Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 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 2005 Plan for the six months ended June 30, 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		497,100		$	1.80		—		$	—
Exercised		(176,137	)	$	1.35		(331,020	)	$	1.81
Canceled		(1,268,635	)	$	4.00		(378,307	)	$	4.06
Outstanding at June 30, 2018		32,728,048		$	3.58		12,109,602		$	3.27
Shares exercisable at June 30, 2018		11,421,018		$	5.68		11,988,227		$	3.25
Shares available for grant at June 30, 2018		23,025,815

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 June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Weighted-average Black- Scholes fair value of stock options granted		$1.33		$0.81		$1.38		$1.03
Risk-free interest rate		2.59%-2.74%		1.63%-1.71%		2.26%-2.74%		1.63%-2.34%
Dividend yield		0%		0%		0%		0%
Volatility		113.68%-114.24%		106.92%-111.46%		113.68%-114.24%		88.91%-111.46%
Expected term (in years)		4.10-4.11		4.18-4.60		4.10-4.14		4.18-7.46
Expected forfeiture rate		0%		0%		0%		0%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the six months ended June 30, 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 June 30, 2018		―		$	―

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 June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development	$	2,489	$	2,534	$	5,587	$	4,748
General and administrative		1,762		1,980		3,909		3,978
Total stock-based compensation expense	$	4,251	$	4,514	$	9,496	$	8,726

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 June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Range of Black-Scholes fair value of ESPP shares granted		$0.66-$3.53		$1.05-$5.47		$0.45-$3.53		$1.05-$5.47
Risk-free interest rate		0.74%-1.79%		0.57%-0.70%		0.66%-1.79%		0.45%-0.70%
Dividend yield		0%		0%		0%		0%
Volatility		59.84%-203.83%		54.67%-267.85%		59.84%-203.83%		45.98%-267.85%
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%