This study is testing if a drug called zelquistinel can help treat depression in adults and if it's safe. Depression is a mental health condition that can make people feel very sad or hopeless. Placebo means a pill that doesn't have the active drug in it. Participants will take either zelquistinel or a placebo for 6 weeks, with weekly clinic visits to check on their depression using a test called the Hamilton depression rating scale-17. This study is double-blind, meaning neither the participants nor the researchers know who is getting the drug or the placebo. This helps keep the results fair.

• Participation lasts up to 98 days, including screening, treatment, and follow-up.
• Weekly clinic visits are required for depression checks.
• Participants must meet certain health criteria and cannot be using certain medications.

If you're between 18 and 64, have been diagnosed with Major Depressive Disorder (MDD), and meet other criteria, you might qualify for this study. Be sure to talk to your doctor and understand all requirements before deciding to join.