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Warrants (Tables)
 9 Months Ended
Sep. 30, 2014
Warrants and Rights Note Disclosure [Abstract]  
Schedule of Warrant Activity
The following is a summary of warrant activity for the six months ended September 30, 2014 (in thousands):
 
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
 
August
2009
Warrants
 
Consultant
and Oxford Warrants
 
Total
Outstanding, January 1, 2014
6,442

 
4,917

 
1,158

 
176

 
290

 
978

 
889

 
14,850

Granted
—

 
—

 
—

 
—

 
—

 
—

 
300

 
300

Exercised
(2,469
)
 
(1,886
)
 
(543
)
 
—

 
(265
)
 
(62
)
 
(469
)
 
(5,694
)
Outstanding, September 30, 2014
3,973

 
3,031

 
615

 
176

 
25

 
—

 
720

 
8,540

Expiration
September 2018
 
December 2017
 
April 2017
 
March 2016
 
March 2016
 
August 2014
 
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 September 30, 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)
3.97

 
3.23

 
2.56

 
1.43

 
1.49

Volatility %
74.39
%
 
76.97
%
 
78.24
%
 
79.41
%
 
79.03
%
Risk-free rate %
1.41
%
 
1.15
%
 
0.85
%
 
0.32
%
 
0.35
%
 
 
As of December 31, 2013
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
 
August
2009
Warrants
Strike price
$
2.50

 
$
1.90

 
$
0.65

 
$
0.65

 
$
2.15

 
$
4.50

Expected term (years)
4.72

 
3.98

 
3.31

 
2.18

 
2.24

 
0.59

Volatility %
71.97
%
 
71.38
%
 
71.71
%
 
73.45
%
 
73.36
%
 
66.85
%
Risk-free rate %
1.61
%
 
1.25
%
 
0.93
%
 
0.45
%
 
0.47
%
 
0.11
%
Changes in Fair Value of Warrant Liability
The changes in fair value of the warrant liability for the nine months ended September 30, 2014 were as follows (in thousands):
 
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
 
August
2009
Warrants
 
Total
Warrant liability, January 1, 2014
$
22,950

 
$
18,060

 
$
5,069

 
$
763

 
$
945

 
$
1,178

 
$
48,965

Fair value of warrants exercised
(12,713
)
 
(10,086
)
 
(2,906
)
 
—

 
(1,159
)
 
(162
)
 
(27,026
)
Change in fair value of warrants
(6,066
)
 
(4,605
)
 
(1,214
)
 
(505
)
 
232

 
(1,016
)
 
(13,174
)
Warrant liability, September 30, 2014
$
4,171

 
$
3,369

 
$
949

 
$
258

 
$
18

 
$
—

 
$
8,765