The Company’s 2008 Stock Incentive Plan (the “2008 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors, consultants and advisors of the Company. The 2008 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The board of directors could also delegate to one or more officers of the Company the power to grant awards to employees and certain officers of the Company. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2008 Plan with service-based vesting conditions generally vest over four years and expire after ten years.

The total number of shares of common stock that were authorized for issuance under the 2008 Plan was 4,039,829 shares. Upon effectiveness of the Company’s 2018 Equity Incentive Plan, the (“2018 Plan”) in July 2018, the remaining 245,557 shares available under the 2008 Plan became available for issuance under the 2018 Plan and no future issuance will be made under the 2008 Plan. Additionally, outstanding options under the 2008 Plan that expired, terminated, are surrendered or canceled without having been fully exercised will be available for future awards under the 2018 Plan.

The exercise price for stock options granted is not less than the fair value of common shares as determined by the board of directors as of the date of grant. The Company’s board of directors valued the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant.

In June 2018, the Company’s stockholders approved the 2018 Plan, which became effective on July 18, 2018. The 2018 Plan provides for the grant of incentive stock options, non-qualified options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2018 Plan is 2,779,544 plus the 245,557 shares of common stock remaining available for issuance under the 2008 Plan as of that date. The number of shares reserved shall be annually increased on January 1, 2019 and each January 1 thereafter through January 1, 2028 by the least of (i) 2,216,368 shares, (ii) 4% of the number of shares of the Company’s common stock outstanding on the first day of the year or (iii) an amount determined by the Company’s board of directors. The shares of common stock underlying any awards that are expired, forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2018 Plan or the 2008 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. In January 2019, the shares available for issuance under the 2018 Plan were increased by 1,032,125 shares pursuant to the annual increase described above. As of September 30, 2019, 1,738,430 shares remained available for future issuance under the 2018 Plan.

In June 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan which became effective on July 18, 2018. A total of 272,504 shares of common stock were reserved for issuance under this plan. The number of shares reserved shall be annually increased on January 1, 2020 and each January 1 thereafter through January 1, 2028 by the least of (i) 545,008 shares, (ii) 1% of the number of shares of the Company’s common stock outstanding on the first day of the year or (iii) an amount determined by the Company’s board of directors.

The following is a summary of stock option activity for the three months ended September 30, 2019,

				Weighted		Weighted
				Average		Average		Aggregate
	Number			Exercise		Contractual		Intrinsic
	of Shares			Price		Term		Value
						(in years)		(in thousands)
Outstanding as of December 31, 2018		3,779,403		$	7.78		8.80	$	110
Granted		1,397,699			9.06
Exercised		(23,906	)		5.54
Forfeited		(280,099	)		9.15
Outstanding as of September 30, 2019		4,873,097		$	8.07		8.44	$	1,575
Vested and expected to vest as of September 30, 2019		4,873,097		$	8.07		8.44	$	1,575
Options exercisable as of September 30, 2019		1,714,765		$	7.08		7.68	$	1,043

During the nine months ended September 30, 2019, the Company granted options to employees and directors for the purchase of 1,397,699 shares of common stock with a weighted average exercise price of $9.06 per share and a weighted average grant-date fair value of $6.42 per share.

The Company estimated the fair value of each stock option award using the Black-Scholes option-pricing model based on the following assumptions:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2019			2018			2019			2018
Risk-free interest rate		1.58	%		2.76	%		2.36	%		2.77	%
Expected volatility		82.22	%		81.14	%		82.24	%		80.96	%
Expected dividend yield		—			—			—			—
Expected term (in years)		6.10			6.05			6.03			6.05

As of September 30, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $18.0 million, which is expected to be recognized over a weighted average period of 2.75 years.

The Company recorded stock-based compensation expense in the following expense categories of its statements of operations and comprehensive loss (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development expenses	$	744	$	566	$	2,000	$	1,068
General and administrative expenses		1,204		678		3,058		1,448
Total	$	1,948	$	1,244	$	5,058	$	2,516