Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018:

	2015 Plan					2005 Plan
	Stock Options			Weighted- Average Exercise Price		Stock Options			Weighted- Average Exercise Price
Outstanding at January 1, 2018		33,675,720		$	3.61		12,818,929		$	3.26
Granted		16,613,409		$	2.24		—		$	—
Exercised		(252,728	)	$	1.35		(563,125	)	$	1.73
Canceled		(2,180,163	)	$	4.18		(602,432	)	$	4.15
Outstanding at December 31, 2018		47,856,238		$	3.12		11,653,372		$	3.29
Shares exercisable at December 31, 2018		15,815,855		$	4.65		11,653,372		$	3.29
Shares available for grant at December 31, 2018		7,821,034

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:

	2018	2017	2016
Weighted average fair value of options granted	$1.74	$1.06	$1.88
Risk-free interest rate	2.26%-3.10%	1.61%-2.34%	0.97%-1.78%
Dividend yield	0%	0%	0%
Volatility	93.31%-115.61%	88.91%-114.10%	57.86%-108.88%
Expected term (in years)	4.07-7.50	4.14-7.46	4.22-7.28
Expected forfeiture rate(1)	N/A	N/A	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, 2018:

	Number of Shares			Per Share Weighted- Average Grant-Date Fair Value
Outstanding and Unvested at January 1, 2018		18,750		$	4.99
Restricted stock granted		―		$	―
Restricted stock vested		―		$	―
Restricted stock forfeited		(18,750	)	$	4.99
Outstanding and Unvested at December 31, 2018		―		$	―

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,
	2018		2017		2016
Research and development	$	10,575	$	11,750	$	11,168
General and administrative		7,739		8,059		7,992
Total stock-based compensation expense	$	18,314	$	19,809	$	19,160

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:

	2018	2017	2016
Range of Black-Scholes fair values of ESPP shares granted	$0.36-$3.53	$0.45-$5.47	$1.86-$4.76
Risk-free interest rate	0.66%-2.24%	0.45%-1.13%	0.22%-0.61%
Dividend yield	0%	0%	0%
Volatility	52.19%-203.83%	45.98%-267.85%	43.03%-86.75%
Expected term (in years)	0.5-2.0	0.5-2.0	0.5-2.0
Expected forfeiture rate(1)	N/A	N/A	5%

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