This is a three-part Phase 2a study. The aim of Part A is to assess the safety and tolerability and preliminary antidepressant efficacy in patients with MDD who are not currently on an antidepressant therapy. The aim of Part B is to assess the antidepressant efficacy, safety and tolerability in patients with MDD who are partial responders while on a current and adequate single SSRI or SNRI treatment. Part C aims to replicate the monotherapy findings of Part A, but with a lower control group dose.

Key Inclusion Criteria:

1. Patients are male or female, of any ethnic origin.

2. Patients are aged between 18 to 65 years, inclusive.

3. Patients meet the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) diagnostic criteria for recurrent MDD without psychotic features, as assessed with the Mini-International Neuropsychiatric Interview (MINI) at Screening.

4. Current moderate to severe MDD diagnosis confirmed with a MADRS-SIGMA

5. Concomitant depression therapy:

   1. (Part A) Patients need to be stable and must not have taken any SSRI or SNRI for at least 6 weeks prior to Screening and must be willing to avoid starting a new pharmacological treatment for MDD until the end of the study procedures and assessments. Discontinuing current treatment is not allowed if done for the purposes of achieving eligibility for this study.
   2. (Part B) Patients need to be on stable treatment with any SSRI or SNRI for at least 6 weeks prior to screening and must be willing to remain on the SSRI or SNRI for the duration of the trial
   3. Patients receiving any form of psychotherapy or counselling must have been receiving therapy at Screening and must be willing to remain in therapy until the end of the study procedures and assessments.

Key Exclusion Criteria:

1. Patient has current or past primary DSM-5 diagnosis of a psychotic disorder or MDD with psychotic features, bipolar, or related disorders. A current diagnosis of PTSD, complex PTSD and borderline personality disorder are exclusionary. Other psychiatric disorders besides MDD should not be the primary disorder.
2. In first degree relatives, a history of schizophrenia, psychosis, bipolar disorder, delusional disorder, paranoid personality disorder or schizoaffective disorder.
3. Current or prior (six weeks before Screening) use of any SSRI/SNRI medication (Part A only).
4. Current or prior (five weeks before Screening) use of any monoamine oxidase inhibitor ([MAO-I]; including phenelzine, tranylcypromine, isocarboxazid, iproniazid, selegiline, rasagiline, the reversible MAO-I moclobemide and the antibiotic linezolid).or any tricyclic antidepressant