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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Stock-based Compensation Expense for Employees and Non-employees

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

 

     Year Ended December 31,  
     2018      2017      2016  
     (in thousands)  

Research and development

    $ 4,499      $ 3,966      $ 3,635  

General and administrative

      2,297        1,939        1,875  
  

 

 

     

 

 

     

 

 

  

Total stock-based compensation

    $ 6,796      $ 5,905      $ 5,510  
  

 

 

     

 

 

     

 

 

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

The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions:

 

     Year Ended December 31,  
     2018     2017     2016  

Expected term (in years)

      5.3 – 7.0       5.3 – 7.0       5.1 – 9.9  

Expected volatility

      88 – 91 %      86 – 96 %      77 – 87 % 

Risk-free interest rate

      2.6 – 3.1 %      1.8 – 2.3 %      1.1 – 2.4 % 

Expected dividend rate

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

A summary of activity under the 2008 Plan, 2015 Plan and 2018 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, 2017

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

Awards authorized under the 2015 Plan

     2,095,808          

Awards authorized under the 2018 Plan

     1,500,000          

Options granted

     (3,566,150 )      3,566,150       2.35      

Options exercised

     —         (120,742 )      1.49      

Options forfeited/cancelled

     411,597       (411,597 )      2.98      
 

 

 

    

 

 

        

Outstanding—December 31, 2018

     1,945,025       10,504,412       $2.86       7.80     $ 545  
 

 

 

    

 

 

        

Exercisable—December 31, 2018

       5,797,657       $3.10       7.00     $ 542  
   

 

 

        

Vested and expected to vest—December 31, 2018

       10,504,412       $2.86       7.80     $ 545