Stock-based compensation

3 Months Ended

Mar. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based compensation

9. Stock-based compensation

In January 2017, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 1.6 million shares as a result of the automatic increase provision of the 2013 Plan. As of March 31, 2017, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.0 million.

Stock-based compensation expense

Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	Three Months Ended March 31,
	2017		2016
Stock options	$	9,342	$	8,926
Restricted stock units		1,956		1,127
Employee stock purchase plan		183		90
	$	11,481	$	10,143

As of March 31, 2017, the Company had $121.7 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.8years.

Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): 

	Three Months Ended March 31,
	2017		2016
Research and development	$	5,654	$	4,681
General and administrative		5,827		5,462
	$	11,481	$	10,143

Stock options

The following table summarizes the stock option activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average exercise price per share
Outstanding at December 31, 2016		3,735		$	52.17
Granted		696		$	76.37
Exercised		(181	)	$	15.18
Canceled or forfeited		(47	)	$	88.88
Outstanding at March 31, 2017		4,203		$	57.35
Exercisable at March 31, 2017		2,070		$	42.63
Vested and expected to vest at March 31, 2017		4,203		$	57.35

During the three months ended March 31, 2017, 0.2 million shares of common stock were exercised, resulting in total proceeds to the Company of $2.7 million. In accordance with the Company’s equity award plans, the shares were issued from a pool of shares reserved for issuance under the equity award plans.

Restricted stock units

The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average grant date fair value
Unvested balance at December 31, 2016		263		$	63.07
Granted		205		$	78.88
Vested		(39	)	$	50.08
Forfeited		(3	)	$	57.10
Unvested balance at March 31, 2017		426		$	71.92

Employee stock purchase plan

On June 3, 2013, the Company’s board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO on June 24, 2013. The 2013 ESPP authorizes the initial issuance of up to a total of 238,000 shares of the Company’s common stock to participating employees. The first offering period under the 2013 ESPP opened on August 1, 2014. During the three months ended March 31, 2017 and 2016, 11,079shares and 9,758 shares of common stock were issued under the 2013 ESPP, respectively.