Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2020	Jul. 31, 2019
Income Statement [Abstract]
Revenue		$ 3,369
Operating expenses:
Research and development	$ 11,165	9,686
General and administrative	3,278	3,247
Total operating expenses	14,443	12,933
Operating loss	(14,443)	(9,564)
Other income:
Interest income	259	590
Foreign currency exchange gain (loss)	438	(453)
Other income	2,932	2,089
Total other income	3,629	2,226
Net loss	(10,814)	(7,338)
Other comprehensive income (loss):
Foreign currency translation adjustments	338	(89)
Unrealized holding (loss) gain on marketable securities	(5)	28
Reclassification adjustment for realized (gain) on marketable securities included in net loss	(70)
Other comprehensive income (loss)	263	(61)
Comprehensive loss	$ (10,551)	$ (7,399)
Net loss per share to common stockholders, basic and diluted	$ (0.61)	$ (0.42)
Weighted average common shares outstanding, basic and diluted	17,848,583	17,488,997