ENHANCEMENT EFFECT OF O-6-FLUOROBENZYLGUANINES ON CHLOROETHYLNITROSOUREA CYTOTOXICITY IN TUMOR-CELLS

O-6-Alkylguanine derivatives sensitize tumor cells to chloroethylnitro sourea (CENU) chemotherapy by inactivation of O-6-methylguanine-DNA me thyltransferase (MGMT), which repairs CENU-induced O-6-alkylguanines i n DNA by accepting the alkyl group at a cysteine moiety. To test the b iological significance of synthesized O-2-fluorobenzylguanine derivati ves, we measured their ability of inactivation of MGMT activity and th eir effects on the cytotoxicity of 1-(4-amino-2-methyl-5-pyrimidinyl)m ethyl- 3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) in compari son with the effects of O-6-benzylguanine and O-6-phenylguanine. The O -6-(4- and 3-fluorobenzyl)guanines considerably reduced the MGMT activ ity of HeLa S3 cell-free extract as did O-6-benzylguanine. In contrast , O-6-(2-fluorobenzyl)guanine and O-6-phenylguanine had less of an eff ect on the activity. Two-hour pretreatment of O-6-(4- and 3-fluorobenz yl)guanines potentiated ACNU cytotoxicity in HeLa S3 cells to a greate r extent than did O-6-(2-fluorobenzyl)guanine and O-6-phenylguanine. T he enhancement effects were consistent with the depletion of MGMT acti vity after the pretreatment of O-6-fluorobenzylguanine derivatives. O- 6-Fluorobenzylguanines with a fluoro-substitution at the 4- or 3-posit ion of the benzyl group were comparable to O-6-benzylguanine and were powerful MGMT inactivators. The chemical features of the O-6-benzyl gr oup are a biologically important determinant in the reaction evolution with MGMT.