Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2017

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

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, 2017:

	2015 Plan				2005 Plan
				Weighted-			Weighted-
				Average			Average
	Stock Options		Exercise Price		Stock Options		Exercise Price
Outstanding at January 1, 2017		25,104,603	$	4.87		14,128,129	$	3.30
Granted		1,138,350	$	1.34		—	$	—
Exercised		—	$	—		(100,000)	$	1.24
Canceled		(2,015,288)	$	4.91		(913,075)	$	3.73
Outstanding at June 30, 2017		24,227,665	$	4.70		13,115,054	$	3.28
Shares exercisable at June 30, 2017		6,177,198	$	7.16		11,687,679	$	3.02
Shares available for grant at June 30, 2017		11,727,335

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 and 2005 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,
	2017		2016		2017		2016
Weighted-average Black-Scholes fair value of stock options granted		$0.81		$2.99		$1.03		$2.47
Risk-free interest rate		1.63%-1.71%		1.07%-1.09%		1.63%-2.34%		1.07%-1.70%
Dividend yield		0%		0%		0%		0%
Volatility		106.92-111.46%		58.10%-58.97%		88.91%-111.46%		57.86%-68.28%
Expected term (in years)		4.18-4.60		4.24-4.26		4.18-7.46		4.24-7.28
Expected forfeiture rate		0%		10.31%		0%		0%-16.33%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the six months ended June 30, 2017:

	Number of		Per Share Weighted-Average Grant-Date
	Shares		Fair Value
Outstanding and Unvested at January 1, 2017		45,000	$	4.99
Restricted stock granted		—	$	—
Restricted stock vested		(26,250)	$	4.99
Restricted stock forfeited		—	$	—
Outstanding and Unvested at June 30, 2017		18,750	$	4.99

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,
	2017		2016		2017		2016
Research and development	$	2,534	$	2,952	$	4,748	$	6,185
General and administrative		1,980		2,305		3,978		4,033
Total stock-based compensation expense	$	4,514	$	5,257	$	8,726	$	10,218

Employee Stock Purchase Plan [Member]

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

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,
	2017		2016		2017		2016
Range of Black-Scholes fair value of ESPP shares granted		$1.05-$5.47		$1.97-$3.88		$1.05-$5.47		$1.86-$3.88
Risk-free interest rate		0.57%-0.70%		0.32%-0.47%		0.45%-0.70%		0.22%-0.47%
Dividend yield		0%		0%		0%		0%
Volatility		54.67%-267.85%		43.03%-86.75%		45.98%-267.85%		43.03%-86.75%
Expected term (in years)		0.5-2.0		0.5-2.0		0.5-2.0		0.5-2.0
Expected forfeiture rate		0%		5%		0%		5%