The following table summarizes the Company’s stock option activity since December 31, 2018:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2018		7,561,719		$	12.26	$	7.23	$	4,958
Granted		2,346,350			5.90
Exercised		(90,125	)		1.61
Forfeited		(1,559,913	)		12.69
Outstanding as of September 30, 2019		8,258,031		$	10.49	$	7.21	$	4,230
Options vested and expected to vest as of September 30, 2019		8,258,031			10.49		7.21		4,230
Options exercisable as of September 30, 2019		4,641,642		$	12.49		5.94	$	4,028

The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2019 and 2018 was $2.12, $4.24, $5.67 and $6.83 per share, respectively.

During the nine months ended September 30, 2019, the Company granted performance-based stock options to employees for the purchase of an aggregate of 1.1 million shares of common stock with a grant date fair value of $4.58 per share. These stock options are exercisable only upon achievement of specified performance targets. As of September 30, 2019, none of these options were exercisable because none of the specified performance targets had been achieved. Because achievement of the specified performance targets was not deemed probable as of September 30, 2019, the Company did not record any expense for these stock options from the dates of issuance through September 30, 2019.

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock unit activity since December 31, 2018:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2018		226,900		$	9.64
Granted		15,000			2.29
Forfeited		(22,500	)		8.77
Vested		(73,500	)		9.98
Unvested restricted stock units as of September 30, 2019		145,900		$	8.85

The Company recorded stock-based compensation expense related to stock options, restricted stock units and the Company’s Employee Stock Purchase Plan (“ESPP”) in the following expense categories of its condensed consolidated statements of operations and comprehensive loss:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development expenses	$	1,033	$	2,246	$	3,574	$	6,436
General and administrative expenses		1,004		2,071		2,625		6,451
	$	2,037	$	4,317	$	6,199	$	12,887

The ESPP provides that eligible employees may contribute up to 15% of their eligible earnings toward the semi-annual purchase of the Company's common stock. The ESPP is qualified under Section 423 of the Internal Revenue Code. The employee's purchase price is derived from a formula based on the closing price of the common stock on the first day of the offering period versus the closing price on the date of purchase (or, if not a trading day, on the immediately preceding trading day). The offering period under the ESPP has a duration of six months, and the purchase price with respect to each offering period beginning on or after such date is, until otherwise amended, equal to 85% of the lesser of (i) the fair market value of the Company's common stock at the commencement of the applicable six-month offering period or (ii) the fair market value of the Company's common stock on the purchase date. The Company recorded an immaterial amount of stock-based compensation expense under the ESPP for the three and nine months ended September 30, 2019.

The total number of available ESPP shares is increased annually, which began in 2016 and will end in 2025. The ESPP allows for share replenishment equal to the lesser of (i) 400,000 shares and (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the preceding calendar year, or an amount determined by the board of directors. As of September 30, 2019, a total of 1.8 million shares were reserved and available for issuance under the ESPP.