Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Apr. 30, 2021	Apr. 30, 2020	Apr. 30, 2019
Income Statement [Abstract]
Revenue		$ 12,690,000	$ 16,127,000
Operating expenses:
Research and development	$ 41,286,000	40,194,000	35,021,000
General and administrative	16,637,000	13,029,000	10,926,000
Total operating expenses	57,923,000	53,223,000	45,947,000
Operating loss	(57,923,000)	(40,533,000)	(29,820,000)
Other income:
Interest income	903,000	1,830,000	1,397,000
Foreign currency exchange gain (loss)	847,000	(367,000)	49,000
Other income	9,929,000	9,830,000	7,682,000
Total other income	11,679,000	11,293,000	9,128,000
Loss before income taxes	(46,244,000)	(29,240,000)	(20,692,000)
Income tax (benefit) expense	0	(124,000)	124,000
Net loss	(46,244,000)	(29,116,000)	(20,816,000)
Other comprehensive income (loss):
Foreign currency translation adjustments	1,076,000	59,000	(1,257,000)
Unrealized holding gain (loss) on available-for-sale securities	(473,000)	285,000	440,000
Reclassification adjustment for realized (gain) on available for sale securities included in net loss	(153,000)	(300,000)
Total other comprehensive income (loss):	450,000	44,000	(817,000)
Comprehensive loss	$ (45,794,000)	$ (29,072,000)	$ (21,633,000)
Net loss per share, basic and diluted	$ (2.42)	$ (1.64)	$ (1.38)
Weighted average common shares outstanding, basic and diluted	19,094,440	17,748,666	15,080,863