fludarabine + cytarabine + busulfan + cyclophosphamide

This is a combination chemotherapy regimen consisting of four agents—fludarabine, cytarabine, busulfan, and cyclophosphamide—used primarily as a conditioning treatment before hematopoietic stem cell transplantation (HSCT) or for the management of certain hematologic malignancies such as leukemia. Each component has a distinct mechanism of action: - Fludarabine is a purine analog antimetabolite that inhibits DNA synthesis by interfering with DNA polymerase and ribonucleotide reductase. - Cytarabine is a pyrimidine analog that inhibits DNA synthesis by incorporating into DNA and inhibiting DNA polymerase. - Busulfan is an alkylating agent that causes cross-linking of DNA strands, leading to cell death. - Cyclophosphamide is also an alkylating agent that interferes with DNA replication and transcription through cross-linking. The combination leverages synergistic effects on malignant cells while providing immunosuppression necessary for successful engraftment in HSCT. This regimen may be used in high-dose or myeloablative settings to eradicate malignant cells and suppress the immune system prior to transplant[2][3][6].