This clinical trial is testing a new medicine, ALTO-203, in people with Major Depressive Disorder (MDD). MDD is a condition that makes people feel very sad and lose interest in things they used to enjoy. We want to see how ALTO-203 works in the body and if it is safe to use. The study has two parts: in the first part, you will take a single dose of ALTO-203 or a placebo (a pill with no medicine) and answer questions about how you feel. In the second part, you'll take daily doses for 28 days. We'll check how safe ALTO-203 is and how your body reacts to it. To join, you must have MDD, not be on antidepressants, and be willing to follow the study rules. You cannot join if you have other mental health issues, like bipolar disorder, or if you are using certain other medicines.

• Participation involves taking medicine and answering some questions.
• The study lasts 28 days with regular check-ups.
• It's important to not use certain other medicines during the study.