THE EFFECTS OF O-6-BENZYLGUANINE AND HYPOXIA ON THE CYTOTOXICITY OF 1,3-BIS(2-CHLOROETHYL)-1-NITROSOUREA IN NITROSOUREA-RESISTANT SF-763 CELLS

O6-Alkylguanine-DNA alkyltransferase (AGT) activity is associated with resistance of brain tumor cell lines to the cytotoxic effects of 1,3- bis(2-chloroethyl)-1-nitrosourea (BCNU). SF-763 cells exhibit high AGT activity and are resistant to BCNU. In this study, we compared the ef fects of the AGT inhibitor O6-benzylguanine (BG) on the cytotoxicity o f BCNU in oxic and hypoxic SF-763 cells; we also measured AGT activity , ornithine decarboxylase (ODC) activity, and polyamine levels to dete rmine if there was any correlation with cell survival as determined by colony-forming efficiency assay. Exponentially growing monolayer cell s were pretreated with 10 muM BG for 2 h under oxic or hypoxic (95% ni trogen/5% CO2) conditions and then exposed to graded concentrations of BCNU for 1 h. BG significantly lowered AGT activity but had no cytoto xic effect in oxic or hypoxic cells; hypoxia alone was not cytotoxic. The cytotoxicity of BCNU was 4 times higher in BG-treated hypoxic cell s than in oxic cells treated with BCNU alone; the BCNU doses required for a 1-log cell kill were 75 and 300 muM, respectively. ODC activity was lowered by hypoxia alone but was not significantly affected by BG in either hypoxic or oxic cells. Polyamine levels were not significant ly affected by hypoxia or BG. These results indicate that pretreatment with BG dramatically lowers AGT activity and increases the cytotoxici ty of BCNU in both oxic and hypoxic SF-763 cells. The mechanism of thi s enhanced cytotoxicity is apparently unrelated to ODC activity or pol yamine levels.