Commitments and Contingencies - Narrative (Details) - USD ($)	3 Months Ended
	Jul. 31, 2018	Jul. 31, 2017
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 188,000	$ 102,000
Corporate headquarters
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 23,000	23,000
Lease expiration date	Nov. 30, 2021
Laboratories and office space
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 0	27,000
Lease expiration date	Oct. 01, 2017
New York laboratory
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 0	52,000
Lease expiration date	May 30, 2017
Rockville, MD
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 151,000	0
Gaithersburg, MD
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 14,000	$ 0