Study to Assess the Efficacy and Safety of NBI-1065845 as an Adjunctive Treatment in Participants With Major Depressive Disorder (MDD)

Key Inclusion Criteria: * Participant has a primary diagnosis of recurrent MDD (moderate or severe) or persistent depressive disorder. * Participant has had an inadequate response to oral antidepressant treatments in the current episode of depression. * Participant must have been taking oral antidepressants for at least 8 weeks and is willing to continue the same oral antidepressants at the same dose and frequency of administration throughout participation in the study. * Total Hamilton Depression Rating Scale-17 Item (HAM-D17) score ≥22 at screening and at study baseline (Day 1). * Willing and able to comply with all study procedures and restrictions in the opinion of the investigator. Key Exclusion Criteria: * A current or prior psychiatric disorder diagnosis in the last 1 year that was the primary focus of treatment other than MDD. * Are considered by the investigator to be at imminent risk of suicide or injury to self or others. * Participants depressive symptoms have previously demonstrated nonresponse to electroconvulsive therapy (ECT) in the current major depressive episode.