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Stock-Based Compensation (Tables)
3 Months Ended
Mar. 31, 2016
Stock-Based Compensation Expense Related to Common Stock Options, Common Stock, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, common stock, restricted common stock and common stock warrants:

 

     Three Months Ended
March 31,
 
     2016      2015  

Research and development

   $ 254       $ 317   

General and administrative

     969         618   
  

 

 

    

 

 

 

Total stock-based compensation expense

   $ 1,223       $ 935   
  

 

 

    

 

 

 
Summary of Stock Option Activity

The following table summarizes the stock option activity in the Company’s equity incentive plans, including non-plan grants to Company executives, from December 31, 2015 through March 31, 2016:

 

     Shares      Weighted Average
Exercise Price
 

Outstanding, December 31, 2015

     3,342,325       $ 5.70   

Granted

     277,500         1.37   

Exercised

     —           —     

Options forfeited/cancelled

     (18,520 )      6.48   
  

 

 

    

Outstanding, March 31, 2016

     3,601,305       $ 5.36   
  

 

 

    
Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of all other options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used:

 

     Three
Months Ended
March 31,
    Three
Months Ended
March 31,
 
     2016     2015  

Risk-free interest rate

     1.7     1.64

Expected life of the options

     6.0 years        6.0 years   

Expected volatility of the underlying stock

     94     104

Expected dividend rate

     0     0
Summary of Restricted Stock Grant Activity

The following table summarizes the restricted stock grant activity in the Company’s equity incentive plans from December 31, 2015 through March 31, 2016:

 

     Shares  

Outstanding, December 31, 2015

     754,605   

Granted

     —     

Exercised

     —     

Options forfeited/cancelled

     —     
  

 

 

 

Outstanding, March 31, 2016

     754,605