Long-term Safety Study of Rapastinel as Adjunctive Therapy in Patients With Major Depressive Disorder

* Meet Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria for MDD * Current major depressive episode of at least 8 weeks and not exceeding 18 months in duration at Screening * Have no more than partial response (\< 50% improvement) to ongoing treatment with a protocol-allowed antidepressant * If female of childbearing potential, have a negative serum β-human chorionic gonadotropin (β-hCG) pregnancy test.

* DSM-5-based diagnosis of any disorder other than MDD that was the primary focus of treatment within 6 months before Screening * Lifetime history of meeting DSM-5 criteria for: * Schizophrenia spectrum or other psychotic disorder * Bipolar or related disorder * Major neurocognitive disorder * 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 * Dissociative disorder * Posttraumatic stress disorder * MDD with psychotic features * Significant suicide risk, as judged by the Investigator.