Psilocybin and Depression - Assessing the Long-term Effects of a Single Administration of Psilocybin on the Psychiatric Symptoms and Brain Activity of Patients With Severe Depression

The main aim of the study is to investigate the possible long-term therapeutic effects of psilocybin on the symptoms of severe depression, as well as the brain mechanisms underlying these changes. Depression severity is assessed before and after (i.e., 1 week, 3 months and 6 months after) a single dose of psilocybin and compared to respective scores of a group receiving an active placebo, ketamine. Brain activity (using functional magnetic resonance imaging) is measured before and one week after drug administration in order to determine whether changes in brain networks related to emotional and self-referential processing correlate with any observed changes in depression scores. Further, blood samples will be obtained from the participants and analyzed in order to reveal gene expression and molecular level correlates underlying rapid antidepressant effects, and to identify biomarkers that predict treatment outcome.