Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 0	$ 13	$ 0	$ 23
Operating expenses:
Research and development	10,730	12,134	25,454	19,487
General and administrative	5,637	4,114	9,735	8,627
Total operating expenses	16,367	16,248	35,189	28,114
Loss from operations	(16,367)	(16,235)	(35,189)	(28,091)
Investment income	13	246	253	523
Foreign currency transaction gain (loss), net	(42)	(10)	40	(40)
Interest expense	(205)	(400)	(478)	(796)
Change in fair value of warrant liabilities	(7,539)	0	(8,385)	0
Other (expense), net	59	5	58	(64)
Net loss	(24,081)	(16,394)	(43,701)	(28,468)
Stock‑based compensation expense	1,481	1,251
Other comprehensive loss:
Foreign currency translation adjustment	31	7	(29)	29
Unrealized gain on securities	0	1	0	3
Total comprehensive loss	$ (24,050)	$ (16,386)	$ (43,730)	$ (28,436)
Net loss per share:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.25)	$ (0.37)	$ (0.46)	$ (0.68)
Weighted average common shares outstanding:
Basic and diluted (in shares)	96,785,915	44,855,083	95,754,714	41,668,902