RETROVIRAL TRANSFER OF A BACTERIAL ALKYLTRANSFERASE GENE INTO MURINE BONE-MARROW PROTECTS AGAINST CHLOROETHYLNITROSOUREA CYTOTOXICITY

The chloroethylnitrosoureas (CENUs) are important antineoplastic drugs for which clinical utility has been restricted by the development of severe delayed myelosuppression in most patients. To investigate the p otential of DNA repair proteins to reduce bone marrow sensitivity to t he CENUs, we transferred the Escherichia coli ada gene, which encodes a M(r) 39,000 O-6-alkylguanine-DNA alkyltransferase (ATase), into muri ne bone marrow cells by the use of a high-titer ecotropic retrovirus, The ada-encoded ATase is resistant to O-6-benzylguanine (O-6-BG), a po tent inhibitor of the mammalian ATases, thus affording the bone marrow an additional level of protection against CENUs, In methylcellulose c ultures, ada-infected hematopoietic progenitor cells were twice as res istant as uninfected cells to the toxic effects of 1,3-bis(2-chloroeth yl)-1-nitrosourea (BCNU) following treatment with O-6-BG. Although sho wing no obvious protective effects against leukopenia, overexpression of the bacterial ATase activity reduced the severity of anemia and thr ombocytopenia in mice treated with O-6-BG and BCNU. These effects, whi ch were maximal at a BCNU dose of 12.5 mg/kg, mere associated with imp roved survival when BCNU was given at this dose, At lower BCNU doses c ytotoxicity was limited in both transduced and control mice, and at hi gher doses the protective effect was saturated due to cytotoxicity. Th ese results suggest that ada gene therapy may be a feasible approach t o amelioration of delayed myelosuppression following O-6-BG plus CENU combination chemotherapy.