Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

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 three months ended March 31, 2019:

	2015 Plan					2005 Plan
	Stock Options			Weighted- Average Exercise Price		Stock Options			Weighted- Average Exercise Price
Outstanding at January 1, 2019		47,856,238		$	3.12		11,653,372		$	3.29
Granted		196,060		$	1.78		—		$	—
Exercised		(30,267	)	$	1.37		(30,000	)	$	0.56
Canceled		(915,624	)	$	4.81		(433,400	)	$	4.63
Outstanding at March 31, 2019		47,106,407		$	3.08		11,189,972		$	3.24
Shares exercisable at March 31, 2019		17,762,576		$	4.70		11,189,972		$	3.24
Shares available for grant at March 31, 2019		5,576,007

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 March 31,
	2019		2018
Weighted-average Black-Scholes fair value of stock options granted	$	1.38	$	1.50
Risk-free interest rate		2.43%-2.55%		2.26%-2.54%
Dividend yield		0%		0%
Volatility		111.65%-126.76%		113.98%-114.12%
Expected term (in years)		4.06-4.50		4.12-4.14
Expected forfeiture rate		0%		0%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock units activity for the three months ended March 31, 2019:

	Number of Shares		Per Share Weighted- Average Grant-Date Fair Value
Outstanding and Unvested at January 1, 2019		―	$	―
Restricted stock units granted		2,964,591	$	0.52
Restricted stock units vested		―	$	―
Restricted stock units forfeited		―	$	―
Outstanding and Unvested at March 31, 2019		2,964,591	$	0.52

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 March 31,
	2019		2018
Research and development	$	3,179	$	3,098
General and administrative		2,379		2,147
Total stock-based compensation expense	$	5,558	$	5,245

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 March 31,
	2019		2018
Range of Black-Scholes fair value of ESPP shares granted		$0.36-$1.74		$0.45-$3.53
Risk-free interest rate		1.20%-2.50%		0.66%-1.79%
Dividend yield		0%		0%
Volatility		52.19%-171.60%		59.84%-203.83%
Expected term (in years)		0.5-2.0		0.5-2.0
Expected forfeiture rate		0%		0%