Inactivation of O-6-alkylguanine-DNA alkyltransferase by 8- substituted O-6-benzylguanine analogs in mice

Purpose: The purpose of this study was to determine the usefulness of various 8-substituted O-6-benzylguanine (BG) analogs as modulators of the DNA repair protein, O-6- alkylguanine-DNA alkyltransferase (AGT). More specifically, the degree of inactivation of AGT in mouse brain, liver, kidney and tumor by O-6-benzyl-8-oxoguanine (8-oxoBG), 8-aza-O(6- )benzylguanine (8-azaBG), O-6-benzyl-8-bromoguanine (8-bromoBG) and O-6-benzyl-8-trifluoromethylguanine (8-tfmBG) was compared to inactivation by BC, a modulator in phase II clinical trials. BG is converted rapidly to 8-oxoBG in rodents, monkeys and humans. It was reasoned that 8-substituted analogs of BG would exhibit different pharmacological properties compared to BG which could influence tissue bioavailability and, thus, the extent of ACT inactivation in vivo. We compared the tissue distribution of these agents and AGT activity following administration of the 8-substituted analogs. Materials and methods: At various time points up to 24 h after i.p. administration of the BG analogs, tissues (i.e. brain, liver, kidney), A549 lung tumor xenografts (i.p.) or D456 brain tumor xenografts (i.c.) were harvested from athymic nude mice for AGT analysis. AGT activity was quantified in tissue extracts using a biochemical assay with [H-3]methylated DNA as a substrate. In addition, concentrations of BG and 8-oxoBG were determined by HPLC with fluorescence detection in mouse tissues following administration of drug. Results: Each of the 8-substituted analogs of BG demonstrated variable AGT inactivation capabilities that were comparable to or better than those of BG especially in kidney and brain tissues. There was a more pronounced depletion of AGT inactivation in brain and D456 brain tumor xenografts following administration of BG compared to 8-oxoBG that could be explained by a much greater concentration of ACT-inactivating drug (BG plus the metabolite 8-oxoBG for mice treated with BG Versus 8-oxoBG for mice treated with 8-oxoBG) present in these tissues. The AUCs for brain, kidney and liver were 3.2, 6.9 and 11.8 times greater for BG than for 8-oxoBG. Conclusions: 8-substituted analogs of BG possess unique ACT-inactivation profiles in vivo that are different from that of BG. The AGT-inhibitory activities of BG and its major metabolite, 8-oxoBG, are related to tissue disposition of both drugs.

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