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)		Aggregate Intrinsic Value (thousands)
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
Options granted		1,091,250			6.26
Options exercised		(44,605	)		5.04				116
Options forfeited		(250,641	)		10.32
Options expired		(17,930	)		51.12
Outstanding — December 31, 2020		2,355,615			8.59		7.96		1,337
Exercisable — December 31, 2020		1,113,791		$	10.45		7.14	$	823
Shares available to be granted — December 31, 2020		1,255,302

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 years ended December 31, 2020 and 2019:

	Year Ended December 31,
	2020			2019
Employee Stock Options:
Expected term (in years)		5.81			5.98
Risk-free interest rate		0.91	%		2.35	%
Dividend yield		—			—
Volatility		113.36	%		90.81	%
Weighted-average fair value of stock options granted	$	5.20		$	5.85

As of December 31, 2020, 1,255,302 shares of common stock were available for future grant and 2,355,615 options to purchase shares of common stock were outstanding. As of December 31, 2020, 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 2.63 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, 2020, a total of 359,545 shares of common stock may be granted in accordance with the terms of the ESPP as of December 31, 2020.

For the year ended December 31, 2020, a total of 38,248 shares of common stock for $0.2 million have been issued to employees participating in the two ESPP purchases of 2020 and 300,444 shares are available for issuance under the ESPP as of December 31, 2020.

Stock-based compensation expense for the ESPP was $0.1 million and $0.1 million for the years ended December 31, 2020 and 2019, respectively, and is included in total stock-based compensation recognized.