Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	4,304	5,320	8,478	10,166
General and administrative	3,075	4,339	6,214	7,256
Total operating expenses	7,379	9,659	14,692	17,422
Loss from operations	(7,379)	(9,659)	(14,692)	(17,422)
Interest income	207	168	307	343
Net loss	$ (7,172)	$ (9,491)	$ (14,385)	$ (17,079)
Net loss per share — basic and diluted	$ (0.26)	$ (0.64)	$ (0.68)	$ (1.16)
Weighted average common shares outstanding—basic and diluted	27,526,065	14,737,236	21,206,269	14,734,775
Comprehensive loss:
Net loss	$ (7,172)	$ (9,491)	$ (14,385)	$ (17,079)
Other comprehensive gain (loss):
Unrealized gain (loss) on investments, net of tax of $0	24	38	29	21
Total other comprehensive gain (loss)	24	38	29	21
Total comprehensive loss	$ (7,148)	$ (9,453)	$ (14,356)	$ (17,058)