busulfan + fludarabine + anti-thymocyte globulin + total lymphoid irradiation

This regimen is a combination of three pharmaceutical agents—busulfan, fludarabine, and anti-thymocyte globulin (ATG)—with total lymphoid irradiation (TLI), used as a conditioning protocol prior to allogeneic hematopoietic stem cell transplantation. Each component has a distinct mechanism: - **Busulfan** is an alkylating agent that induces DNA cross-linking and cell death, primarily targeting rapidly dividing hematopoietic cells. - **Fludarabine** is a purine analog that inhibits DNA synthesis by interfering with DNA polymerase and ribonucleotide reductase, leading to immunosuppression and cytotoxicity in lymphocytes. - **Anti-thymocyte globulin** (ATG) is a polyclonal antibody preparation targeting human T cells, resulting in profound T-cell depletion via complement-mediated lysis and opsonization. - **Total lymphoid irradiation** delivers targeted radiation to the lymphatic system to further deplete immune cells. The combination aims for effective myeloablation or immunosuppression while reducing toxicity compared to traditional regimens. It is primarily used as preparative therapy for patients undergoing allogeneic stem cell transplantation for conditions such as severe combined immunodeficiency (SCID), other severe T-cell immune deficiencies, acute myeloid leukemia (AML), myelodysplastic syndromes (MDS), chronic myeloid leukemia (CML), or other hematologic malignancies[1][2][5]. The regimen may be tailored in intensity depending on patient factors.