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Stock Options (Tables)
 12 Months Ended
Dec. 31, 2017
Stock Options [Abstract]  
Non-Qualified Stock Options
The grant awards were made pursuant to the NASDAQ inducement grant exception as a material component of employment compensation.
 
Issue Date
 
25% Vesting Date
 
Executive 
 
Number
of Options
November 16, 2015
 
October 30, 2016
 
SVP, General Counsel and Secretary
 
80,000
November 1, 2016
 
October 17, 2017
 
Chief Operating Officer
 
150,000
March 1, 2017
 
February 28, 2018
 
SVP, Regulatory Affairs
 
80,000
November 1, 2017
 
October 31, 2018
 
Chief Financial Officer
 
200,000
Stock-Based Compensation Expense
The Company's stock-based compensation expense was recognized in operating expense as follows:

  
Year Ended December 31,
 
  
2017
  
2016
  
2015
 
Stock-Based Compensation
         
Research and development
 
$
2,687,975
  
$
2,177,643
  
$
1,129,556
 
General and administrative
  
3,494,866
   
3,127,427
   
1,769,748
 
Total
 
$
6,182,841
  
$
5,305,070
  
$
2,899,304
 
Assumptions Used to Value Stock Options and Warrants Granted
The fair value of options and warrants granted during the years ended December 31, 2017, 2016 and 2015 was estimated using the Black-Scholes option valuation model utilizing the following assumptions:

  
Year Ended December 31,
 
  
2017
  
2016
  
2015
 
  
Weighted
Average
  
Weighted
Average
  
Weighted
Average
 
Volatility
  
88.87
%
  
77.20
%
  
79.80
%
Risk-Free Interest Rate
  
1.88
%
  
1.39
%
  
1.74
%
Expected Term in Years
  
6.00
   
6.02
   
6.05
 
Dividend Rate
  
0.00
%
  
0.00
%
  
0.00
%
Fair Value of Option on Grant Date
 
$
6.93
  
$
5.39
  
$
5.42
 
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, 2015
  
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
 
                 
Options outstanding at December 31, 2016
  
5,316,511
  
$
5.84
         
Granted
  
1,365,400
   
9.39
         
Exercised
  
(35,366
)
  
3.34
         
Forfeited
  
(183,750
)
  
9.46
         
Options outstanding at December 31, 2017
  
6,462,795
  
$
6.50
   
7.13
  
$
20,467,335
 
Vested and expected to vest at December 31, 2017
  
6,462,795
  
$
6.50
   
7.13
  
$
20,467,335
 
Exercisable at December 31, 2017
  
4,066,066
  
$
5.14
   
6.21
  
$
18,100,589