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Stock-Based Compensation (Tables)
3 Months Ended
Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity

The following table summarizes stock option activity for the three months ended March 31, 2017:

 

     Number of
Options
     Weighted-
Average
Exercise
Price
     Weighted-
Average
Remaining
Contractual
Term
( Years)
     Aggregate
Intrinsic
Value
 

Outstanding — January 1, 2017

     2,829,301      $ 6.89         $ —    

Granted

     365,555      $ 2.78        

Exercised

     (136,249    $ 2.19        

Forfeited or expired

     (54,822    $ 7.69        
  

 

 

          

Outstanding — March 31, 2017

     3,003,785      $ 6.59        8.08      $ 1,496  
  

 

 

          

Exercisable — March 31, 2017

     1,437,144      $ 6.99        7.23      $ 846  
  

 

 

          

Vested and expected to vest — March 31, 2017

     2,956,125      $ 6.56        8.08      $ 1,485  
  

 

 

          
Calculation of Fair Value Assumptions Using Black Scholes Option Model

The estimated fair values of employee stock options granted during the three months ended March 31, 2017 and 2016, were determined on the date of grant using the Black-Scholes option-pricing model with the following assumptions:

 

     Three Months Ended March 31,
     2017    2016

Expected option life (years)

   5.80 – 6.71    6.22

Risk-free interest rate

   2.10% - 2.23%    1.61% -1.94%

Expected volatility

   76.6% - 79.9%    70.0%

Expected dividend yield

   0%    0%
Stock-Based Compensation Expense

The following table presents total stock-based compensation expense for the three months ended March 31, 2017:

 

     Three Months Ended
March 31,
 
     2017      2016  

Research and development

   $ 153      $ 214  

General and administrative

     462        1,019  
  

 

 

    

 

 

 

Total stock based compensation expense

   $ 615      $ 1,233