Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (53,335)	$ (42,015)	$ (47,867)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	6,796	5,905	5,510
Depreciation	111	258	197
Non-cash restructuring charges			811
Other	(68)	31	15
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,341)	(115)	(362)
Accrued liabilities	2,770	990	(1,713)
Accounts payable	(48)	(1,217)	2,246
Net cash used in operating activities	(45,115)	(36,163)	(41,163)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(118)	(92)	(214)
Proceeds from sale of property and equipment			43
Net cash used in investing activities	(118)	(92)	(171)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon follow-on offering, net of offering costs	45,996	27,422
Proceeds from issuance of term loan, net of issuance costs	4,955
Proceeds from exercise of common stock options	180	166	211
Payment of deferred offering costs			(15)
Net cash provided by financing activities	51,131	27,588	196
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(88)	(4)	(60)
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	5,810	(8,671)	(41,198)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH-Beginning of period	100,536	109,207	150,405
CASH, CASH EQUIVALENTS AND RESTRICTED CASH-End of period	106,346	100,536	109,207
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes, net	15	260	107
Cash paid for interest	87
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Property and equipment purchases included in accounts payable and accrued liabilities	11	$ 4	$ 85
Issuance of common stock warrant	$ 116