Condensed Consolidated Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Operating expenses:
Research and development	$ 5,905us-gaap_ResearchAndDevelopmentExpense	$ 7,690us-gaap_ResearchAndDevelopmentExpense
General and administrative	3,437us-gaap_GeneralAndAdministrativeExpense	2,470us-gaap_GeneralAndAdministrativeExpense
Total operating expenses	9,342us-gaap_OperatingExpenses	10,160us-gaap_OperatingExpenses
Loss from operations	(9,342)us-gaap_OperatingIncomeLoss	(10,160)us-gaap_OperatingIncomeLoss
Other (expense) income:
Interest expense	(280)us-gaap_InterestExpense	(260)us-gaap_InterestExpense
Interest income, net	16	14
Other (expense) income, net	(16)us-gaap_OtherNonoperatingIncomeExpense	(2)us-gaap_OtherNonoperatingIncomeExpense
Net loss	(9,622)us-gaap_NetIncomeLoss	(10,408)us-gaap_NetIncomeLoss
Other comprehensive (loss) income:
Net unrealized (loss) gain on marketable securities	6	(4)
Comprehensive loss	$ (9,616)us-gaap_ComprehensiveIncomeNetOfTax	$ (10,412)us-gaap_ComprehensiveIncomeNetOfTax
Basic and diluted net loss per common share (in dollars per share)	$ (0.29)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.32)us-gaap_EarningsPerShareBasicAndDiluted
Weighted average common shares outstanding used to calculate basic and diluted net loss per common share (in shares)	32,992,178	32,966,471

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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