Inclusion Criteria:  De novo patients  The patient has a primary diagnosis of MDD according to DSM-5™ although co-morbid anxiety disorders will be permitted (except Post Traumatic Stress Disorder (PTSD) and Obsessive Compulsive Disorder (OCD)). The patient has a CDRS-R total score ≥45 at the Screening and Baseline Visits. The patient has a Clinical Global Impression - Severity of Illness (CGI-S) ≥4 at the Screening and Baseline Visit  Exclusion Criteria:  The patient receives ongoing current psychotherapy that is planned to be intensified. Interpersonal psychotherapy (IPT) or cognitive behavioural therapy (CBT) are not allowed. The patient presents with, or has a history of, an Axis I (DSM-5TM) diagnosis of Bipolar Disorder, PTSD, OCD, Autism, Pervasive Developmental Disorder (PDD), or Schizophrenia or Schizoaffective Disorder. The patient has a diagnosis of attention-deficit/hyperactivity disorder (ADHD) and is not maintained on a stable dose of a methylphenidate or amphetamine for a minimum of 4 weeks prior to the study treatment. The patient has attempted suicide or is at significant risk of suicide  Other inclusion and exclusion criteria may apply.