Business decision to stop the program.

The study will evaluate the efficacy, safety, and tolerability of 450 milligrams (mg) and 900 milligrams (mg) of Rapastinel, compared to placebo in participants with major depressive disorder (MDD).

Inclusion Criteria:

• 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 Visit 1
• Treatment naive in the current episode or have inadequate response to 1-3 antidepressant therapies given at adequate dose and duration in the current episode
• 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:
• 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

A disclosure restriction on the PI is that the sponsor can review results communications prior to public release and can embargo communications regarding trial results for a period that is less than or equal to 90 days from the time submitted to the sponsor for review. The sponsor cannot require changes to the communication and cannot extend the embargo.