Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 2,354	$ 10,336	$ 9,185	$ 18,473
General and administrative	2,268	1,970	6,802	6,299
Total operating expenses	4,622	12,306	15,987	24,772
Loss from operations	(4,622)	(12,306)	(15,987)	(24,772)
Other income:
Investment income	113	83	311	161
Other (expense) income, net	(1)	(84)	206	198
Total other income (expense)	112	(1)	517	359
Net loss	(4,510)	(12,307)	(15,470)	(24,413)
Foreign currency translation adjustment		0	(1)
Unrealized gain (loss) on available-for-sale investments	3		19	(7)
Comprehensive loss	(4,507)	(12,307)	(15,452)	(24,420)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(4,510)	(12,307)	(15,470)	(24,413)
Accretion of convertible preferred stock to redemption value		(6,747)		(6,747)
Net loss attributable to common stockholders	$ (4,510)	$ (19,054)	$ (15,470)	$ (31,160)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (0.25)	$ (1.08)	$ (0.87)	$ (1.82)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders - basic and diluted (in shares)	17,824,186	17,574,371	17,725,095	17,158,032
Included in operating expenses, above, are the following amounts for non-cash stock-based compensation expense:
Allocated share-based compensation	$ 904	$ 747	$ 2,647	$ 2,459
Research and Development Expense [Member]
Included in operating expenses, above, are the following amounts for non-cash stock-based compensation expense:
Allocated share-based compensation	298	235	877	841
General and Administrative Expense [Member]
Included in operating expenses, above, are the following amounts for non-cash stock-based compensation expense:
Allocated share-based compensation	$ 606	$ 512	$ 1,770	$ 1,618