Inclusion Criteria:
Main selection criteria at screening visit:
- The patient has a Major Depressive Episode (MDE) associated to Major Depressive Disorder (MDD) diagnosed according to Diagnostic and Statistical Manual of Mental Disorders, 4th edition, Text Revision (DSM-IV-TR™). The current MDE should be confirmed using the Mini International Neuropsychiatric Interview (MINI)
- The patient has an inadequate response to at least one antidepressant treatment (including the treatment the patient is taking at screening) in the current MDE, as documented by self-report as less than a pre-defined percentage response on the Massachusetts General Hospital Antidepressant Treatment Response Questionnaire (ATRQ).
- Pre-defined MADRS total score
- Pre-defined CGI-S total score
- The patient has had the current MDE for ≥10 weeks
- The patient is receiving a SSRI or SNRI antidepressant treatment for ≥6 weeks, at same dosage for ≥2 weeks
- Pre-defined Inventory of Depressive Symptomatology - Clinician-Rated - 30 items (IDS-C 30) Item 6 Mood (irritable) score
Main inclusion criteria at baseline visit:
- The patient still fulfils DSM-IV-TR™ criteria for MDE
- Pre-defined MADRS total score
- Pre-defined CGI-I (Lead-in period) score
- The patient received the same SSRI or SNRI antidepressant treatment at adequate doses during the entire lead-in period
- The patient has less than a pre-defined percentage decrease in MADRS score at the end of the lead-in period as compared to the screening visit
- The patient still has the pre-defined IDS-C 30 Item 6 Mood (Irritable) score
Exclusion Criteria:
- The patient has any current psychiatric disorder or Axis I disorder (DSM-IV-TR™ criteria), established as the principal diagnosis, other than MDD. All anxiety disorders, impulse-control disorders such as intermittent explosive disorder, as well as non-suicidal self-injury are not excluded diagnoses as far as they are not considered as principal diagnosis.
- The patient has a current Axis II (DSM-IV-TR™) diagnosis of antisocial, borderline, paranoid, schizoid, schizotypical, or histrionic personality disorder.
- The patient has experienced/experiences hallucinations, delusions, or any psychotic symptomatology in the current MDE.
- The patient, in the opinion of the investigator or based on C-SSRS rating, is at significant risk of suicide.
- The patient has started formal cognitive or behavioural therapy or systematic psychotherapy within 6 weeks prior to screening, or plans to start such therapy during the study. Any ongoing formal psychotherapy initiated more than 6 weeks prior to screening should be continued with the same methodology and at the same frequency and intensity during the entire study.
- The patient has a current diagnosis or history of substance abuse or dependence (excluding nicotine or caffeine) or alcohol abuse or dependence (DSM-IV-TR™ criteria) <6 months prior to the Screening Visit.
- The patient reports adjunctive treatment with an antipsychotic medication together with an antidepressant for 3 weeks or more during the current MDE.
- The patient has received electroconvulsive therapy (ECT) <6 months prior to the Screening Visit or at any time during the current MDE (if its duration is longer than 6 months).
- The patient has had vagus nerve stimulation or a deep brain stimulation device implanted for the management of depression.
Other inclusion and exclusion criteria may apply.