CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Alliance revenue		$ 1,448		$ 1,448
Revenue, Product and Service [Extensible List]	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax
Operating expenses:
Research and development	$ 5,379	$ 10,024	$ 14,889	$ 18,126
General and administrative	2,632	2,895	6,274	6,038
Restructuring costs		45		176
Total operating expenses	8,011	12,964	21,163	24,340
Loss from operations	(8,011)	(11,516)	(21,163)	(22,892)
Other income (expense):
Interest income	27	339	152	743
Warrant revaluation (loss) gain	(913)		188
Future tranche right revaluation (loss) gain	(15,349)		5,362
Foreign currency exchange gain (loss)	20	1	52	(1)
Net loss	$ (24,226)	$ (11,176)	$ (15,409)	$ (22,150)
Net loss per share applicable to common stockholders (Note 12)
Basic	$ (0.72)	$ (0.39)	$ (0.48)	$ (0.79)
Diluted	$ (0.72)	$ (0.39)	$ (0.52)	$ (0.79)
Weighted-average number of common shares used in computing net loss per share applicable to common stockholders
Basic	33,583	28,461	31,941	28,070
Diluted	33,583	28,461	34,123	28,070
Comprehensive loss:
Net loss	$ (24,226)	$ (11,176)	$ (15,409)	$ (22,150)
Unrealized gain on available-for-sale securities				2
Total other comprehensive income				2
Comprehensive loss	$ (24,226)	$ (11,176)	$ (15,409)	$ (22,148)