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Stock Options (Tables)
 12 Months Ended
Dec. 31, 2016
Stock Options [Abstract]  
Stock-Based Compensation Expense
The Company’s stock-based compensation expense was recognized in operating expense as follows:
 
 
 
Year Ended December 31,
 
 
 
2016
  
2015
  
2014
 
Stock-Based Compensation
         
Research and development
 
$
2,177,643
  
$
1,129,556
  
$
569,132
 
General and administrative
  
3,127,427
   
1,769,748
   
728,853
 
Total
 
$
5,305,070
  
$
2,899,304
  
$
1,297,985
 
Assumptions Used to Value Stock Options and Warrants Granted
The fair value of options and warrants granted during the years ended December 31, 2016, 2015 and 2014 was estimated using the Black-Scholes option valuation model utilizing the following assumptions:

 
 
For the year ended December 31,
 
 
 
2016
  
2015
  
2014
 
 
 
Weighted
Average
  
Weighted
Average
  
Weighted
Average
 
 
         
Volatility
  
77.20
%
  
79.80
%
  
75.54
%
Risk-Free Interest Rate
  
1.39
%
  
1.74
%
  
1.96
%
Expected Term in Years
  
6.02
   
6.05
   
5.78
 
Dividend Rate
  
0.00
%
  
0.00
%
  
0.00
%
Fair Value of Option on Grant Date
 
$
5.39
  
$
5.42
  
$
5.35
 
Stock Option Activity
The following table summarizes the number of options outstanding and the weighted average exercise price:
 
 
 
Number of
Shares
  
Weighted
Average
Exercise
Price
  
Weighted
Average
Remaining
Contractual
Life in Years
  
Aggregate
Intrinsic
Value
 
 
            
Options outstanding at January 1, 2014
  
1,993,301
  
$
2.00
       
Granted
  
452,410
  
$
8.15
       
Exercised
  
-
   
-
       
Forfeited
  
-
   
-
       
Options outstanding at December 31, 2014
  
2,445,711
  
$
3.13
   
8.18
  
$
6,247,407
 
Vested and expected to vest at December 31, 2014
  
2,408,395
  
$
3.12
   
8.17
  
$
6,175,025
 
Exercisable at December 31, 2014
  
1,413,335
  
$
2.46
   
7.86
  
$
4,162,373
 
 
                
Options outstanding at December 31, 2014
  
2,445,711
  
$
3.13
         
Granted
  
1,902,609
   
7.87
         
Exercised
  
(4,753
)
  
0.23
         
Forfeited
  
(30,640
)
  
7.98
         
Expirations
  
(10,660
)
  
8.28
         
Options outstanding at December 31, 2015
  
4,302,267
  
$
5.19
   
8.14
  
$
31,659,550
 
Vested and expected to vest at December 31, 2015
  
4,213,091
  
$
5.14
   
8.12
  
$
31,202,132
 
Exercisable at December 31, 2015
  
1,857,077
  
$
2.83
   
7.05
  
$
17,952,965
 
 
                
Options outstanding at December 31, 2015
  
4,302,267
  
$
5.19
         
Granted
  
1,211,400
   
8.09
         
Exercised
  
(63,639
)
  
4.62
         
Forfeited
  
(133,517
)
  
5.79
         
Options outstanding at December 31, 2016
  
5,316,511
  
$
5.84
   
7.60
  
$
35,599,646
 
Vested and expected to vest at December 31, 2016
  
5,235,931
  
$
5.80
   
7.58
  
$
35,246,927
 
Exercisable at December 31, 2016
  
3,027,112
  
$
4.41
   
6.76
  
$
24,559,384