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, 2016		140,990		$	128.25		3.93		—
Options granted		742,000		$	4.56
Options forfeited		(61,249	)	$	165.63
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
Exercisable — December 31, 2018		414,584		$	17.98
Vested and expected to vest — December 31, 2018		1,361,977		$	12.04
Shares available to be granted — December 31, 2018		1,295,144

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 6.04 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 weighted average grant date fair value of employee stock options was $9.97 and $4.01 for the years ended December 31, 2018 and 2017, respectively, and was estimated using the following weighted-average assumptions for the years ended December 31, 2018 and 2017:

	Year Ended December 31,
	2018			2017
Employee Stock Options:
Expected term (in years)		6.04			6.03
Risk-free interest rate		2.67	%		2.03	%
Dividend yield		—			—
Volatility		92.61	%		110.29	%

During the year ended December 31, 2018, no options were issued to consultants. During the year ended December 31, 2017, options to purchase 20,000 shares of common stock were issued to consultants that vest over one to four years with a weighted-average exercise price of $4.69 per share. During the years ended December 31, 2018, and 2017, the Company recorded stock-based compensation expense attributable to these nonemployee stock awards of $0.09 million and $0.02 million, respectively.

The estimated grant-date fair value of the nonemployee stock options was determined using the Black-Scholes valuation model and the following assumptions:

	Year Ended December 31,
	2018		2017
Research and development	$	590	$	164
General and administrative⁽¹⁾		1,990		699
Total stock-based compensation	$	2,580	$	863

(1)

2018 includes $0.1 million in modification stock-based compensation expense related to a Board member’s separation agreement.

As of December 31, 2018, the Company had unrecognized employee stock-based compensation expense of $6.5 million, related to unvested stock option awards, which is expected to be recognized over an estimated weighted-average period of 2.78 years.

In June 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”). The 2018 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.

A total of 120,000 shares of common stock may be granted in accordance with the terms of the ESPP. As of December 31, 2018, no shares of common stock have been issued to employees participating in the ESPP and 120,000 shares are available for issuance under the ESPP. Compensation expense, using Black-Scholes, for the ESPP was $0.03 million as of December 31, 2018.