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

Inclusion Criteria:

• Age ≥ 50 years old

• Presence of a hematologic malignancy for which an alloHCT is indicated with areduced 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 toinclusion

• Karnofsky index ≥ 70%

• Availability of a sibling donor, an unrelated stem-cell donor or a familialhaploidentical donor

• Written informed consent

Exclusion Criteria:

• Patients planned to receive a non-myeloablative conditioning regimen (2 Gray totalbody irradiation (TBI) +/- purine analog, fludarabine + cyclophosphamide orequivalent)

• 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/kgIV) + 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 tophysician'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 fullparticipation in the study, including administration of study drug and attendingrequired study visits; pose a significant risk to the subject; or interfere withinterpretation of study data.

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