Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2020	Oct. 31, 2019	Oct. 31, 2020	Oct. 31, 2019
Income Statement [Abstract]
Revenue		$ 3,920		$ 7,289
Operating expenses:
Research and development	$ 9,148	9,789	$ 20,313	19,476
General and administrative	3,633	3,420	6,912	6,665
Total operating expenses	12,781	13,209	27,225	26,141
Operating loss	(12,781)	(9,289)	(27,225)	(18,852)
Other income:
Interest income	193	505	451	1,095
Foreign currency exchange gain (loss)	(24)	560	414	108
Other income	2,186	2,321	5,119	4,408
Total other income	2,355	3,386	5,984	5,611
Net loss	(10,426)	(5,903)	(21,241)	(13,241)
Other comprehensive income (loss):
Foreign currency translation adjustments	(127)	495	211	406
Unrealized holding (loss) gain on marketable securities	(164)	28	(169)	56
Reclassification adjustment for realized (gain) on marketable securities included in net loss	(46)		(116)
Other comprehensive income (loss)	(337)	523	(74)	462
Comprehensive loss	$ (10,763)	$ (5,380)	$ (21,315)	$ (12,779)
Net loss per share, basic and diluted	$ (0.58)	$ (0.33)	$ (1.19)	$ (0.75)
Weighted average common shares outstanding, basic and diluted	17,907,393	17,823,302	17,877,988	17,656,150