CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 1,183	$ 3,042	$ 7,248	$ 8,415
Operating expenses:
Research and development	3,337	4,135	13,585	8,571
General and administrative	3,901	1,206	7,290	4,694
Total operating expenses	7,238	5,341	20,875	13,265
Loss from operations	(6,055)	(2,299)	(13,627)	(4,850)
Investment and other income, net	54	23	64	45
Gain on extinguishment	2,318	0	2,318	0
Interest expense	(1,098)	(267)	(1,982)	(769)
Change in fair value of derivative liability	0	0	(11)	0
Change in fair value of warrant liability	0	2	223	8
Net loss	(4,781)	(2,541)	(13,015)	(5,566)
Reduction (accretion) of redeemable convertible preferred stock to redemption value	(82)	(966)	10,274	(5,071)
Net loss attributable to common stockholders	$ (4,863)	$ (3,507)	$ (2,741)	$ (10,637)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (1.65)	$ (5.98)	$ (1.98)	$ (18.13)
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted (in shares)	2,943,896	586,738	1,382,592	586,701