This randomized clinical trial will evaluate two approaches of GvHD prophylaxis; the standard of care GVHD prophylaxis regimen (methotrexate/calcineurin inhibitors) and post-transplant cyclophosphamide with calcineurin inhibitors for their efficacy as a new GVHD prophylaxis strategy.

An open-label randomized clinical trial will be performed. Eligible patients are females and males, between 14 and 65 years undergoing allo-HCT for treatment of Acute Leukemia (AML or ALL). Patients must have a matching related peripheral blood stem cell donor. Patients from or referred to the KFSH&RC for allogeneic transplant consideration will be offered the opportunity to participate in this trial.

Treatment Description:

Patients will be randomized on one of the arms; an intervention or a standard of care arm.

Inclusion Criteria:

• Patients with Acute Leukemias (AML, ALL) in morphologic complete remission with or without hematologic recovery
• Patients must have a fully matched (8/8) related donor willing to donate peripheral blood stem cells and must meet institutional criteria for donation
• Planned Myeloablative conditioning regimen
• Cardiac function: ejection fraction at rest ≥ 50% by MUGA or TTE
• Estimated creatinine clearance greater than 50 mL/minute
• Pulmonary function: DLCO ≥ 50% (adjusted for hemoglobin), and FVC and FEV1 ≥ 50%
• Liver function: total bilirubin < 2x the upper limit of normal (unless elevated bilirubin is attributed to Gilbert's Syndrome) and ALT/AST < 2.5x the upper normal limit
• Signed informed consent

Exclusion Criteria:

• Karnofsky or Lansky Performance Score < 70%.
• Active disease
• Patients with uncontrolled bacterial, viral, or fungal infections
• Presence of fluid collection (ascites, pleural or pericardial effusion) that interferes with methotrexate clearance or makes methotrexate use contraindicated
• Patients seropositive for HIV-1 or -2
• Patients seropositive for HTLV-I or -II
• Patients with active Hepatitis B or C viral replication by PCR
• Women who are pregnant (positive serum or urine βHCG) or breastfeeding
• Females with childbearing potential (FCBP) or men who have sexual contact with FCBP unwilling to use effective forms of birth control or abstinence for one year after transplantation
• History of uncontrolled autoimmune disease or on active treatment
• Patients with prior malignancies, except resected non-melanoma skin cancer or treated cervical carcinoma in situ; cancer treated with curative intent ≥ 5 years previously will be allowed; cancer treated with curative intent < 5 years previously will not be allowed.