This study is testing a new treatment, called ALTO-203, for people with Major Depressive Disorder (MDD). MDD is a condition where people feel very sad and lose interest in activities. The study will check how the body and brain respond to the treatment. There are two parts: a single-dose part and a 28-day multiple-dose part. In the single-dose part, participants will take ALTO-203 or a placebo (a pill with no medicine) and answer questions and do tasks to see how they feel. In the multiple-dose part, participants take ALTO-203 or placebo for 28 days. The study will make sure the treatment is safe.

• The study lasts for 28 days, with different visits needed.
• Participants will answer questions and do tasks to see how they feel.
• Safety of ALTO-203 will be closely watched.

To join, you need to have MDD, not be taking antidepressants, and be willing to follow the study rules. You can't join if you have other mental illnesses or certain medical conditions, or if you are part of another study.