Property and Equipment - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	Apr. 30, 2020	Apr. 30, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment at cost	$ 3,411	$ 3,309
Less: Accumulated depreciation	(1,368)	(896)
Property and equipment, net	2,043	2,413
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment at cost	1,543	1,391
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment at cost	36	36
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment at cost	185	175
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment at cost	$ 1,647	$ 1,707