The Company maintains the 2008 Stock Incentive Plan, or 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.

In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 1,210,256 shares of common stock for future issuance under the 2016 Plan and ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expire, lapse or terminate become available under the 2016 Plan as shares available for future grants.

Additionally, pursuant to the terms of the 2016 Plan, the Board is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2021 and 2020, the number of shares of common stock that may be issued under the 2016 Plan was increased by 4,322,850 and 3,453,022 shares, respectively. As of March 31, 2021, 3,248,520 shares remain available for future issuance under the 2016 Plan.

In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan was adopted by the Board without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 1,175,000 shares of its common stock for issuance. In March 2019, the Board approved the amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of March 31, 2021, there are 1,358,333 shares available for future grant under the 2018 Inducement Incentive Award Plan.

Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):


	Three Months Ended March 31,
	2021		2020
Research and development	$	754	$	623
General and administrative	1,026		786
Total stock-based compensation expense	$	1,780	$	1,409

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:


	Three Months Ended March 31,
	2021			2020
Risk-free interest rate	0.44		%	1.73		%
Dividend yield	—			—
Expected term	5.28			6.08
Expected volatility	83.26		%	87.96		%
Weighted-average fair value of common stock	$	3.13		$	2.30

The weighted average grant date fair value of stock options granted to employees during the three months ended March 31, 2021 and 2020 was $1.99 and $1.69, respectively.

As of March 31, 2021 total unrecognized compensation expense related to unvested employee stock options was $11.8 million, which is expected to be recognized over a weighted average period of 2.7 years.


				Weighted-average
				remaining	Aggregate
	Number of	Weighted-average		contractual term	intrinsic value
	options	exercise price ($)		(in years)	(in thousands)
Employees








Outstanding at December 31, 2020	7,302,176	$	3.98	8.43	$	4,456
Granted	2,628,000	$	3.13
Exercised	(153,278)	$	1.47
Forfeited	(62,577)	$	5.19
Outstanding at March 31, 2021	9,714,321	$	3.78	8.65	$	15,665

Vested at March 31, 2021	3,064,130	$	5.46	7.63	$	4,313
Vested and expected to vest at March 31, 2021	8,920,548	$	3.88	8.59	$	14,231

Non-employee consultants








Outstanding at December 31, 2020	473,073	$	5.89	5.23	$	86
Granted	—	$	—
Exercised	—	$	—
Forfeited	—	$	—
Outstanding at March 31, 2021	473,073	$	5.89	4.98	$	366

Vested at March 31, 2021	462,186	$	5.82	4.92	$	366
Vested and expected to vest at March 31, 2021	473,073	$	5.89	4.98	$	366

In addition, during the first quarter of 2021, the Company awarded 197,500 restricted stock units to executives under the 2016 Plan, of which 98,750 were determined to be granted as of the award date consistent with ASC 718. The remaining 98,750 restricted stock units do not have a defined performance metric as of the award date, resulting in the restricted stock units being reserved for future issuance as of March 31, 2021. These restricted stock units will vest in two equal installments on the dates

an applicable performance condition is achieved, on or prior to December 31, 2021. If the performance conditions are not satisfied on or prior to December 31, 2021, the restricted stock units will be forfeited for no consideration.

The restricted stock units granted had a weighted average fair value of $2.99 per share based on the closing price of the Company’s common stock on the date of grant. The restricted stock units were valued at approximately $1.4 million on their grant date. 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% for restricted stock awards to employees based on historical attrition trends.

Unrecognized compensation expense for all restricted stock units was $1.5 million as of March 31, 2021, which is expected to be recognized over a weighted average period of 3.1 years.


	Number of shares	Weighted average grant date fair value ($)
Unvested at December 31, 2020	87,500	$	6.03
Granted	468,550	2.99
Vested	(10,937)	6.03
Reserved for issuance	98,750	2.99
Forfeited	(800)	2.99
Unvested at March 31, 2021	643,063	$	3.35

In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 173,076 shares of common stock for future issuance under the ESPP to participating employees. In January 2021 and 2020, the number of shares of common stock authorized for issuance under the ESPP was increased by 1,080,711 shares and 863,254 shares, respectively. During the three months ended March 31, 2021, the Company issued 34,696 shares of common stock under the ESPP. As of March 31, 2021, 2,546,464 shares remain available for future issuance under the ESPP.