The Company maintains the 2008 Stock Incentive Plan (the “2008 Plan”) for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non‑qualified stock option and restricted stock awards as determined by the Board. At inception of the 2008 Plan, a total of 2,213,412 shares of common stock were authorized for grants under the 2008 Plan. The Company ceased granting awards under the 2008 Plan upon the effectiveness of the 2016 Plan (as defined below); however, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan and the applicable 2008 Plan agreement. Shares subject to awards that were granted under the 2008 Plan and that expire, lapse or terminate following the

On June 7, 2016, the Company’s stockholders approved the 2016 Incentive Award Plan (the “2016 Plan”), which became effective June 21, 2016. The 2016 Plan provides for the granting of incentive and non‑qualified stock option, restricted stock and other stock and cash-based awards as determined by the Board. Shares subject to awards that are granted under the 2016 Plan and that expire, lapse or terminate are available for future grants under the 2016 Plan. At inception of the 2016 Plan, a total of 1,210,256 shares of common stock were authorized for future issuance under the 2016 Plan. The number of shares of common stock that may be issued under the 2016 Plan automatically increases on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Board. During the year ended December 31, 2019 and 2018, the number of shares of common stock that may be issued under the 2016 Plan was increased by 898,871 shares and 893,730 shares, respectively. As of December 31, 2019, 267,612 shares remain available for future issuance under the 2016 Plan.

The 2008 Plan and 2016 Plan provide that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2008 Plan and 2016 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date.

The Company’s 2018 Employment Inducement Incentive Award Plan (the “Inducement Incentive Award Plan”), which was adopted by the Board on September 25, 2018 without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules ("Rule 5635(c)(4)"), provides for the grant of equity-based awards in the form of non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock or cash based awards. In accordance with Rule 5635(c)(4), awards under the Inducement Incentive Award Plan may only be made to a newly hired employee who has not previously been a member of the Board, or an employee who is being rehired following a bona fide period of non-employment by the Company, as a material inducement to the employee’s entering into employment with the Company. The Company reserved 1,175,000 shares of its common stock for issuance under the Inducement Incentive Award Plan. On March 25, 2019, the Board approved the amendment and restatement of the Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of December 31, 2019, there are 750,000 shares available for future grant under the Inducement Incentive Award Plan.

The fair value of each option award was estimated on the grant date using the Black‑Scholes option pricing model. Expected volatilities are based on historical volatilities from guideline companies because the Company's common stock has not traded for a period that is at least equal to the expected term of its stock option awards. The Company uses the “simplified” method to estimate the expected life of options granted and are expected to be outstanding. The risk‑free interest rate used is the rate for a U.S. Treasury zero coupon issue with a remaining life consistent with the options expected life on the grant date. The Company has not paid and does not expect to pay in the foreseeable future, any cash dividends. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on awards that are expected to vest.

The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:



	Year Ended December 31,
	2019			2018			2017
Risk-free interest rate	1.86		%	2.85		%	2.03		%
Dividend yield	—			—			—
Expected term	5.94			6.06			5.90
Expected volatility	87.66		%	85.17		%	84.52		%
Weighted-average fair value of common stock	$	2.01		$	8.45		$	15.32

The weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2019, 2018 and 2017 was $1.47, $6.17, and $10.97 respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2019, 2018 and 2017 was $0.1 million, $0.6 million, and $2.6 million respectively.

As of December 31, 2019 and December 31, 2018, total unrecognized compensation expense related to unvested employee stock options was $9.8 million and $9.8 million, respectively, which is expected to be recognized over a weighted average period of 2.5 years and 3.1 years, respectively.

Effective November 20, 2019, the Company amended the terms of the options granted to a non-employee director in connection with his resignation from the board of directors, whereby he was entitled to receive immediate vesting of his outstanding, unvested stock options (141,328 shares), provided that the options will become exercisable in accordance with their original vesting schedules irrespective of his termination of service, and an extension of the right to exercise each of his vested stock options (234,772 shares) until the final expiration date of the applicable option, resulting in a modification. The subsequent stock-based compensation amount recognized as of December 31, 2019 was $0.1 million.

