The study aims to see if adding a new medicine, ABX-002, helps adults with major depressive disorder who are not fully helped by their current antidepressant. This trial is called double-blind, meaning neither doctors nor patients know who gets the real medicine or a placebo (a fake pill). The study lasts about 91 days and has three parts:

• Screening & Randomization: Check if you qualify and decide if you'll get ABX-002 or a placebo (35 days).
• Treatment Period: Take the assigned pill every day (42 days).
• Safety Follow-up: Ensure you're safe after treatment (2 weeks).

To join, you must have depression that hasn't lasted more than 12 months and have been on the same antidepressant for at least 6 weeks. You cannot join if you have certain mental health disorders like schizophrenia or bipolar disorder, or if you're pregnant. The study will closely watch your health and mood, and you'll need to visit the research center several times. Participating in this study means you'll help researchers learn more about treating depression.