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Stock-Based Compensation Expense (Tables)
 12 Months Ended
Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Summary of Stock Option Activity

The following table summarizes stock option activity:

 

 

 

Number of

Options

 

 

Weighted-

Average

Exercise

Price Per Share

 

 

Weighted-

Average

Remaining

Contractual

Life

(in Years)

 

 

Weighted-

Average

Grant

Date Fair

Value

 

Outstanding as of December 31, 2012

 

 

893,039

 

 

$

3.29

 

 

 

8.11

 

 

 

 

 

Granted

 

 

170,269

 

 

$

3.51

 

 

 

 

 

 

$

1.37

 

Exercised

 

 

(20,161

)

 

$

3.14

 

 

 

 

 

 

 

 

 

Canceled

 

 

(288,073

)

 

$

3.40

 

 

 

 

 

 

 

 

 

Outstanding as of December 31, 2013

 

 

755,074

 

 

$

3.30

 

 

 

7.62

 

 

 

 

 

Granted

 

 

865,516

 

 

$

4.94

 

 

 

 

 

 

$

4.02

 

Exercised

 

 

—

 

 

$

—

 

 

 

 

 

 

 

 

 

Canceled

 

 

(509,846

)

 

$

3.27

 

 

 

 

 

 

 

 

 

Outstanding as of December 31, 2014

 

 

1,110,744

 

 

$

4.59

 

 

 

8.72

 

 

 

 

 

Granted

 

 

766,882

 

 

$

15.04

 

 

 

 

 

 

$

11.60

 

Options assumed in the Merger

 

 

551,363

 

 

$

30.11

 

 

 

 

 

 

 

 

 

Exercised

 

 

(38,570

)

 

$

4.20

 

 

 

 

 

 

 

 

 

Canceled

 

 

(20,572

)

 

$

28.09

 

 

 

 

 

 

 

 

 

Outstanding as of December 31, 2015

 

 

2,369,847

 

 

$

13.99

 

 

 

8.28

 

 

 

 

 

Vested and expected to vest as of December 31, 2015

 

 

2,309,228

 

 

$

14.05

 

 

 

8.26

 

 

 

 

 

Vested and exercisable as of December 31, 2015

 

 

1,155,771

 

 

$

17.11

 

 

 

7.52

 

 

 

 

 

 
Summary of Intrinsic Values Related to Options

The following table presents intrinsic values related to options as of December 31, 2015 (in thousands):

 

 

 

As of

December 31,

2015

 

Intrinsic value of options outstanding

 

$

6,314

 

Intrinsic value of vested and expected to vest

 

 

6,191

 

Intrinsic value of vested and exercisable

 

 

3,840

 

 
Summary of Stock-Based Compensation Expense

Stock-based compensation expense related to options granted was recorded as follows (in thousands):

 

 

 

Year Ended December 31,

 

 

 

2015

 

 

2014

 

 

2013

 

Research and development

 

$

624

 

 

$

149

 

 

$

188

 

General and administrative

 

 

1,629

 

 

 

564

 

 

 

235

 

Total

 

$

2,253

 

 

$

713

 

 

$

423

 

 
Summary of Assumptions Used to Value Stock Options on Date of Grant

The following table presents the assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options granted in the periods presented:

 

 

 

Year Ended December 31,

 

 

2015

 

2014

 

2013

Risk-free interest rate

 

1.56% — 1.93%

 

1.83% — 2.29%

 

1.10% — 1.68%

Expected price volatility

 

90.00% — 95.80%

 

94.50% — 105.40%

 

95.75% — 96.13%

Expected term (in years)

 

6.2

 

7.0

 

0.9 — 10.0

Expected dividend yield

 

— %

 

— %

 

— %