The study aims to find out if adding **ABX-002** to a current antidepressant helps adults with major depressive disorder (MDD), a condition causing long-lasting sadness or loss of interest. This is a **double-blind** trial, meaning neither the participants nor the researchers know who is receiving the real medicine or a **placebo**, a substance with no active treatment. The study has three stages: Screening, Treatment, and Follow-Up, lasting about 35 days; a 42-day Treatment Period; and a 2-week Safety Follow-Up.

Participants need to have been on a stable dose of a selective serotonin reuptake inhibitor (SSRI) or serotonin norepinephrine reuptake inhibitor (SNRI) for at least 6 weeks. They should not have a history of certain mental health disorders like schizophrenia or bipolar disorder, nor should they be at risk for certain medical conditions like heart disease.

• **Duration**: Approximately 79 days from start to finish.
• **Visits**: Regular visits are required for monitoring.
• **Risks**: Potential side effects from medication or placebo.