The primary objective will be to evaluate effectiveness, safety and acceptability of FMT on depressive symptoms at 2 months follow-up in SZ patients with resistant MD.

Background. Major Depression (MD) will move into the first place of cause of the global burden of disease by 2030. Conventional treatments (antidepressants) are only partly effective with frequent side effects. More specifically, MD is underdiagnosed and undertreated in schizophrenia (SZ) despite its high impact on prognosis and quality of life in SZ patients. Even when treated, 44% of the patients remain major depressed under conventional antidepressants.

FEMADSZ makes the hypothesis that this is due to the fact that the source of MD relies outside the brain, in the gut microbiota. The associations between microbiota disturbances are now well demonstrated in rodent models (>60 studies) and indirect data in humans clearly suggest that the microbiota hypothesis of MD is the most promising track in psychiatry. The primary objective will be to evaluate effectiveness, safety and acceptability of FMT on depressive symptoms at 2 months follow-up in SZ patients with resistant MD. Methods: This is a pilot, single-arm, prospective, open-label proof-of-concept trial, including SZ patients with resistant MD. Fifteen patients will be enrolled and administered a Fecal Microbiota Transplantation. The main outcome measure will be depressive score at 2 months, adverse events and acceptability. Each patient will receive a FDG-PET scan, a routine blood draw and a metagenomic microbiota analysis before and after intervention. Expected results. This pilot study will help determining the effectiveness of FMT and the neurobiologic and microbiologic mechanisms associated with response to treatment.

Inclusion Criteria:

• patients aged 18 to 65 years
• with the current diagnosis of shizophrenia

Exclusion Criteria:

• history of lifetime FMT
• History of gut surgery