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Warrants (Tables)
12 Months Ended
Dec. 31, 2015
Warrants and Rights Note Disclosure [Abstract]  
Schedule of Warrant Activity
The following is a summary of warrant activity for the years ended December 31, 2015 and 2014 (in thousands):
 
 
March
2015
Warrants
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
Other Warrant Issuances
 
Consultant
and Oxford Warrants
 
Total
Outstanding, January 1, 2014

 
6,442

 
4,917

 
1,158

 
1,444

 
889

 
14,850

Granted

 

 

 

 

 
300

 
300

Exercised

 
(2,469
)
 
(1,886
)
 
(543
)
 
(327
)
 
(469
)
 
(5,694
)
Expired

 

 

 

 
(916
)
 

 
(916
)
Outstanding, December 31, 2014

 
3,973

 
3,031

 
615

 
201

 
720

 
8,540

Granted
14,006

 

 

 

 

 

 
14,006

Expired

 

 

 

 

 
(238
)
 
(238
)
Outstanding, December 31, 2015
14,006

 
3,973

 
3,031

 
615

 
201

 
482

 
22,308

Expiration
March 2020
 
September 2018
 
December 2017
 
April 2017
 
Varies 2014-2016
 
Varies 2014-2020
 
 
Fair Value of Warrants is Estimated Using Black-Scholes Option Pricing Model
The fair value of the warrants is estimated using an appropriate pricing model with the following inputs:
 
 
As of December 31, 2015
 
March
2015
Warrants
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
Strike price
$
2.08

 
$
2.50

 
$
1.83

 
$
0.65

 
$
0.65

 
$
2.02

Expected term (years)
4.22

 
2.72

 
1.98

 
1.31

 
0.18

 
0.24

Volatility %
75.85
%
 
74.70
%
 
76.37
%
 
65.60
%
 
47.98
%
 
71.41
%
Risk-free rate %
1.58
%
 
1.24
%
 
1.05
%
 
0.77
%
 
%
 
%
 
 
As of December 31, 2014
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
Strike price
$
2.50

 
$
1.90

 
$
0.65

 
$
0.65

 
$
2.15

Expected term (years)
3.72

 
2.98

 
2.31

 
1.18

 
1.24

Volatility %
75.60
%
 
76.85
%
 
78.24
%
 
77.38
%
 
77.12
%
Risk-free rate %
1.30
%
 
1.09
%
 
0.80
%
 
0.32
%
 
0.35
%
Changes in Fair Value of Warrant Liability
The changes in fair value of the warrant liability for the years ended December 31, 2015 and 2014 were as follows (in thousands):
 
 
March
2015
Warrants
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
Other Warrant Issuances
 
Total
Warrant liability, January 1, 2014
$

 
$
22,950

 
$
18,060

 
$
5,069

 
$
2,886

 
$
48,965

Fair value of warrants granted

 

 

 

 

 

Fair value of warrants exercised

 
(12,713
)
 
(10,086
)
 
(2,906
)
 
(1,321
)
 
(27,026
)
Change in fair value of warrants

 
(7,677
)
 
(5,947
)
 
(1,538
)
 
(1,394
)
 
(16,556
)
Warrant liability, December 31, 2014

 
2,560

 
2,027

 
625

 
171

 
5,383

Fair value of warrants granted
10,296

 

 

 

 

 
10,296

Fair value of warrants exercised

 

 

 

 

 

Change in fair value of warrants
41

 
(627
)
 
(462
)
 
(88
)
 
(25
)
 
(1,161
)
Warrant liability, December 31, 2015
$
10,337

 
$
1,933

 
$
1,565

 
$
537

 
$
146

 
$
14,518