Inclusion Criteria:  Established diagnosis of Major Depressive Disorder (MDD) as confirmed at the time of screening by the Mini International Neuropsychiatric Interview (MINI), with a duration of current depressive episode ≥ 8 weeks and ≤ 18 months at the time of screening visit. At least moderate severity of MDD confirmed by a trained site-based rater, at screening and Visit 2 day 1 (i.e. prior to randomisation). In the current episode, patients have shown insufficient treatment response (defined by less than 50 % response to one or at maximum to two antidepressant drugs of adequate dose and treatment duration (according to Summary of Product Characteristics) as evaluated by Antidepressant Treatment Response Questionnaire (ATRQ). Documented ongoing monotherapy treatment of ≥ 8 weeks at randomisation (i.e. Visit 2 day 1) with a protocol specified Selective Serotonin Reuptake Inhibitors (SSRI) or Serotonin-Norepinephrine Reuptake Inhibitors (SNRI) at adequate dose (according to ATRQ assessment and as confirmed per detectable drug levels in the screening blood sampling at Visit 1). Male or female patients, aged 18 to 65 years at screening. If Women of childbearing potential (WOCBP) are included, they have to be able and willing to use two methods of contraception which include one highly effective method of birth control per ICH M3 (R2) that result in a low failure rate of less than 1 %, plus one additional barrier method. If men who are able to father a child, are willing to participate, they have to use an adequate form of effective contraception for the duration of study participation and for at least 28 days after treatment has ended. Signed and dated written informed consent in accordance with ICH-GCP and local legislation prior to admission to the trial.  Exclusion Criteria:  Patient had met diagnostic criteria per Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) for schizophrenia, schizoaffective disorder, schizophreniform disorder, bipolar disorder, delusional disorder or MDD with psychotic features at any time point in the patient's life. Diagnosis with antisocial, paranoid, schizoid, schizotypical or Borderline personality disorder as per DSM-5 criteria at the time of screening visit. Any other personality disorder at screening visit that significantly affects current psychiatric status and is likely to impact trial participation, as per the judgement of investigator. Diagnosis of any other mental disorder (in addition to those as described in Exclusion Criterion #1 and #2) that was the primary focus of treatment within 6 months prior to screening, as per clinical discretion of the investigator. Patients with a Body Mass Index (weight [kg]/height [m]²) lower than 18 kg/m² at screening. Diagnosis of a moderate to severe substance-related disorder within the last 6 months before screening visit (with exception of caffeine and tobacco). Use of Ketamine/S-Ketamine for the current ongoing depressive episode. Stable treatment with benzodiazepines and/or nonbenzodiazepine hypnotics. (Note: As needed (PRN) use of benzodiazepines and/or nonbenzodiazepine hypnotics may be acceptable except for the trial period Day -1 to Day 2).  Further exclusion criteria apply