SPL026 With or Without SSRIs in Participants With MDD

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