Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

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

Schedule of summary of option activity

The following is a summary of option activity under the 2015 Plan and 2005 Plan for the six months ended June 30, 2019:

	2015 Plan			2005 Plan
		Weighted-			Weighted-
		Average			Average
	Stock Options	Exercise Price		Stock Options	Exercise Price
Outstanding at January 1, 2019	2,392,567	$	62.41	582,616	$	65.72
Granted	12,602	$	30.02	—	$	—
Exercised	(1,514)	$	27.42	(1,500)	$	11.20
Canceled	(169,647)	$	59.70	(24,376)	$	90.94
Outstanding at June 30, 2019	2,234,008	$	62.46	556,740	$	64.77
Shares exercisable at June 30, 2019	945,971	$	91.02	556,740	$	64.77
Shares available for grant at June 30, 2019	1,402,965

Schedule of assumptions used to estimate grant date fair value of stock options 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 using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Weighted-average Black-Scholes fair value of stock options granted		$8.50		$26.61		$23.43		$27.55
Risk-free interest rate		1.81% - 2.33%		2.59% - 2.74%		1.81% - 2.55%		2.26% - 2.74%
Dividend yield		0%		0%		0%		0%
Volatility		127.32% - 128.49%		113.68% - 114.24%		111.65% - 128.49%		113.98% - 114.12%
Expected term (in years)		4.04 - 4.05		4.10 - 4.11		4.04 - 4.50		4.10 - 4.14
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, 2019:

		Per Share
		Weighted-
		Average
	Number of	Grant-Date
	Shares	Fair Value
Outstanding and Unvested at January 1, 2019	—	$	—
Restricted stock units granted	148,211	$	10.40
Restricted stock units vested	—	$	—
Restricted stock units forfeited	(3,112)	$	10.40
Outstanding and Unvested at June 30, 2019	145,099	$	10.40

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,
	2019		2018		2019		2018
Research and development	$	2,444	$	2,489	$	5,624	$	5,588
General and administrative		2,176		1,762		4,555		3,909
Total stock-based compensation expense	$	4,620	$	4,251	$	10,179	$	9,497

Employee Stock Option [Member]

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

Schedule of assumptions used to estimate grant date fair value of stock options 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,
	2019		2018		2019		2018
Range of Black-Scholes fair value of ESPP shares granted		$9.45- $34.78		$13.21 - $70.64		$7.25 - $34.78		$8.99 - $70.64
Risk-free interest rate		1.34% - 2.50%		0.74% - 1.79%		1.20% - 2.50%		0.66% - 1.79%
Dividend yield		0%		0%		0%		0%
Volatility		59.71% - 171.60%		59.84% - 203.83%		52.19% - 171.60%		59.84% - 203.83%
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%