Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Income Statement [Abstract]
Revenue	$ 12,690	$ 16,127	$ 8,394
Operating expenses:
Research and development	40,194	35,021	18,237
General and administrative	13,029	10,926	8,862
Total operating expenses	53,223	45,947	27,099
Operating loss	(40,533)	(29,820)	(18,705)
Other income:
Interest income	1,830	1,397	82
Foreign currency exchange gain (loss)	(367)	49	(1,574)
Other income	9,830	7,682	4,392
Total other income	11,293	9,128	2,900
Loss before income taxes	(29,240)	(20,692)	(15,805)
Income tax (benefit) expense	(124)	124
Net loss	(29,116)	(20,816)	(15,805)
Other comprehensive income (loss):
Foreign currency translation adjustments	59	(1,257)	1,534
Unrealized holding gains on available-for-sale securities	285	440
Reclassification adjustment for realized (gain) on available for sale securities included in net loss	(300)
Comprehensive loss	$ (29,072)	$ (21,633)	$ (14,271)
Net loss per share to common stockholders, basic and diluted	$ (1.64)	$ (1.38)	$ (1.53)
Weighted average common shares outstanding, basic and diluted	17,748,666	15,080,863	10,321,780