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 ceils 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 ‐benrylhypoxanthine on the sensitivity of tumor cell lines to I‐(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 μM 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 ‐Benzylhypoxanthine at the high concentration (100 μM) 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 on enhancing ACNU cytotoxicity 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.