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Stock-Based Compensation (Tables)
9 Months Ended
Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity
The following table summarizes stock opti
o
n activity for the nine months ended September 30, 2019:
 
   
Number of
Options
   
Weighted-
Average
Exercise
Price
   
Weighted-
Average
Remaining
Contractual
Term
( Years)
   
Aggregate
Intrinsic
Value
 
Outstanding — January 1, 2019
   972,569   $23.85     
0
   $
0
 
Granted
   651,600   $1.06    0    0 
Exercised
   0   $0    0    0 
Forfeited or expired
   (535,493  $23.77    0    0 
   
 
 
                
Outstanding — September 30, 2019
   1,088,676   $10.25    8.86   $0 
   
 
 
                
Exercisable — September 30, 2019
   355,845   $26.24    7.77   $0 
   
 
 
                
Vested and expected to vest — September 30, 2019
   1,082,710   $10.28    8.86   $0 
   
 
 
                
Calculation of Fair Value Assumptions Using Black Scholes Option Model
The estimated fair values of employee stock options granted during the three and nine months ended September 30, 2019 and 2018, were determined on the date of grant using the Black-Scholes option-pricing model with the following assumptions:
 
   
Three Months Ended
September 30,
  
Nine Months Ended
September 30,
 
   
2019
   
2018
  
2019
  
2018
 
Expected option life (years)
   0    6.05   6.02   5.58 
Risk-free interest rate
   0    2.79  2.22  2.77
Expected volatility
   0    78.91  74.14  79.67
Expected dividend yield
   0    0  0  0
Stock-Based Compensation Expense
The following table presents total stock-based compensation expense for the three and nine months ended September 30, 2019 and 2018:
 
   
Three Months Ended
September 30,
   
Nine months Ended
September 30,
 
   
2019
   
2018
   
2019
   
2018
 
Research and development
  $14   $244   $81   $760 
General and administrative
   171    416   $1,654   $1,598 
   
 
 
   
 
 
   
 
 
   
 
 
 
Total stock based compensation expense
  $185   $660   $1,735   $2,358