Stock-based compensation

3 Months Ended

Mar. 31, 2020

Share-based Payment Arrangement [Abstract]

Stock-based compensation

Stock-based compensationIn January 2020 and 2019, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.2 million and 2.2 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of March 31, 2020, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.5 million.

Stock-based compensation expense

The Company recognized stock-based compensation expense totaling $36.3 million and $32.3 million for the three months ended March 31, 2020 and 2019, respectively. Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	For the three months ended March 31,
	2020	2019
Stock options	24,440	23,183
Restricted stock units	11,853	8,881
Employee stock purchase plan	—	277
	36,293	32,341

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

	For the three months ended March 31
	2020	2019
Research and development	16,269	15,516
Selling, general and administrative	20,024	16,825
	36,293	32,341

As of March 31, 2020, the Company had $361.8 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units, and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.7 years.

Stock option activity

The following table summarizes the stock option activity under the Company’s equity award plans:

	Shares (in thousands)	Weighted- average exercise price per share
Outstanding at December 31, 2019	5,483	$	116.30
Granted	1,147	$	75.65
Exercised	(20)	$	37.71
Canceled, forfeited, or expired	(132)	$	148.55
Outstanding at March 31, 2020	6,478	$	108.68
Exercisable at March 31, 2020	3,248	$	99.92
Vested and expected to vest at March 31, 2020	6,478	$	108.68

During the three months ended March 31, 2020, less than 0.1 million shares of common stock were exercised, resulting in total proceeds to the Company of $0.8 million.

Restricted stock unit activity

The following table summarizes the restricted stock unit activity under the Company’s equity award plans:

	Shares (in thousands)	Weighted- average grant date fair value
Unvested balance at December 31, 2019	1,127	$	146.10
Granted	709	$	77.69
Vested	(204)	$	132.15
Forfeited	(47)	$	150.73
Unvested balance at March 31, 2020	1,585	$	117.15

Employee stock purchase plan

On June 3, 2013, the Company adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which authorized the initial issuance of up to a total of 0.2 million shares of the Company’s common stock to participating employees. During each of the three months ended March 31, 2020 and 2019, less than 0.1 million shares of common stock were issued under the 2013 ESPP.