Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 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 the 2005 Plan for the year ended December 31, 2017:

	2015 Plan				2005 Plan
	Stock Options		Weighted- Average Exercise Price		Stock Options		Weighted- Average Exercise Price
Outstanding at January 1, 2017		25,104,603	$	4.87		14,128,129	$	3.30
Granted		12,411,543	$	1.37		—	$	—
Exercised		—	$	—		(115,000)	$	1.25
Canceled		(3,840,426)	$	4.65		(1,194,200)	$	3.92
Outstanding at December 31, 2017		33,675,720	$	3.61		12,818,929	$	3.26
Shares exercisable at December 31, 2017		8,550,717	$	5.87		11,659,554	$	3.04
Shares available for grant at December 31, 2017		2,279,280

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 or the date upon which the 2015 Plan was approved by the Company’s stockholders for stock options granted prior to that time using the Black-Scholes option-pricing model with the following assumptions:

	2017		2016		2015
Weighted average fair value of options granted		$1.06		$1.88		$4.38
Risk-free interest rate		1.61%-2.34%		0.97%-1.78%		1.19%-2.13%
Dividend yield		0%		0%		0%
Volatility		88.91%-114.10%		57.86%-108.88%		53.58%-68.39%
Expected term (in years)		4.14-7.46		4.22-7.28		3.98-7.34
Expected forfeiture rate(1)		N/A		0%-16.33%		0%-16.33%

(1) See Note 3 regarding the Company’s adoption of ASU 2016-09 in 2017.

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the year ended December 31, 2017:

	Number of Shares		Per Share Weighted- Average Grant-Date 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 December 31, 2017		18,750	$	4.99

Stock-Based Compensation Expense

The Company recorded stock-based compensation expense for awards issued under the above mentioned plans in the consolidated statements of operations as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Research and development	$	11,750	$	11,168	$	6,771
General and administrative		8,059		7,992		6,660
Total stock-based compensation expense	$	19,809	$	19,160	$	13,431

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:

	2017		2016		2015
Range of Black-Scholes fair values of ESPP shares granted		$0.45-$5.47		$1.86-$4.76		$1.06-$3.38
Risk-free interest rate		0.45%-1.13%		0.22%-0.61%		0.05%-0.35%
Dividend yield		0%		0%		0%
Volatility		45.98%-267.85%		43.03%-86.75%		40.79%-64.24%
Expected term (in years)		0.5-2.0		0.5-2.0		0.5-2.0
Expected forfeiture rate(1)		N/A		5%		5%

 

(1) See Note 3 regarding the Company’s adoption of ASU 2016-09 in 2017.