Inclusion Criteria:  Recurrent MDD diagnosis with a current episode duration of at least 12 weeks and no more than 24 months. Inadequate response to treatment in the current MDD episode, defined as <50% reduction in depression severity in response to at least 1 and no more than 2 antidepressant trials at an adequate dose and duration in the current MDE as specified in the ATRQ. Stable regimen of antidepressant treatment for at least 8 weeks prior to Baseline, with no clinically meaningful change (defined as no increase in dose and no decrease in dose ≥25% for tolerability) during that period. 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.