Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (7,876)	$ (9,418)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	88	92
Stock-based compensation expense	505	548
Accretion of discount/premium on investment securities	25	38
Non-cash interest expense	50	79
Changes in assets and liabilities:
Prepaid expenses and other current assets	(19)	(38)
Other assets		(2)
Accounts payable	(468)	230
Accrued expenses	(454)	(663)
Deferred rent	44	(13)
Net cash used in operating activities	(8,105)	(9,147)
Investing activities
Purchases of available-for-sale securities		(32,111)
Sales and maturities of available-for-sale securities	14,910	3,332
Purchases of property and equipment	(9)	(290)
Net cash provided by (used in) investing activities	14,901	(29,069)
Financing activities
Proceeds from at-the-market offering, net of issuance costs	2,215
Proceeds from exercise of common stock options and warrants	21	87
Payments on borrowing	(833)	(833)
Net cash provided by (used in) financing activities	1,403	(746)
Net increase (decrease) in cash and cash equivalents	8,199	(38,962)
Cash and cash equivalents, beginning of period	23,596
Cash and cash equivalents, end of period	31,795	23,818
Supplemental disclosure of cash flow information
Cash paid for interest	104	163
Non-cash financing activities
Fixed asset purchases included in accounts payable	$ 5	$ 68