The main aim of the study is to test the safety and tolerability of single doses of SPL026 (N,N-dimethyltryptamine [DMT] fumarate, a psychedelic tryptamine) in patients currently taking a selective serotonin reuptake inhibitor (SSRI) for their depression, but for whom the SSRI is not fully relieving their depression.

Inclusion Criteria:

• MDD diagnosis
• Previously tried at least one approved method of treatment for their depression
• No monoamine oxidase-inhibitor class antidepressants for at least 3 months
• Deemed healthy on the basis of a clinical history, physical examination, ECG, vital signs, laboratory tests of blood and urine
• No psychedelic drug use in the 6 months before dosing until the end of the study
• Willing to follow the contraception requirements of the trial
• Willing to be contacted by email and video call, and have online access
• Able to give fully informed written consent
• Test Cohort only: currently on a stable dose of an unspecified single SSRI alone and not in combination with any other psychiatric medications, for at least 6 weeks prior to Screening with no intention of making any changes
• Control Cohort only: no antidepressant medication for 6 months before dosing

Exclusion Criteria:

• Substance use disorder
• Current or clinically relevant history of a psychotic disorder, or first-degree relatives with a clinically relevant history of a psychotic disorder
• Significant history of mania
• Significant risk of suicide
• Clinically relevant abnormal findings at the screening assessment
• Blood pressure, heart rate, or QTcF outside the acceptable ranges
• Acute or chronic illness (other than MDD) or infection
• Clinically relevant abnormal medical history or concurrent medical condition (other than MDD)
• Use of any serotonergic psychedelics within 6 months prior to dosing
• Patients of child-bearing potential who are pregnant or lactating, or who are sexually active and not using a reliable method of contraception
• History of a severe adverse reaction to any drug or sensitivity to serotonergic psychedelic drugs
• Use of over-the-counter or prescribed medication (excluding oral contraceptives, hormone replacement therapy and SSRIs [Test Cohort only]) within previous 28 day before dose of trial medication; or paracetamol or ibuprofen within 4 hours prior to dosing