Study of Adjunctive or Monotherapy Rapastinel Treatment in Patients With Major Depressive Disorder (MDD)

Inclusion Criteria: * Completion of lead-in study * Adequate therapeutic benefit in the lead-in study to justify continuation of treatment with rapastinel in the judgement of the investigator Exclusion Criteria: * Suicide risk, as determined by meeting any of the following criteria: 1. A suicide attempt within the past year 2. Significant risk, as judged by the investigator, based on the psychiatric interview or information collected in the Columbia- Suicide Severity Rating Scale (C-SSRS) at any visit in the lead-in study 3. Montgomery-Asberg Depression Rating Scale (MADRS) Item 10 score ≥ 5 at any visit during participation in the lead-in study where MADRS was conducted. * At imminent risk of injuring self or others or causing significant damage to property, as judged by the investigator * Females who are pregnant, breastfeeding, or planning to become pregnant or breastfeed during participation * Females of childbearing potential and male partners of females of childbearing potential, not using a reliable means of contraception