Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 4,646	$ 0	$ 4,646	$ 23
Operating expenses:
Research and development	13,960	8,104	39,414	27,591
General and administrative	4,420	3,690	14,155	12,317
Total operating expenses	18,380	11,794	53,569	39,908
Loss from operations	(13,734)	(11,794)	(48,923)	(39,885)
Investment income	4	184	257	707
Loss on extinguishment of debt	(461)	0	(461)	0
Foreign currency transaction gain (loss), net	43	7	83	(33)
Interest expense	(365)	(388)	(843)	(1,184)
Change in fair value of warrant liabilities	4,779	0	(3,606)	0
Other income (expense), net	5	(3)	63	(67)
Net loss	(9,729)	(11,994)	(53,430)	(40,462)
Other comprehensive loss:
Foreign currency translation adjustment	(32)	(5)	(61)	24
Unrealized gain on securities	0	(3)	0	0
Total comprehensive loss	$ (9,761)	$ (12,002)	$ (53,491)	$ (40,438)
Net loss per share:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.09)	$ (0.26)	$ (0.54)	$ (0.94)
Weighted average common shares outstanding:
Basic and diluted (in shares)	105,325,788	46,407,846	98,968,359	43,265,909