,900		$	7.85
Options exercised		(41,219	)	$	4.57			$	159
Options forfeited		(182,722	)	$	8.36
Options expired		(11,395	)	$	96.49
Outstanding — December 31, 2019		1,577,541		$	10.85		8.15	$	1,350
Exercisable — December 31, 2019		740,356		$	13.30
Shares available to be granted — December 31, 2019		1,014,126

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 and is 5.98 years based on the average between the vesting period and the contractual life of the option. 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 year ended December 31, 2019 and 2018:

	Year Ended December 31,
	2019			2018
Employee Stock Options:
Expected term (in years)		5.98			6.04
Risk-free interest rate		2.35	%		2.67	%
Dividend yield		—			—
Volatility		90.81	%		92.61	%
Weighted-average fair value of stock options granted	$	5.85		$	9.97

	Year Ended December 31,
	2019		2018
Research and development	$	1,052	$	606
General and administrative⁽¹⁾		2,152		2,000
Total stock-based compensation	$	3,204	$	2,606

(1)	Included $0.03 and $0.1 million related to modifications of certain Board member stock options in the years ended December 31, 2019 and 2018, respectively. These modifications extended the post-termination exercise period of certain options.

Also included in general and administrative stock-based compensation for the year ended December 31, 2019 and 2018 is expense related to 24,235 and 6,790 shares of common stock issued to certain board members in lieu of their cash compensation, respectively.

As of December 31, 2019, 1,014,126 shares of common stock were available for future grant and 1,577,541 options to purchase shares of common stock were outstanding. As of December 31, 2019, the Company had unrecognized employee stock-based compensation expense of $5.3 million, related to unvested stock option awards, which is expected to be recognized over an estimated weighted-average period of 2.41 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, 2019, a total of 239,545 shares of common stock may be granted in accordance with the terms of the ESPP. As of December 31, 2019, 20,853 shares of common stock have been issued to employees participating in the ESPP and 218,692 shares are available for issuance under the ESPP. Compensation expense, using Black-Scholes, for the ESPP was $0.1 million and $0.03 million as of December 31, 2019 and 2018, respectively, and is included in stock-based compensation expense.