**Study Summary:** This study is testing a medicine called **milsaperidone** to see if it helps people with **Major Depressive Disorder (MDD)** feel better when taken with their regular antidepressants. MDD is a condition where people feel very sad or lose interest in activities they once enjoyed. **Adjunctive therapy** means adding another treatment to help the main treatment work better. A **placebo** is a pill that looks like the medicine but doesn't have any active ingredients, used to compare results. **Eligibility:** To join, participants need to be between 18 and 65 years old and diagnosed with MDD, but not responding well to current antidepressants. They should not have any other major psychiatric disorders. **Key Points:** - **Study Length:** Participants will be involved for a specific period; details will be provided during the screening. - **Visits:** Regular visits to the study site for check-ups and assessments will be required. - **Compensation:** Participants may receive compensation for their time and travel expenses. This study aims to find better ways to help those who don’t feel better with their current treatment. Always talk to your doctor to see if joining is right for you.