Inclusion Criteria:  Recurrent MDD diagnosis with a current episode duration of at least 8 weeks and no more than 24 months. HAM-D17 total score of ≥23 at Screening and Baseline. Body mass index (BMI) between 18 and 38 kg/m2 (inclusive).  Exclusion Criteria:  Lifetime history of seizures, including febrile seizures. Neurodegenerative disorder (eg, Alzheimer's disease, Parkinson's disease, multiple sclerosis, or Huntington's disease). Lifetime history of bipolar disorder, a psychotic disorder (eg, schizophrenia or schizoaffective disorder), or obsessive compulsive disorder or a history of a psychotic mood episode in last 2 years. Any current psychiatric disorder (other than MDD). Lifetime history of treatment resistant depression. Received electroconvulsive therapy (ECT) or vagus nerve stimulation (VNS) within the last year or transcranial magnetic stimulation (TMS) within the last 6 months prior to Screening. Daily consumption of more than 2 standard alcohol-containing beverages for males or more than 1 standard alcohol-containing beverages for females.