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 (single or recurrent episode as defined by DSM-5 TR \[if single episode, duration of ≥4 weeks and ≤24 months\] and established as per evaluation by the Investigator. The first MDD episode must have occurred prior to age 60. * Moderate to severe depression at Screening and Baseline, independently confirmed. * Participants have been on a stable dose of antidepressant medication (label specified) at an adequate dose in the last 4 weeks prior to Screening and has had an inadequate response (less than 50% improvement), as judged by the Investigator. * Participant has a body mass index (BMI) of 40 kg/m2 or less (BMI ≤40 kg/m2), inclusive, at Screening. * Participant is able to refrain from nicotine use during the dosing session (up to 8 hours). * Participants capable of producing sperm must use a condom plus spermicide during the trial and for 12 weeks after their final dose of trial medication, if their partner is a person of childbearing potential. * Participants of childbearing potential who have a partner capable of producing sperm must agree to use a highly effective method of contraception in combination with the use of a condom plus spermicide during the trial and for 12 weeks after their final dose of trial medication. Such participants must have a negative pregnancy test at Screening and Day 1 prior to dosing. * Part…