Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

Schedule of summary of option activity

The following is a summary of stock options and stock appreciation rights activity under the 2015 Plan and 2005 Plan for the six months ended June 30, 2020:

	2015 Plan			2005 Plan
		Weighted-			Weighted-
		Average			Average
	Stock Options	Exercise Price		Stock Options	Exercise Price
Outstanding at January 1, 2020	3,388,701	$	35.64	501,780	$	64.19
Granted	2,820,041	$	22.28	—	$	—
Exercised	(209,959)	$	30.35	(76,856)	$	33.56
Canceled	(32,254)	$	41.18	(22,329)	$	52.26
Outstanding at June 30, 2020	5,966,529	$	29.50	402,595	$	70.71
Shares exercisable at June 30, 2020	953,073	$	85.48	402,595	$	70.71
Shares available for grant at June 30, 2020	3,098,569

Schedule of assumptions used to estimate grant date fair value of stock options and stock appreciation rights granted using black-scholes option-pricing model

The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Range of Black-Scholes fair value of ESPP shares granted		$2.87-$21.80		$9.45-$34.78		$2.57-$35.00		$7.25-$34.78
Risk-free interest rate		1.5%-2.6%		1.3%-2.5%		1.5%-2.6%		1.2%-2.5%
Dividend yield		0%		0%		0%		0%
Volatility		66.6%-150.9%		59.7%-171.6%		66.6%-154.4%		52.2%-171.6%
Expected term (in years)		0.5-2.0		0.5-2.0		0.5-2.0		0.5-2.0
Expected forfeiture rate		0%		0%		0%		0%

Schedule of summary of restricted stock units activity

The following is a summary of restricted stock units activity for the six months ended June 30, 2020:

		Per Share
		Weighted-
		Average
	Number of	Grant-Date
	Shares	Fair Value
Outstanding and Unvested at January 1, 2020	1,102,311	$	5.95
Restricted stock units granted	457,204	$	75.97
Restricted stock units vested	(47,563)	$	18.61
Restricted stock units forfeited	(29,425)	$	25.46
Outstanding and Unvested at June 30, 2020	1,482,527	$	26.75

Schedule of stock-based compensation expense

The Company recorded all stock-based compensation expense in the consolidated statements of operations as follows (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Research and development	$	4,098	$	2,444	$	6,005	$	5,624
General and administrative		3,834		2,176		5,892		4,555
Total stock-based compensation expense	$	7,932	$	4,620	$	11,897	$	10,179

Stock options

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

Schedule of assumptions used to estimate grant date fair value of stock options and stock appreciation rights granted using black-scholes option-pricing model

The fair value of stock options granted under the 2015 Plan was estimated at the date of grant or the date upon which the 2015 Plan was approved by the Company’s stockholders for stock options discussed above using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Weighted-average Black-Scholes fair value of stock options granted		$77.41		$8.50		$76.99		$23.43
Risk-free interest rate		0.3%-0.6%		1.8%-2.3%		0.3%-1.5%		1.8%-2.6%
Dividend yield		0%		0%		0%		0%
Volatility		116.0%-151.5%		127.3%-128.5%		116.0%-151.5%		111.6%-128.5%
Expected term (in years)		3.9-7.6		4.0		3.9-7.6		4.0-4.5
Expected forfeiture rate		0%		0%		0%		0%