To evaluate the efficacy, safety, and tolerability of NBI-1070770 compared to placebo on improving symptoms of depression in participants with major depressive disorder (MDD).

Key Inclusion Criteria:

Participants must meet all of the following inclusion criteria:

• Primary diagnosis of recurrent MDD or persistent MDD.
• Participants currently receiving pharmacological treatment for depression must have been taking current antidepressant medication(s) for ≥8 weeks prior to screening and must be willing and able to continue with current antidepressant medication(s).
• Participants must be willing and able to comply with all study procedures and restrictions.

Key Exclusion Criteria:

Participants will be excluded from the study if they meet any of the following criteria:

• Participant is pregnant or breastfeeding or plan to become pregnant during the study.
• Participant has an unstable medical condition or unstable chronic disease.
• Participant has a history of neurological abnormalities.
• Participant has a current or prior psychiatric disorder diagnosis that was the primary focus of treatment other than MDD.
• Participant's depressive symptoms have previously demonstrated nonresponse to an adequate course of treatment with electroconvulsive therapy.
• Participant has an alcohol or substance use disorder.

Note: Other inclusion/exclusion criteria may apply