7. Stock incentive plans

On June 5, 2018, the Company’s stockholders approved the 2018 Stock Option and Incentive Plan (the “2018 Plan”), which became effective on June 20, 2018. The 2018 Plan provides for an annual increase, to be added on the first day of each fiscal year, of up to 4% of the Company’s outstanding shares of common stock as of the last day of the prior year. On January 1, 2020, 1,349,165 shares of common stock were added to the 2018 Plan. The number of shares available for grant under the Company’s 2018 Plan as of March 31, 2020 was 2,758,037.

Stock‑based compensation expense

Non-cash stock-based compensation expense recognized in the accompanying condensed statements of operations relating to both stock options, restricted stock awards, and restricted stock units for the three months ended March 31, 2020 and 2019 was as follows (in thousands):


	Three months ended
	March 31,
	2020		2019
Research and development	$	456	$	421
General and administrative		1,875		1,579
Total stock‑based compensation expense	$	2,331	$	2,000

Stock options

The table below summarizes activity related to stock options (in thousands, except per share amounts):


				Weighted‑
		Weighted‑		average
		average		remaining	Aggregate
		exercise		contractual	intrinsic
Options	Shares	price		term	value
Outstanding, December 31, 2019	4,798	$	8.75	8.29	$	1,159
Granted	1,939		3.09
Exercised	(104)		2.34
Forfeited and canceled	(214)		6.12
Outstanding, March 31, 2020	6,419	$	7.23	8.48	$	102
Vested and expected to vest at March 31, 2020	6,419	$	7.23	8.48	$	102
Exercisable at March 31, 2020	2,439	$	6.96	7.45	$	100

During the three months ended March 31, 2020 and 2019, the Company granted 1.9 million and 1.0 million stock options, respectively and these options had a weighted-average grant-date fair value of $2.04 and $12.60 per share, respectively. The weighted-average grant-date fair value of options was determined using the Black-Scholes option-pricing model. The assumptions used in the Black-Scholes option-pricing model for options granted during the three months ended March 31, 2020 were similar to those as described in the Annual Report. As of March 31, 2020, there was $19.0 million of total unrecognized stock-based compensation expense related to non-vested stock options which is expected to be recognized over a weighted-average period of 2.75 years. The options have a ten-year life and generally vest over a period of four years, subject to continuous employment.

Restricted stock awards

In March 2020, the Company issued 4,976 restricted stock awards to a board member that vested immediately on the date of grant. Non-cash restricted stock award expense recognized in the accompanying condensed statements of operations was less than $0.1 million for each of the three months ended March 31, 2020 and 2019, respectively. The total fair value of shares that vested in the three months ended March 31, 2020 was less than $0.1 million. At March 31, 2020, there are no unvested restricted stock awards.

Restricted stock units

In May 2019, the Company issued an aggregate of 1,183,400 restricted stock units to employees. The restricted stock units vest in two years from the date of grant. The Company at any time may accelerate the vesting of the restricted stock units. Such shares are not accounted for as outstanding until they vest.

The table below summarizes activity related to restricted stock units (in thousands, except per share amounts):


		Weighted‑
		average
		grant date
		fair value
	Shares	per share
Unvested as of December 31, 2019	1,032	$	3.63
Issued	—	$	—
Vested	(6)		3.63
Forfeited and canceled	(59)		3.63
Unvested as of March 31, 2020	967	$	3.63

Non-cash restricted stock unit award expense recognized in the accompanying condensed statements of operations was $0.4 million and $0.0 million for the three months ended March 31, 2020 and 2019, respectively. At March 31, 2020, there was $2.0 million of unrecognized compensation related to 966,600 unvested restricted stock units that will be recognized as expense over a weighted-average period of 1.13 years.