Stock-based compensation

6 Months Ended

Jun. 30, 2020

Share-based Payment Arrangement [Abstract]

In 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 June 30, 2020, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.4 million.

Stock-based compensation expense

The Company recognized stock-based compensation expense totaling $48.5 million and $84.8 million for the three and six months ended June 30, 2020, respectively. The Company recognized stock-based compensation expense totaling $55.1 million and $87.5 million for the three and six months ended June 30, 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 June 30,						For the six months ended June 30,
	2020		2019		2020		2019
Stock options	$	26,073	$	24,800	$	50,513	$	47,983
Restricted stock units	14,143		30,012		25,996		38,893
Employee stock purchase plan and other	8,313		299		8,313		576
	$	48,529	$	55,111	$	84,822	$	87,452

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 June 30,						For the six months ended June 30,
	2020		2019		2020		2019
Research and development	$	23,098	$	29,694	$	39,367	$	45,210
Selling, general and administrative	25,431		25,417		45,455		42,242
	$	48,529	$	55,111	$	84,822	$	87,452

As of June 30, 2020, the Company had approximately $332.0 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted-average period of approximately 2.5 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	1,306	$	72.50
Exercised	(27)	$	40.91
Canceled, forfeited, or expired	(296)	$	135.10
Outstanding at June 30, 2020	6,466	$	106.90
Exercisable at June 30, 2020	3,442	$	102.07
Vested and expected to vest at June 30, 2020	6,466	$	106.90

During the six months ended June 30, 2020, less than 0.1 million stock options were exercised, resulting in total proceeds to the Company of $1.1 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	879	$	72.59
Vested	(318)	$	143.73
Forfeited	(95)	$	132.84
Unvested balance at June 30, 2020	1,593	$	106.81

Employee stock purchase plan

In June 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 six months ended June 30, 2020 and 2019, less than 0.1 million shares of common stock were issued under the 2013 ESPP.