Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Income Statement [Abstract]
Revenue	$ 2,331	$ 248	$ 3,554	$ 1,390
Operating Expenses:
Research and development	4,548	3,339	12,385	9,670
General and administrative	2,129	5,026	6,905	8,973
Total operating expenses	6,677	8,365	19,290	18,643
Operating loss	(4,346)	(8,117)	(15,736)	(17,253)
Other income (expense):
Interest income	14	7	17	31
Foreign currency exchange gain (loss)	(1,887)	(195)	(1,836)	1,511
Other income	985	661	2,407	1,310
Total other income (expense)	(888)	473	588	2,852
Net loss	(5,234)	(7,644)	(15,148)	(14,401)
Other comprehensive loss:
Foreign currency translation adjustments	2,434	166	2,507	(2,832)
Comprehensive loss	$ (2,800)	$ (7,478)	$ (12,641)	$ (17,233)
Net loss per share to common stockholders, basic and diluted	$ (0.49)	$ (1.03)	$ (1.49)	$ (5.50)
Weighted average common shares outstanding, basic and diluted	10,788,556	7,657,874	10,168,520	3,013,073