Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2020	Jan. 31, 2019
Income Statement [Abstract]
Revenue	$ 1,577	$ 3,890	$ 8,866	$ 13,201
Operating expenses:
Research and development	11,233	7,650	30,709	23,882
General and administrative	3,068	2,900	9,733	7,879
Total operating expenses	14,301	10,550	40,442	31,761
Operating loss	(12,724)	(6,660)	(31,576)	(18,560)
Other income:
Interest income	372	723	1,467	1,016
Foreign currency exchange gain	138	248	245	83
Other income	2,923	1,733	7,332	5,171
Total other income	3,433	2,704	9,044	6,270
Net loss	(9,291)	(3,956)	(22,532)	(12,290)
Other comprehensive income (loss):
Foreign currency translation adjustments	142	(97)	548	(1,224)
Unrealized holding gains on available-for-sale securities	40	232	225	232
Reclassification adjustment for realized (gain) on available-for-sale securities included in net loss	(100)		(229)
Other comprehensive income (loss)	82	135	544	(992)
Comprehensive loss	$ (9,209)	$ (3,821)	$ (21,988)	$ (13,282)
Net loss per share to common stockholders, basic and diluted	$ (0.52)	$ (0.23)	$ (1.27)	$ (0.85)
Weighted average common shares outstanding, basic and diluted	17,838,872	17,231,449	17,717,057	14,379,872