Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Income Statement [Abstract]
Grant income	$ 1,504,000	$ 2,133,000	$ 1,804,000
Operating expenses:
Research and development expenses	12,666,000	14,661,000	8,285,000
General and administrative expenses	11,177,000	2,653,000	1,608,000
Total operating expenses	23,843,000	17,314,000	9,893,000
Operating loss	(22,339,000)	(15,181,000)	(8,089,000)
Other income:
Interest income	36,000	50,000	19,000
Foreign currency exchange rate gain	1,371,000	1,661,000
Other income	2,329,000	2,034,000	844,000
Total other income	3,736,000	3,745,000	863,000
Net loss	(18,603,000)	(11,436,000)	(7,226,000)
Other comprehensive income (loss):
Foreign currency translation adjustments	(2,568,000)	(2,240,000)	(156,000)
Comprehensive loss	$ (21,171,000)	$ (13,676,000)	$ (7,382,000)
Net loss per share attributable to common stockholders, basic and diluted	$ (4.47)	$ (26.17)	$ (34.94)
Weighted average common shares outstanding, basic and diluted	4,646,764	591,298	263,358