Warrants (Tables)

3 Months Ended

Mar. 31, 2016

Warrants and Rights Note Disclosure [Abstract]

Schedule of Warrant Activity

The following is a summary of warrant activity for the three months ended March 31, 2016 (in thousands):

	January 2016 Warrants	March 2015 Warrants	September 2013 Warrants	December 2012 Warrants	Other Equity Financing Warrants		Consultant and Oxford Warrants	Total
Outstanding, January 1, 2016	—	14,006	3,973	3,031	816		482	22,308
Issued	13,643	—	—	—	—		—	13,643
Exercised	—	—	—	—	(145	)	—	(145	)
Expired	—	—	—	—	(31	)	—	(31	)
Outstanding, March 31, 2016	13,643	14,006	3,973	3,031	640		482	35,775
Expiration	January 2021	March 2020	September 2018	December 2017	Varies 2016-2017		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 March 31, 2016
	January 2016 Warrants			March 2015 Warrants			September 2013 Warrants			December 2012 Warrants			April 2011 Warrants			March 2010 Warrants
Strike price	$	1.42		$	2.08		$	2.50		$	1.75		$	0.65		$	1.92
Expected term (years)	4.77			3.97			2.47			1.73			1.06			0.50
Volatility %	77.91		%	78.82		%	79.51		%	75.03		%	83.64		%	75.90		%
Risk-free rate %	1.17		%	1.03		%	0.80		%	0.69		%	0.60		%	0.39		%

	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			1.00
Volatility %	75.85		%	74.70		%	76.37		%	65.60		%	47.98		%	71.41		%
Risk-free rate %	1.58		%	1.24		%	1.05		%	0.77		%	—		%	—		%

Changes in Fair Value of Warrant Liability

The changes in fair value of the warrant liability for the three months ended March 31, 2016 were as follows (in thousands):

	January 2016 Warrants		March 2015 Warrants			September 2013 Warrants			December 2012 Warrants			April 2011 Warrants			Other Equity Financing Warrants			Total
Warrant liability, January 1, 2016	$	—	$	10,337		$	1,933		$	1,565		$	537		$	146		$	14,518
Fair value of warrants issued	5,590		—			—			—			—			—			5,590
Fair value of warrants exercised	—		—			—			—			—			(46		)	(46		)
Change in fair value of warrants	5,686		(1,134		)	(231		)	(305		)	(48		)	(96		)	3,872
Warrant liability, March 31, 2016	$	11,276	$	9,203		$	1,702		$	1,260		$	489		$	4		$	23,934