fludarabine + OKT3 + treosulfan + thiotepa

This is a multi-agent combination regimen consisting of four drugs: - **Fludarabine** is a purine analog antimetabolite that inhibits DNA synthesis, primarily used as an antineoplastic and immunosuppressive agent. It induces cytotoxicity in lymphocytes and is commonly used in conditioning regimens for hematopoietic stem cell transplantation and the treatment of hematologic malignancies such as chronic lymphocytic leukemia (CLL) and low-grade lymphomas[1][5][7]. - **OKT3** (muromonab-CD3) is a murine monoclonal antibody targeting the CD3 receptor on T cells. It acts by blocking T cell function, leading to profound immunosuppression, and was historically used to prevent or treat acute organ transplant rejection[6][9]. Its use has been largely discontinued in many regions due to adverse effects. - **Treosulfan** is an alkylating agent with both myeloablative and immunosuppressive properties. It crosslinks DNA strands, leading to apoptosis of rapidly dividing cells including hematopoietic progenitors. Treosulfan is approved for use in conditioning regimens prior to allogeneic stem cell transplantation, especially in patients with acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS)[7]. - **Thiotepa** is another alkylating agent that crosslinks DNA, inhibiting replication and transcription. It has been used for various cancers (breast, ovarian, bladder), but also as part of high-dose chemotherapy/conditioning regimens before bone marrow or stem cell transplantation[10]. The combination of these agents provides potent cytoreduction and immunoablation prior to allogeneic hematopoietic stem cell transplantation—especially for patients at higher risk who require intensified conditioning but may not tolerate standard regimens.