Enhancement effect of O6-fluorobenzylguanines on chloroethylnitrosourea cytotoxicity in tumor cells.

O6-Alkylguanine derivatives sensitize tumor cells to chloroethylnitrosourea (CENU) chemotherapy by inactivation of O6-methylguanine-DNA methyltransferase (MGMT), which repairs CENU-induced O6-alkylguanines in DNA by accepting the alkyl group at a cysteine moiety. To test the biological significance of synthesized O6-fluorobenzylguanine derivatives, we measured their ability of inactivation of MGMT activity and their effects on the cytotoxicity of 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) in comparison with the effects of O6-benzylguanine and O6-phenylguanine. The O6-(4- and 3-fluorobenzyl)guanines considerably reduced the MGMT activity of HeLa S3 cell-free extract as did O6-benzylguanine. In contrast, O6-(2-fluorobenzyl)guanine and O6-phenylguanine had less of an effect on the activity. Two-hour pretreatment of O6-(4- and 3-fluorobenzyl) guanines potentiated ACNU cytotoxicity in HeLa S3 cells to a greater extent than did O6-(2-fluorobenzyl)guanine and O6-phenylguanine. The enhancement effects were consistent with the depletion of MGMT activity after the pretreatment of O6-fluorobenzylguanine derivatives. O6-Fluorobenzylguanines with a fluoro-substitution at the 4- or 3-position of the benzyl group were comparable to O6-benzylguanine and were powerful MGMT inactivators. The chemical features of the O6-benzyl group are a biologically important determinant in the reaction evolution with MGMT.