The estimated grant date fair values of non-employee stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:



	Year Ended December 31,
	2019		2018		2017
Risk-free interest rate	1.92	%	2.77	%	—	%
Dividend yield	—		—		—
Expected life (in years)	5.33		5.81		0.00
Expected volatility	88.60	%	85.86	%	—	%

As of December 31, 2019 and 2018 total unrecognized compensation expense related to unvested non‑employee stock options was less than $0.1 million and $1.1 million, respectively, which is expected to be recognized over a weighted average period of 0.4 years and 2.2 years, respectively.

The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:



					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee awards
Outstanding at December 31, 2018	3,681,575		$	9.49	7.77	$	300
Granted	3,876,255		$	2.01
Exercised	(125,600	)	$	1.20
Forfeited	(1,108,634	)	$	10.38
Outstanding at December 31, 2019	6,323,596		$	4.91	8.71	$	1,716

Vested at December 31, 2019	1,485,228		$	10.02	6.73	$	90
Vested and expected to vest at December 31, 2019	5,843,053		$	5.10	8.65	$	1,500

Non‑employee awards
Outstanding at December 31, 2018	412,404		$	6.44	6.42	$	28
Granted	73,489		$	1.86
Exercised	—		$	—
Forfeited	(12,820	)	$	0.47
Outstanding at December 31, 2019	473,073		$	5.89	6.23	$	38

Vested at December 31, 2019	330,528		$	5.61	5.06	$	21
Vested and expected to vest at December 31, 2019	473,073		$	5.89	6.23	$	38

During the first quarter of 2019, the Company awarded 100,000 restricted stock units under the Inducement Incentive Award Plan, of which 50,000 were determined to be granted and 50,000 were reserved for issuance consistent with ASC Topic 718, Compensation-Stock Compensation (ASC 718). The 50,000 granted restricted stock units had a fair value of $2.29 per share based on the closing price of the Company’s common stock on the date of grant. These restricted stock units were valued at approximately $0.1 million, and will vest on the date an applicable performance condition is achieved on or prior to December

The 50,000 reserved restricted stock units did not have defined performance criteria until August 6, 2019, at which time, they were deemed both granted and vested. These restricted stock units had a fair value of $1.65 per share. These restricted stock units were valued at approximately $0.1 million and fully expensed as of December 31, 2019.

On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 21, 2016. The ESPP is intended to qualify as an "employee stock purchase plan" under Section 423 of the Internal Revenue Code of 1986 with the purpose of providing employees with an opportunity to purchase the Company's common stock through accumulated payroll deductions.

Under the ESPP, the Company has set two six-month offering periods during each calendar year, one beginning March 1^st and the other beginning September 1^st of each calendar year, during which employees may elect to have up to 25% of their eligible compensation deducted on each payday on an after-tax basis for use in purchasing the Company's common stock on the last trading day of each offering period, subject to limits imposed by the Internal Revenue Code. The purchase price of the shares may not be less than 85% of the fair market value on the first or last trading day of the offering period, whichever is lower. The first ESPP offering period began on March 1, 2017.

At inception of the ESPP, a total of 173,076 shares of common stock were authorized and reserved for future issuance under the ESPP. The number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors. During the year ended December 31, 2019 and 2018, the number of shares of common stock that may be issued under the ESPP was increased by 224,717 shares and 223,432 shares, respectively. During the year ended December 31, 2019, the Company issued 17,205 shares of common stock under the ESPP. As of December 31, 2019, 747,406 shares remain available for future issuance under the ESPP.

The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):



	Year Ended December 31,
	2019		2018		2017
Research and development	$	2,079	$	2,453	$	1,779
General and administrative	3,082		3,261		2,302
Total stock-based compensation expense	$	5,161	$	5,714	$	4,081