ENHANCING EFFECT OF O-6-ALKYLGUANINE DERIVATIVES ON CHLOROETHYLNITROSOUREA CYTOTOXICITY TOWARD TUMOR-CELLS

O-6-Alkylguanine derivatives are well known as chemical modulators of the DNA repair enzyme O-6-methylguanine-DNA methyltransferase (MGMT). Depletion of the enzyme by these derivatives leads to increase sensitivity of tumor cells to chloroethylnitrosoureas. We tested the effect of O-6-methylguanine, O-6-benzylguanine, O-6-(p-methylbenzyl)guanine, O-6-(p-chlorobenzyl)guanine, O-6-(p-methoxybenzyl)guanine, O-6-methylhypoxanthine and O-6-benzylhypoxanthine on the sensitivity of tumor cell lines to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3- nitrosourea hydrochloride (ACNU) using a colorimetric cytotoxicity assay. The sensitivity of MGMT-proficient tumor cells including HeLa S3, C6-1, C6-2/ACNU, U-138 MG and U-373 MG cells was greatly enhanced by 2 hr pretreatment of 10-100 mu M O-6-benzylguanine, O-6-(p-methylbenzyl)guanine and O-6-(p-chlorobenzyl)guanine, but not by O-6-methylguanine or O-6-methylhypoxanthine. O-6-(p-methoxybenzyl)guanine moderately sensitized the 2 cell lines, HeLa S3 and C6-1, tested in our study to ACNU cytotoxicity. O-6-Benzylhypozanthine at the high concentration (100 mu M) sensitized, to some extent, 3 MGMT-proficient cell lines. Lesser degrees of enhancement by the O-6-benzylguanine derivatives were noted in MGMT-deficient tumor cells. Biological effects of O-6-alkylguanine derivatives in enhancing ACNU cytoxicity of tumor cells suggest that the exocyclic 2-amino and O-6-benzyl groups in O-6-benzylguanine skeleton are both essential for the inhibition of MGMT activity. (C) 1994 Wiley-Liss, Inc.