Oral Pooled Fecal Microbiotherapy to Prevent Allogeneic Hematopoietic Cell Transplantation Complications (PHOEBUS Trial)

- Age ≥ 50 years old

- Presence of a hematologic malignancy for which an alloHCT is indicated with a reduced toxicity or reduced intensity conditioning regimen

- Patients with polynuclear neutrophils > 0.5 G/L

- Patients having received wide spectrum antibiotics within the last 90 days prior to inclusion

- Karnofsky index ≥ 70%

- Availability of a sibling donor, an unrelated stem-cell donor or a familial haploidentical donor

- Written informed consent

- Patients planned to receive a non-myeloablative conditioning regimen (2 Gray total body irradiation (TBI) +/- purine analog, fludarabine + cyclophosphamide or equivalent)

- Patients planned to receive a conventional myeloablative conditioning regimen (e.g. high dose cyclophosphamide and high dose TBI (≥10Gy); high dose busulfan (12.8 mg/kg IV) + high dose cyclophosphamide)

- Patients receiving a manipulated graft (in-vitro T-cell depletion)

- Patients planned to receive a conditioning regimen with alemtuzumab

- Patients planned to receive alloHCT with cord blood cells

- Patients planned to receive alloHCT from unrelated donor with >= 3/10 HLA-mismatches

- Patients receiving a large spectrum antibiotic at time of randomization

- Patients planned to receive vedolizumab or abatacept for GvHD prophylaxis

- Creatinine clearance <30 mL/min

- Bilirubin or amino-transferases abnormalities contra-indicating alloHCT

- Cardiac ejection fraction less than 40%

- Pulmonary impairment with <50% lung carbon monoxide diffusing capacity (DLCO)

- Pregnancy

- Confirmed or suspected intestinal ischemia

- Confirmed or suspected toxic megacolon or gastrointestinal perforation

- Any history of gastro-intestinal surgery in the past 3 months

- Any history of chronic digestive disease (Crohn's disease, ulcerative colitis, inflammatory bowel disease or other relevant digestive condition according to physician's judgement)

- Known allergy or intolerance to trehalose or maltodextrin

- Patients with EBV-IgG negative serology

- Any condition that would, in the investigator's judgment, interfere with full participation in the study, including administration of study drug and attending required study visits; pose a significant risk to the subject; or interfere with interpretation of study data.

- Vulnerable patients such as: persons deprived of liberty, persons in Intensive Care Unit unable to provide informed consent prior to the intervention.