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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2018
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,		  
     2018      2017      2018      2017  
     (in thousands)  

Research and development

    $ 1,164      $ 1,008      $ 2,162      $ 2,021  

General and administrative

      618        503        1,119        1,017  
  

 

 

     

 

 

     

 

 

     

 

 

  

Total stock-based compensation

    $ 1,782      $ 1,511      $ 3,281      $ 3,038  
  

 

 

     

 

 

     

 

 

     

 

 

  
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,		  
     2018     2017     2018     2017  

Expected term (in years)

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

Expected volatility

      88 – 91 %      86 – 87 %      88 – 91 %      86 – 96 % 

Risk-free interest rate

      2.6 – 2.9 %      1.8 – 1.9 %      2.6 – 2.9 %      1.8 – 2.3 % 

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:

 

     Shares
Available
for Grant		     Options Outstanding  
    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, 2017

      1,503,770       7,470,601     $ 3.09        8.12      $ 12,363  

Awards authorized

      2,095,808            

Options granted

      (3,276,750 )      3,276,750       2.38        

Options exercised

      —         (88,958 )      1.50        

Options forfeited/cancelled

      88,434       (88,434 )      4.70        
  

 

 

    

 

 

          

Outstanding—June 30, 2018

      411,262       10,569,959     $ 2.87        8.27      $ 9,736  
  

 

 

    

 

 

          

Exercisable—June 30, 2018

        4,977,704     $ 3.03        7.36      $ 5,520  
    

 

 

          

Vested and expected to vest—June 30, 2018

        10,569,959     $ 2.87        8.27      $ 9,736