Key Inclusion Criteria:
1. Patient is male or female, of any ethnic origin.
2. Patient is aged between 18 to 65 years, inclusive.
3. Patient has a body mass index (BMI) of 18.0 to 35.0 kg/m2, inclusive.
4. Patient is ≥50 kg.
5. Patient meets the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) diagnostic criteria for recurrent MDD without psychotic features based on the Mini-International Neuropsychiatric Interview (MINI) at Screening. Comorbid anxiety disorders (e.g., social anxiety disorder, panic disorder, generalised anxiety disorder, specific phobia, agoraphobia) and cluster C personality disorders (avoidant, dependent and obsessive-compulsive) are allowed, provided that MDD is considered the primary diagnosis.
6. Current moderate to severe MDD as confirmed with a MADRS-SIGMA total score \>22 and CGI-S score \>3 at Screening and Day -1.
7. Patient is either not currently taking antidepressants (and hasn't for at least 6 weeks prior to Screening) or is being treated with an SSRI or SNRI antidepressant drug according to national guidelines during the current MDD episode.
a. If the patient is currently being treated with SSRI or SNRI antidepressants, these have been prescribed at a stable dose and the dose has remained unchanged for at least 6 weeks prior to Screening. However, the following medications are not permitted during the study at any time: NMDA receptor antagonists (including ketamine, esketamine) and 5-HT2A receptor agonists (including psilocybin, DMT, 5-MeO-DMT). No augmentation strategies will be permitted.
8. Changes in current drug treatment or psychological treatment for depression are not foreseen for the duration of the study.
Key Exclusion Criteria:
1. Current or recent history of clinically significant suicidal ideation or behaviours as defined by:
1. Suicidal ideation as endorsed on items 4 or 5 on the C-SSRS within 6 months prior to Screening, or
2. Suicidal behaviours within 1 year prior to Screening, or
3. Clinical assessment of significant suicidal risk. Patients with a prior suicide attempt of any sort, or prior serious suicidal ideation/plan \>6 months ago, should be carefully screened for current suicidal ideation and only included at the discretion of the Investigator.
2. Involuntary psychiatric hospitalisation in the current episode. Previous involuntary psychiatric hospitalisation should be carefully considered and only included at the discretion of the Investigator.
3. Lifetime diagnosis of any DSM-5 psychotic disorders, bipolar or related disorders, post-traumatic stress disorder (PTSD), complex-PTSD and borderline personality disorder. Other psychiatric disorders besides MDD should not be the primary disorder.
4. Patient has failed previous treatment with rapidly acting antidepressant drugs, such as NMDA receptor antagonists (e.g., ketamine, esketamine) or 5-HT2A receptor agonists (e.g., psilocybin, DMT, 5-MeO-DMT) or neuromodulating treatments, such as electroconvulsive therapy, transcranial magnetic stimulation, vagus nerve stimulation, or deep brain stimulation.
5. Patient is currently or has recently (within 6 weeks prior to Day 1) been treated with antipsychotic medication.
6. Use of psychoactive substances (including ketamine, esketamine or psychedelics, excluding cannabis) during the 6 weeks prior to Screening.
