title: Identification of the thiophene ring of methapyrilene as a novel bioactivation-dependent hepatic toxicophore.
abstract: Methapyrilene (MP), a 2-thiophene H(1)-receptor antagonist, is a model toxicant in the genomic and proteomic analyses of hepatotoxicity. In rats, it causes an unusual periportal necrosis that is hypothetically attributed to chemically reactive and cytotoxic metabolites. We have characterized the bioactivation of MP by hepatic microsomes and primary rat hepatocytes, and we established a possible causal linkage with cytotoxicity. Methapyrilene tritiated at C-2 of the diaminoethane moiety ([3H]MP) was metabolized via an NADPH-dependent pathway to intermediates that combined irreversibly with microsomes (rat > mouse approximately human). This binding was attenuated by the cytochrome P450 (P450) inhibitor 1-aminobenzotriazole and thiols but not by trapping agents for iminium ions and aldehydes. Reactive intermediates were trapped as thioether adducts of monooxygenated MP. Mass spectrometric and hydrogen/deuterium exchange analysis of the glutathione adduct produced by rat liver microsomes indicated that the metabolite was most probably a thioether of MP S-oxide substituted in the thiophene ring. The glutathione adduct was formed by rat hepatocytes and eliminated in bile by rats administered [3H]MP intravenously. MP produced concentration- and time-dependent cytotoxicity, depleted glutathione, and underwent irreversible binding to the hepatocytes before a significant increase in cell damage was observed. P450 inhibitors reduced turnover of the drug, production of the glutathione adduct, irreversible binding, and cytotoxicity but inhibited glutathione depletion selectively. MP underwent lesser turnover and bioactivation in mouse hepatocytes and was not cytotoxic. Analogs with phenyl and p-methoxyphenyl rings were much less hepatocytotoxic than MP. Hepatotoxicity in rats was diminished by predosing with 1-aminobenzotriazole. For the first time, a thiophene ring substituent is identified as a bioactivation-dependent toxicophore in hepatocytes.
authors.affiliation: Department of Pharmacology and Therapeutics, the University of Liverpool, Liverpool, United Kingdom.
authors.forename: Emma E
authors.initials: EE
authors.lastname: Graham
authors.forename: Rachel J
authors.initials: RJ
authors.lastname: Walsh
authors.forename: Charlotte M
authors.initials: CM
authors.lastname: Hirst
authors.forename: James L
authors.initials: JL
authors.lastname: Maggs
authors.forename: Scott
authors.initials: S
authors.lastname: Martin
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Wild
authors.forename: Ian D
authors.initials: ID
authors.lastname: Wilson
authors.forename: John R
authors.initials: JR
authors.lastname: Harding
authors.forename: J Gerald
authors.initials: JG
authors.lastname: Kenna
authors.forename: Raimund M
authors.initials: RM
authors.lastname: Peter
authors.forename: Dominic P
authors.initials: DP
authors.lastname: Williams
authors.forename: B Kevin
authors.initials: BK
authors.lastname: Park
mesh_terms: D000818:Animals; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D056486:Chemical and Drug Induced Liver Injury / Q000209:etiology* / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D005978:Glutathione / Q000378:metabolism; D022781:Hepatocytes / Q000187:drug effects* / Q000378:metabolism; D006634:Histamine H1 Antagonists / Q000737:chemistry / Q000493:pharmacokinetics / Q000633:toxicity; D006801:Humans; D008297:Male; D008701:Methapyrilene / Q000737:chemistry / Q000493:pharmacokinetics / Q000633:toxicity; D051379:Mice; D008815:Mice, Inbred Strains; D008862:Microsomes, Liver / Q000187:drug effects* / Q000378:metabolism; D015394:Molecular Structure; D051381:Rats; D017208:Rats, Wistar; D013876:Thiophenes / Q000737:chemistry / Q000378:metabolism*
chemical_list: D006634:Histamine H1 Antagonists; D013876:Thiophenes; D008701:Methapyrilene; D005978:Glutathione