A Study of a Deuterated Psilocin Analog (CYB003) in Humans With Major Depressive Disorder

Inclusion Criteria: * Participants must meet all the following criteria to be included in the trial: * Age18 to 85 years. * Participant has a diagnosis of MDD. * Moderate to severe depression at Screening. * Participants have been on a stable dose of antidepressant medication at an adequate dose in the last 4 weeks prior to Screening. * Participant has a body mass index (BMI) of 40 kg/m2 or less (BMI ≤40 kg/m2), inclusive, at Screening. * Participants with well controlled hypertension. * Participant is able to refrain from smoking during the dosing session. * Participants must use a condom plus spermicide during the trial and for 12 weeks afterwards. Participants of childbearing potential must agree to use a highly effective method of and a negative pregnancy test at Screening and Day -1 prior to dosing. * Participants of non-childbearing potential who are or were capable of producing eggs (ova) must have been postmenopausal or permanently sterile following hysterectomy, bilateral salpingectomy, or bilateral oophorectomy. * Participants have provided written informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form. Exclusion Criteria * Participants with any of the following characteristics/conditions will be excluded from trial participation: * Current or previously diagnosed schizophrenia spectrum or other psychotic disorders. * Family history of schizophrenia, schizoaffective disorder, or bipolar disorder type 1 (first-degree relatives). * Significant suicide risk within 12 months of Screening. * Current or previous diagnosis of treatment-resistant MDD. * Has had electroconvulsive treatment, transcranial magnetic stimulation, deep brain stimulation, or vagal nerve stimulation for any episode of MDD in the last 6 months. * Currently receiving a monoamine oxidase inhibitor, tricyclic antidepressants, mirtazapine, trazodone, moclobemide, buspirone, or an antipsychotic or mood stabilizer. * Clinically relevant history of abnormal physical health interfering with the trial (including but not limited to, neurological, cardiovascular, respiratory, gastrointestinal \[including dyspepsia or gastroesophageal reflux disease\], hepatic, or renal disorder). * Has hypothyroidism or hyperthyroidism, unless controlled on appropriate medication. * Current diagnosis of uncontrolled hypertension or an arrhythmia, or clinically relevant abnormal results for heart rate. * Participants have a presence or relevant history of organic brain disorders. * Participant is taking or has taken OTC doses of 5 hydroxytryptophan or St John's Wort within prior to trial medication administration. * Strenuous exercise prior to each clinic visit. * Donation of blood or plasma within 4 weeks prior to first dosing and until 4 weeks after final dosing. * Participants capable of producing sperm who will not abstain from sperm donation between first dosing and 12 weeks after final dosing. * Participants of childbearing potential who are pregnant, breastfeeding, planning to conceive or unwilling to abstain from egg (ova) donation between first dosing and 12 weeks after final dosing. * History of serotonin syndrome. * Unwilling to consent to audio and video recording of psychological support and dosing sessions.