Study of Monotherapy Rapastinel in the Prevention of Relapse in Patients With Major Depressive Disorder (MDD)

Inclusion Criteria: - * Completion of Study RAP-MD-30, RAP-MD-31, or RAP-MD-32 * If female of childbearing potential, have a negative serum β-human chorionic gonadotropin (β-hCG) pregnancy test Exclusion Criteria: - * DSM-5-based diagnosis of any disorder other than MDD that was the primary focus of treatment within 6 months before Visit 1 * Lifetime history of meeting DSM-5 criteria for: * 1.Schizophrenia spectrum or other psychotic disorder * 2.Bipolar or related disorder * 3.Major neurocognitive disorder * 4.Neurodevelopmental disorder of greater than mild severity or of a severity that impacts the participant's ability to consent, follow study directions, or otherwise safely participate in the study * 5.Dissociative disorder * 6.Posttraumatic stress disorder * 7.MDD with psychotic features * Significant suicide risk, as judged by the Investigator