The purpose of this study is to determine the efficacy and safety of milsaperidone compared to placebo as adjunctive therapy in patients with Major Depressive Disorder

Inclusion Criteria:

• Male or female patient 18 to 65 years of age, inclusive;
• Meets DSM-5-TR criteria for MDD
• Currently having an inadequate response to antidepressant therapy as confirmed by the Investigator using the Antidepressant Treatment Response Questionnaire (ATRQ)

Exclusion Criteria:

• Within the patients lifetime, has a confirmed DSM-5-TR psychiatric diagnosis other than MDD
• Within 6 months of Screening, has a confirmed DSM-5-TR psychiatric diagnosis other than MDD
• Within 12 months of Screening, has had any other psychiatric condition (other than MDD) that has been the main focus of treatment