This study is looking to see if a medicine called brenipatide can help adults with Major Depressive Disorder (MDD). MDD is a condition causing long-lasting sadness and loss of interest. Participants will be compared to a placebo (a pill with no medicine) to check if brenipatide helps delay the return of depression symptoms. The study has three parts: a one-month screening, at least 12 months of treatment, and two months of follow-up. The study might end early if depression symptoms worsen or if participants decide to leave.

• Participants must be willing to attend study visits and follow instructions, like self-injecting the medication.
• Participants cannot have other mental health issues like schizophrenia or bipolar disorder.
• The study will last at least 15 months, with regular visits needed.