Commitments and Contingencies - Narrative (Details) - USD ($)	9 Months Ended
	Jan. 31, 2019	Jan. 31, 2018
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 583,000	$ 480,000
Corporate headquarters
Commitments and Contingencies [Line Items]
Operating leases, rent expense	68,000	67,000
Laboratories and office space
Commitments and Contingencies [Line Items]
Operating leases, rent expense	0	59,000
New York laboratory
Commitments and Contingencies [Line Items]
Operating leases, rent expense	0	52,000
Rockville, MD
Commitments and Contingencies [Line Items]
Operating leases, rent expense	453,000	302,000
Gaithersburg, MD
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 41,000	$ 0