EXPERIMENTAL TUMOR-THERAPY IN MICE USING THE CYCLOPHOSPHAMIDE-ACTIVATING CYTOCHROME-P450 2B1 GENE

Most malignant tumors of the central nervous system do not respond wel l to chemotherapy. The anticancer drug cyclophosphamide (CPA) is large ly ineffective against these neoplasms as its-conversion to DNA-alkyla ting, cytotoxic metabolites is restricted primarily to the liver and t hese metabolites do not readily cross the blood-brain barrier. Here, w e show that brain tumor cells can be sensitized to the cytotoxic effec ts of CPA, both in culture and in vivo, by introduction of the hepatic enzyme responsible for the activation of CPA, cytochrome P450 2B1. St able transfection of rat C6 glioma cells with the P450 2B1 gene render ed the cultured tumor cells sensitive to CPA. Further, C6 cells bearin g this gene were more sensitive than parental cells to the cytotoxic a ction of CPA when grown subcutaneously in the flanks of athymic mice. Murine fibroblasts producing a retrovirus vector encoding P450 2B1 and expressing this enzyme were then prepared and grafted into the brains of athymic mice seeded with rat C6 gliomas. Intrathecal administratio n of CPA prevented the development of meningeal neoplasia and led to p artial regression of the parenchymal tumor mass. By contrast, C6 gliom a-bearing mice receiving fibroblasts expressing the Escherichia coil l acZ gene and CPA exhibited extensive meningeal tumors and parenchymal solid brain tumors. The in situ activation of CPA by cytochrome P450 2 B1 provides a novel approach not only for brain tumor gene therapy, bu t also for negative, drug-conditional selection of other defined cell populations.