CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (5,072)	$ (10,644)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	184	95
Share-based compensation	933	614
Realized (gain) loss on investments	(1)
Unrealized foreign currency transaction (gain) loss	(95)	8
Accretion of discount on marketable securities	(355)	(45)
Changes in operating assets and liabilities:
Other receivables	(80)	(208)
Prepaid expenses and other current assets	(448)	604
Tax incentive receivables	(62)	(45)
Other assets	433
Accounts payable	(3,008)	(2,469)
Accrued expenses and other current liabilities	(3,364)	165
Deferred rent		(36)
Other long-term liabilities	11
Net cash used in operating activities	(10,924)	(11,961)
Cash flows from investing activities:
Purchases of marketable securities	(25,835)	(22,825)
Proceeds from maturities of marketable securities	50,587
Purchases of property and equipment	(81)
Net cash provided by (used in) investing activities	24,671	(22,825)
Cash flows from financing activities:
Proceeds from the issuance of common stock, net of issuance costs	1,525
Payment of offering costs	(16)
Proceeds from stock option exercises	59
Net cash provided by financing activities	1,568
Net increase (decrease) in cash and cash equivalents	15,315	(34,786)
Cash, cash equivalents and restricted cash at beginning of period	34,080	87,338
Cash, cash equivalents and restricted cash at end of period	49,395	$ 52,552
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	$ 75