Potentiation of the Cytotoxicity of Chloroethylnitrosourea by O6-Arylmethylguanines.

Abstract

It was reported recently that monomeric O6-benzylguanine (1) acts as an alternative substrate for a DNA repair enzyme, O6-alkylguanine-DNA alkyltransferase (AGT), and that therefore pretreatment of cells with 1 induces depletion of AGT resulting in an enhanced cytotoxic response to alkylating antitumor agents. In order to study the interaction of O6-benzylguanine derivatives with AGT and to obtain greater AGT depletion, we synthesized the following O6-arylmethylguanine derivatives and related compounds: O6-(4-, 3- and 2-fluorobenzyl)guanines (2, 3, 4), O6-(4-, 3- and 2-trifluoromethylbenzyl)guanines (5, 6, 7), O6-(4-, 3- and 2-pyridylmethyl)guanines (8, 9, 10), O6-(2- and 1-naphthylmethyl)guanines (11, 12), O6-biphenylmethylguanine (13), S and Se analogues of O6-benzylguanine (14, 15) and O6-phenylguanine (16). Ten of these are new compounds. All these compounds were tested for their potentiation of N'-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-N-(2-chloroethyl)-N-nitrosou rea (ACNU) cytotoxicity using HeLa S3 and C6-1 cells. Compounds 2, 3, 5, 8, 9, 11 and 13 were active, as was 1. Compounds 7 and 12, with a substituent at the alpha position of the benzyl group, and compound 10, the alpha-nitrogen analogue of 1, were almost completely devoid of potentiating activity. These results suggest that the alpha-position of the O6-benzyl group plays an important role in the interaction of O6-benzylguanines with AGT. Of the other compounds, 4 and 6 exhibited very weak activity and 14, 15 and 16 were inactive. Possible reasons for these differences in activity are discussed in relation to the biomimetic dealkylation rates of O6-benzylguanine derivatives and the chemical characteristics of their substituents.

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

“…ACNU was supplied by Sankyo (Tokyo, Japan). 06-Arylmethylguanines employed were Ob-benzylguanine, 06-(4-, 3-, and 2-fluorobenzyl)guanines, 06-(4-, 3-, and 2-trifluoromethylbenzyl)guanines, 06-(4-, 3-, and 2-pyridylmethy1)guanines, and Oh-(2-and 1-naphtylmethyl)guanines, and these were synthesized as described previously (Kohda et al, 1995). Structures and identification numbers of the 06-arylmethylguanine derivatives are shown in Figure 1.…”

“…Thus, the dose required to inhibit the growth of cells by 50% was expressed as ICso, and the D M F was calculated as the ratio of ICso of cells treated with ACNU alone versus the ICso of cells that were treated with 06-arylmethylguanines and subsequent ACNU. Results have been reported (Kohda et al, 1995) and some of them are quoted in Table 11.…”

“…To date, Oh-benzylguanine is known as one of the most effective inactivators among the derivatives tested (Dolan et al, 1990(Dolan et al, , 1991. In this study, we report the syntheses of several 06-arylmethylguanine derivatives and their potentiation effect on the cytotoxicity of ACNU for HeLa S3 cells (Kohda et al, 1995). Oh-Benzylguanine derivatives had potentiation activities for ACNU cytotoxicity; however, introduction of a substituent at the a-position of the benzyl moiety or replacement of the a-carbon of the benzyl moiety by a nitrogen resulted in a loss of the potentiation activities.…”

O6-(Fluorobenzyl)Guanine and Chloroethylnitrosourea in Xenografted Rat Brain Tumor In Vivo

“…1) used in the present study has been reported previously (24). Briefly, a mixture of a tert-BuOH solution containing Na, a fluorobenzyl alcohol (4-fluorobenzyl alcohol, 3-fluorobenzyl alcohol, or 2-fluorobenzyl alcohol), and 2-amino-6-chloropurine, was heated and then evaporated to remove the tert-BuOH.…”

Synthesis of radiolabeled O6‐benzylguanine derivatives as new potential PET tumor imaging agents for the DNA repair protein O6‐alkylguanine‐DNA alkyltransferase

“…20 However, these three classes of inhibitors are not ready for in vivo studies, since it is difficult to make BG derivatives labeled at the 8-position; the toxicity of O 6 -(hetarylmethyl)guanines remains to be investigated; and BP derivatives are not as effective as O 6 -BG in mouse liver or spleen due to their short half-life in vivo. 21 The potentiation effect order of the substituted O 6 -BG analogs is para > meta > ortho, 22,23 most likely because the steric effect at the ortho-position plays a more important role than the electronic effect. Preliminary findings from the biological assay of unlabeled 1a, 1b, and 1c warrant further evaluation of radiolabeled 1a, 1b, and 1c as new potential PET tumor imaging agents for the AGT in vivo.…”