This study is checking how well a new drug, **NBI-1070770**, works and how safe it is for people with major depressive disorder (**MDD**). MDD is a condition that makes people feel very sad for long periods. The study will compare NBI-1070770 to a placebo, which looks like the real drug but doesn't have any medicine in it. To join, you need to have MDD and be on antidepressant medicines for at least 8 weeks before the study starts. You also need to agree to follow all the study rules. If you are pregnant or have certain medical problems, you can't join. Also, if your depression didn't get better with past treatments like shock therapy, you won't be able to join. The study will not accept people with alcohol or substance problems.

• The study might have several visits and procedures to check your health.
• You'll need to keep taking your current depression medicine during the study.
• There might be some risks, like side effects from the study drug.