Unaudited Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (12,291)	$ (26,219)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,290	1,281
Non-cash interest expense	863	1,031
Amortization of debt issuance costs and debt discount	305	390
Depreciation and amortization expense	78	83
Fair value adjustment related to derivative and warrant liability	(453)	9,741
Write-off of deferred offering costs	10
Change in assets and liabilities:
Prepaid expenses and other assets	186	(1,650)
Operating lease right-of-use assets	(1,785)
Accounts payable and accrued expenses	(1,027)	1,268
Operating lease liabilities	2,537
Other liabilities	(779)	(24)
Net cash used in operating activities	(11,066)	(14,099)
Cash flows from investing activities:
Purchases of property and equipment	(17)
Maturities of marketable securities	3,260	9,229
Net cash provided by investing activities	3,243	9,229
Cash flows from financing activities:
Proceeds from equity line of credit	2,721
Proceeds from at-the-market offering, net of commissions		2,798
Repayment of obligations under capital lease		(49)
Repayment of principal on finance lease liabilities	(51)
Net cash provided by financing activities	2,670	2,749
Net decrease in cash, cash equivalents and restricted cash	(5,153)	(2,121)
Cash, cash equivalents and restricted cash, beginning of period	19,119	11,971
Cash, cash equivalents and restricted cash, end of period	13,966	9,850
Supplemental cash flow information:
Cash paid for interest	2,287	2,618
Deferred offering costs included in accounts payable and accrued expenses	105
Trade date receivables		1,511
Property and equipment included in accounts payable and accrued expenses		$ 3