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, 2019, 1,341,436 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 September 30, 2019 was 2,809,289 which includes 505,046 shares of the Company’s common stock reserved under the Company’s 2015 Stock Option and Grant Plan (the “2015 Plan”) that became available for issuance upon the effectiveness of the 2018 Plan. No future issuance will be made under the 2015 Plan.

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 and nine months ended September 30, 2019 and 2018 was as follows (in thousands):


	Three months ended				Nine months ended
	September 30,				September 30,
	2019		2018		2019		2018
Research and development	$	816	$	242	$	2,181	$	616
General and administrative		1,825		478		4,785		1,368
Total stock‑based compensation expense	$	2,641	$	720	$	6,966	$	1,984

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, 2018	3,959	$	7.46	8.79	$	35,984
Granted	1,663		12.77
Exercised	(142)		1.99
Forfeited and canceled	(522)		11.91
Outstanding, September 30, 2019	4,958	$	8.93	8.45	$	1,356
Vested and expected to vest at September 30, 2019	4,958	$	8.93	8.45	$	1,356
Exercisable at September 30, 2019	1,877	$	5.37	7.90	$	968

During the nine months ended September 30, 2019 and 2018, the Company granted 1.7 million and 2.4 million stock options, respectively and these options had a weighted-average grant-date fair value of $8.36 and $9.95 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 and nine months ended September 30, 2019 were similar to those as described in the Annual Report. As of September 30, 2019, there was $20.4 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 2.90 years. The options have a ten-year life and generally vest over a period of four years, subject to continuous employment.

Restricted stock awards

Non-cash restricted stock award expense recognized in the accompanying condensed statements of operations was $0.1 million for each of the three months ended September 30, 2019 and 2018 and $0.3 million for each of the nine months ended September 30, 2019 and 2018. The total fair value of shares that vested in the nine months ended September 30, 2019 was $0.2 million. At September 30, 2019, there was less than $0.1 million of unrecognized compensation cost related to 3,632 unvested restricted stock awards that will be recognized as expense over a weighted-average period of less than 0.01 years.

Restricted stock units

In May 2019, the Company issued an aggregate of 1,183,400 shares of 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. There are 2,166 shares of common stock underlying restricted stock units outstanding as of September 30, 2019.

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, 2018	-	$	-
Issued	1,183	$	3.63
Vested	(2)		3.63
Forfeited and canceled	(26)		3.63
Unvested as of September 30, 2019	1,155	$	3.63

Non-cash restricted stock unit award expense recognized in the accompanying condensed statements of operations was $0.6 million and $0.8 million for the three and nine months ended September 30, 2019, respectively. At September 30, 2019, there was $3.4 million of unrecognized compensation related to 1,154,934 unvested restricted stock units that will be recognized as expense over a weighted-average period of 1.63 years.