This study tests a new medicine called **CYB003** for people with **Major Depressive Disorder (MDD)**, which is a type of depression that makes someone feel very sad for a long time. The study wants to see if CYB003 is safe and if it works better than a placebo, which is a pill with no medicine in it. People between 18 and 85 years old who have moderate to severe depression and are already taking antidepressants can join. **Key Points:** - The study requires multiple visits and participants must use certain safety measures (like using condoms) during and after the study. - Participants cannot join if they have certain health problems like schizophrenia, uncontrolled high blood pressure, or if they recently had certain depression treatments. - Participants will not be paid, but the study will cover costs related to participation. To join, you must not have a history of serious mental health issues beyond MDD, and you should not be planning to donate blood or sperm during the study. It's important to follow all study rules and give your consent to be recorded during sessions.