Differential inactivation of O6‐methylguanine‐dna methyltransferase activity by O6‐arylmethylguanines

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Abstract
Activity of O6-methylguanine-DNA methyltransferase (MGMT) is well related with drug resistance of tumor cells to chloroethylnitrosoureas (CENUs), because MGMT removes CENU-induced O6-alkylguanines in DNA by accepting the alkyl group at a cysteine moiety. Inactivation of MGMT is a feasible way to overcome MGMT-related resistance of tumor cells to CENUs. O6-Benzylguanine is known to be a strong depleter of MGMT. We previously reported the potentiation effect of O6-arylmethylguanine derivatives on cytotoxicity of 1 -(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU), a CENU, for HeLa S3 cells. In this study, we tested the activity of these O6-arylmethylguanine derivatives as depleters of MGMT using HeLa S3 cell-free extract. The O6-arylmethylguanine derivatives tested were O6-benzylguanine (I), O6-(4-, 3-, and 2-fluorobenzyl)guanines (2–4), O6-(4, 3-, and 2-trifluoromethylbenzyl)guanines (5–7), O6-(4-, 3-, and 2-pyridylmethyl)guanines (8–10), and O6-(2- and 1 -naphtylmethyl)guanines (11, 12). Among these, compounds 1–3, 5, 8, 9 and 11 showed a strong MGMT depletion activity, whereas compounds 4, 6, 7, 10 and 12 were inactive. These inactive compounds, except for 6, have a substituent at the a position of the benzyl group (4, 7, 12) or are an α nitrogen analogue of 1 (10). There was a good relation (r = -0.856, p 0.001) between the MGMT depletion activity of O6-arylmethylguanine derivatives and their potentiation activity of ACNU cytotoxicity. These results suggest that the a position of the benzyl group plays an important role in the interaction of O6-arylmethylguanine derivatives with MGMT to result in the inactivation of MGMT. Potent MGMT inactivators (1–3, 5, 8, 9, 11) sensitize tumor cells to CENU chemotherapy.

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