The purpose of this study is to evaluate the efficacy of NBI-1065845 compared with placebo in participants with MDD on improving symptoms of depression.

Key Inclusion Criteria:

Participants must meet all of these criteria for inclusion in the study:

1. The participant has completed written informed consent.
2. At the time of signing the informed consent, participant must be 18 to 65 years of age, inclusive.
3. The participant has a primary diagnosis of recurrent Major Depressive Disorder (MDD) or persistent depressive disorder.
4. Participant must have had inadequate response to antidepressant treatment.
5. Participant must have a Total Hamilton Depression Rating Scale-17 Item (HAMD-17) score ≥ 22 at screening.
6. Participants currently receiving pharmacological treatment for depression must have been taking current antidepressant medication(s) for ≥8 weeks prior to screening. Subjects not currently receiving pharmacologic treatment for depression must have received the most recent antidepressant medication(s) for ≥8 weeks in the current episode of depression.
7. Participants must be willing and able to comply with all study procedures.

Key Exclusion Criteria:

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

1. Participant is pregnant or breastfeeding or plans to become pregnant during the study.
2. Participant has an unstable medical condition or unstable chronic disease.
3. Participant has a history of neurological abnormalities.
4. Participant has a current or prior psychiatric disorder that was the primary focus of treatment other than MDD.
5. The participant's depressive symptoms have previously demonstrated nonresponse to an adequate course of treatment with electroconvulsive therapy (ECT).
6. The participant has an alcohol or substance use disorder.
7. In the Investigator's opinion, the participant is not capable of adhering to the protocol requirements.