Commitments and Contingencies - Narrative (Details) - USD ($)	9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 480,000	$ 297,000
Corporate headquarters
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 67,000	64,000
Lease expiration date	Nov. 30, 2021
Laboratories and office space
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 59,000	78,000
Lease expiration date	Oct. 01, 2017
New York laboratory
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 52,000	155,000
Lease expiration date	May 30, 2017
Rockville, MD
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 302,000	$ 0