Stock-based Compensation

6 Months Ended

Jun. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

7. Stock-based Compensation

In January 2020, the Board of Directors authorized an additional 1,479,696 shares to be issued under the 2015 Equity Incentive Plan (the 2015 Plan). As of June 30, 2020, the total number of shares of common stock authorized for issuance under the 2015 Plan and the 2014 Stock Plan (the 2014 Plan) was 12,412,917, of which 2,309,229 remained available for future grants under the 2015 Plan.

Stock-based Compensation Expense

The Company’s stock-based compensation expense by award type was as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Stock options	$	8,086	$	6,844	$	15,865	$	12,296
Restricted stock units		—		69		—		136
Employee stock purchase plan		230		186		468		385
	$	8,316	$	7,099	$	16,333	$	12,817

As of June 30, 2020, the Company had $77.3 million of unrecognized stock-based compensation expense related to stock options and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.7 years.

The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive loss as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	4,284	$	3,526	$	8,331	$	5,873
General and administrative		4,032		3,573		8,002		6,944
	$	8,316	$	7,099	$	16,333	$	12,817

Stock Options

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2019		5,544		$	28.79		7.5	$	86,509
Granted		1,448		$	38.68
Exercised		(281	)	$	14.14
Cancelled or forfeited		(215	)	$	45.93
Outstanding at June 30, 2020		6,496		$	31.06		7.6	$	64,890
Exercisable at June 30, 2020		3,387		$	21.79		6.4	$	60,687
Vested and expected to vest at June 30, 2020		6,496		$	31.06		7.6	$	64,890

(a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

The weighted-average grant date fair value per share of options granted during the six months ended June 30, 2020 was $24.35. During the six months ended June 30, 2020, the total number of stock options exercised was 281,468, resulting in total proceeds of $4.0 million. The total intrinsic value of options exercised during the six months ended June 30, 2020 was $7.5 million.

Employee Stock Purchase Plan

In January 2020, the Board of Directors authorized an additional 369,924 shares to be issued under the 2015 ESPP. As of June 30, 2020, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 623,924, of which 486,068 remained available for future issuance. During the six months ended June 30, 2020, 17,442 shares of common stock were issued under the 2015 ESPP.