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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Stock-based Compensation Expense for Employees and Non-employees

In the accompanying condensed consolidated statements of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows:

 

     Three Months Ended
June 30,		      Six Months Ended
June 30,		  
     2017      2016      2017      2016  
     (in thousands)  

Research and development

    $ 1,008      $ 807      $ 2,021      $ 1,761  

General and administrative

      503        496        1,017        905  
  

 

 

     

 

 

     

 

 

     

 

 

  

Total stock-based compensation

    $ 1,511      $ 1,303      $ 3,038      $ 2,666  
  

 

 

     

 

 

     

 

 

     

 

 

  
Schedule of Estimated Grant-date Fair Values of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions

The estimated grant-date fair values of all of the Company’s stock-based awards were calculated using the Black-Scholes option pricing model, based on assumptions as follows:

 

     Three Months Ended
June 30,		     Six Months Ended
June 30,		  
     2017     2016     2017     2016  

Expected term (in years)

      5.3 – 6.1       5.3 – 7.0       5.3 – 7.0       5.3 – 9.9  

Expected volatility

      86 – 87 %      77 – 85 %      86 – 96 %      77 – 85 % 

Risk-free interest rate

      1.8 – 1.9 %      1.1 – 1.7 %      1.8 – 2.3 %      1.1 – 1.9 % 

Expected dividend rate

      — %      — %      — %      — % 
Summary of Stock-Based Compensation Activity

A summary of activity under the 2008 Plan and 2015 Plan and related information is as follows:

 

           Options Outstanding  
     Shares
Available
for Grant		     Number
of Shares
Outstanding		     Weighted-
Average
Exercise
Price Per
Share		      Weighted-
Average
Remaining
Contractual
Term
(Years)		      Aggregate
Intrinsic
Value of
Outstanding
Options
(in thousands)		  

Outstanding — December 31, 2016

      1,392,521       6,543,654     $ 3.44        8.82      $ 888  

Awards authorized

      1,214,837            

Options granted

      (1,393,775 )      1,393,775       1.43        

Options exercised

      —         (49,861 )      0.46        

Options forfeited/cancelled

      171,525       (171,525 )      4.39        
  

 

 

    

 

 

          

Outstanding — June 30, 2017

      1,385,108       7,716,043     $ 3.07        8.60      $ 337  
  

 

 

    

 

 

          

Exercisable — June 30, 2017

        3,479,127     $ 2.91        7.92      $ 332  
    

 

 

          

Vested and expected to vest — June 30, 2017

        7,716,043     $ 3.07        8.60      $ 337