This study is testing a new medicine called **CYB003** to help people with **Major Depressive Disorder (MDD)**. MDD is a condition where people feel very sad or hopeless for a long time. The medicine is compared to a *placebo*, which looks like real medicine but doesn't have any active ingredients, to see which works better. Participants should be between 18 and 85 years old and have tried other depression medicines that didn't work well enough. They should also not have certain other mental health conditions like schizophrenia or bipolar disorder. **Key Points:** - Participants will need to refrain from nicotine use for up to 8 hours during dosing sessions. - People who join the study must agree not to donate blood or sperm (if applicable) for a certain time period. - People with specific health conditions or taking particular medications cannot participate. Before joining, participants will discuss with doctors to ensure it's safe for them and will sign a consent form to show they understand and agree to the study rules. For more details, visit the study website: [Study Website](https://embrace-mdd-trial.com/).