Conditioning Regimens

Conditioning regimens, also referred to as preparative regimens, are combinations of chemotherapy or radiation therapy designed to prepare the patient’s body to receive the donor’s bone marrow. The purpose of the conditioning regimen varies according to the type of stem cell transplant to be received by the patient. For autologous transplants, the conditioning regimen is designed so that increasing doses of chemotherapy with or without irradiation are administered to destroy more malignant cells. For allogeneic transplants, the conditioning regimen is designed not only to destroy residual malignant cells, but also to provide immunosuppression to prevent the patient’s body from rejecting the donor’s stem cells.

Historically, the most common types of conditioning regimens for patients receiving allogeneic stem cell transplants have been cyclophosphamide with total body irradiation or busulfan and cyclophosphamide. The conditioning regimens for those receiving autologous stem cell transplantation vary markedly among centers and, to some extent, on the basis of tumor type.

Today cyclophosphamide, cytarabine (Ara-C) and etoposide (VP-16) are chemotherapeutic drugs commonly used in conditioning regiments for patients receiving allogeneic transplants. Total body irradiation (TBI) is also commonly used in most of these regimens. Sometimes busulfan is used instead of TBI when patients cannot receive additional radiation therapy. In some cases, antibodies such as OKT3 or anti-thymocyte globulin (ATG) are administered to prevent graft rejection. These antibody preparations are designed to recognize T-cells, which are part of the immune system, and destroy them so that they cannot cause graft rejection.

For patients receiving autologous stem cell transplants, the conditioning regimens may consist of cyclophosphamide, etoposide (VP-16), carmustine (BCNU), cytarabine (Ara-C), melphalan, cisplatin, carboplatin, and busulfan. In many cases, total body irradiation before transplantation is not administered; instead, radiation to localized sites of disease is administered after autologous transplantation. This approach allows higher doses of radiation to be given to specific sites where most tumor cells were found before transplantation. As part of conditioning regimens, chemotherapeutic drugs are usually administered in doses that are 3 to 8 times higher than standard doses given without stem cell support. The increases in doses are permissible because the initial toxicity (destruction of the patient’s marrow) will be overcome by the transplantation of stem cells.

The side effects resulting from a conditioning regimen typically vary and depend on the chemotherapeutic drugs used, and whether radiation therapy is administered.