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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2016
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 nine months ended September 30, 2016:
 
 
 
2015 Plan
 
2005 Plan
 
 
 
Stock Options
 
Weighted-Average 
Exercise Price
 
Stock Options
 
Weighted-Average
Exercise Price
 
Outstanding at January 1, 2016
 
 
8,357,003
 
$
8.97
 
 
15,450,542
 
$
3.31
 
Granted
 
 
10,384,437
 
$
5.10
 
 
—
 
$
—
 
Exercised
 
 
—
 
$
—
 
 
(694,163)
 
$
2.15
 
Canceled
 
 
(598,687)
 
$
7.29
 
 
(247,375)
 
$
4.75
 
Outstanding at September 30, 2016
 
 
18,142,753
 
$
6.81
 
 
14,509,004
 
$
3.34
 
Shares exercisable at
September 30, 2016
 
 
2,297,355
 
$
8.56
 
 
10,559,379
 
$
2.83
 
Shares available for grant at September 30, 2016
 
 
12,812,247
 
 
 
 
 
 
 
 
 
 
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
September 30,
 
Nine Months Ended
September 30,
 
 
 
2016
 
2015
 
2016
 
2015
 
Weighted-average Black- Scholes fair value of stock options granted
 
 
$3.34
 
 
$4.70
 
 
$2.50
 
 
$4.43
 
Risk-free interest rate
 
 
0.97%-1.09%
 
 
1.32%-1.39%
 
 
0.97%-1.70%
 
 
1.19%-2.13%
 
Dividend yield
 
 
0%
 
 
0%
 
 
0%
 
 
0%
 
Volatility
 
 
58.58%-59.02%
 
 
54.93%-57.17%
 
 
57.86%-68.28%
 
 
53.58%-68.39%
 
Expected term (in years)
 
 
4.24
 
 
4.27-4.60
 
 
4.24-7.28
 
 
3.98-7.34
 
Expected forfeiture rate
 
 
10.31%
 
 
14.18%-16.33%
 
 
0%-16.33%
 
 
0%-16.33%
 
Summary of Restricted Stock Awards Activity
The following is a summary of restricted stock awards activity for the nine months ended September 30, 2016:
 
 
 
Number of
Shares
 
Per Share Weighted-Average Grant-Date
Fair Value
 
Outstanding and Unvested at January 1, 2016
 
 
25,000
 
$
8.72
 
Restricted stock granted
 
 
45,000
 
$
4.99
 
Restricted stock vested
 
 
—
 
$
—
 
Restricted stock forfeited
 
 
(25,000)
 
$
8.72
 
Outstanding and Unvested at September 30, 2016
 
 
45,000
 
$
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 
September 30,
 
Nine Months Ended 
September 30,
 
 
 
2016
 
2015
 
2016
 
2015
 
Research and development
 
$
2,917
 
$
2,240
 
$
9,108
 
$
4,361
 
General and administrative
 
 
2,239
 
 
2,525
 
 
6,272
 
 
4,917
 
Total stock-based compensation expense
 
$
5,156
 
$
4,765
 
$
15,380
 
$
9,278
 
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 September 30,
 
Nine Months Ended September 30,
 
 
 
2016
 
2015
 
2016
 
2015
 
Range of Black-Scholes fair value of ESPP shares granted
 
 
$1.97-$4.76
 
 
$1.20-$3.38
 
 
$1.86-$4.76
 
 
$1.06-$3.38
 
Risk-free interest rate
 
 
0.32%-0.61%
 
 
0.07%-0.35%
 
 
0.22%-0.61%
 
 
0.05%-0.35%
 
Dividend yield
 
 
0%
 
 
0%
 
 
0%
 
 
0%
 
Volatility
 
 
43.03%-86.75%
 
 
40.79%-64.24%
 
 
43.03%-86.75%
 
 
40.79%-64.24%
 
Expected term (in years)
 
 
0.5-2.0
 
 
0.5-2.0
 
 
0.5-2.0
 
 
0.5-2.0
 
Expected forfeiture rate
 
 
5%
 
 
5%
 
 
5%
 
 
5%