Key Inclusion Criteria: - Patients designated to undergo allogeneic peripheral blood or bone marrow stem cell transplantation following the diagnosis of one of the primary diseases in early or intermediate disease status (i.e., acute myeloid leukemia, acute lymphoid leukemia, and myelodysplastic syndrome) - Patients with an unrelated HLA-A,-B, -C and -DRBI matched donor - Patients with a Karnofsky Performance Score ≥ 70% Key Exclusion Criteria: - Clinically significant concomitant diseases (i.e., cardiac, pulmonary, renal and CNS) - Bacterial, viral, or fungal infections - Known positive for Hepatitis B surfaces antigen, or Hepatitis C antibody, or who have been tested positive for HIV - Patients with any concurrent malignancy. Patients with prior malignancy being curatively treated and disease-free for less than 10 years - Known contraindications to the administration of rabbit immunoglobulin antibodies - Hypersensitivity to methylprednisolone, tacrolimus, methotrexate or any excipients contains in these products