Condensed Consolidated Statement 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
Income Statement [Abstract]
Revenue from collaboration agreements	$ 8,447	$ 5,364	$ 24,933	$ 13,172
Operating expenses:
Research and development	23,935	12,948	58,007	32,042
General and administrative	2,861	2,417	8,472	6,083
Total operating expenses	26,796	15,365	66,479	38,125
Operating loss	(18,349)	(10,001)	(41,546)	(24,953)
Other income (expense), net:
Interest income (expense)	(352)	558	(170)	1,454
Amortization of debt discount	(203)		(229)
Change in fair value of warrant liability	(3,141)		(3,141)
Other (expense) income, net	43	(1)	44	323
Total other income (expense), net	(3,653)	557	(3,496)	1,777
Loss before income taxes	(22,002)	(9,444)	(45,042)	(23,176)
Income tax expense (benefit)	(167)	650	(502)	900
Net loss	(21,835)	(10,094)	(44,540)	(24,076)
Other comprehensive loss:
Unrealized gain (loss) on short-term investments	(10)	5	8	(4)
Comprehensive loss	(21,845)	(10,089)	(44,532)	(24,080)
Accrual of preferred stock dividends	(5,212)	(2,201)	(10,363)	(6,531)
Net loss attributable to common stockholders	$ (27,047)	$ (12,295)	$ (54,903)	$ (30,607)
Net loss per share attributable to common stockholders—basic and diluted (Note 12)	$ (17.55)	$ (8.93)	$ (36.76)	$ (22.59)
Weighted-average number of shares used in computed net loss per share —basic and diluted	1,540,902	1,376,365	1,493,521	1,354,734