Comparing Post-Transplant Cyclophosphamide As GVHD Prophylaxis to Standard of Care for Acute Leukemia Patients

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.