In vitro biotransformation and investigation of metabolic enzymes possibly responsible for the metabolism of bisdesoxyolaquindox in the liver fractions of rats, chicken, and pigs

Abstract

Bisdesoxyolaquindox is a reduced metabolite of olaquindox which is used as a medicinal feed additive in veterinary medicine. The relevant metabolism studies of bisdesoxyolaquindox have been carried out for the first time in rat, chicken, and pig liver subcellular fractions in order to understand the metabolic enzymes that are possibly responsible for the metabolism of olaquindox. The metabolites were characterized by high-performance liquid chromatography combined with hybrid ion trap/time-of-flight mass spectrometry. The major metabolic pathways of bisdesoxyolaquindox in the three species were the oxidation of hydroxyl to bisdesoxyolaquindox-2'-carboxyl acid (O10) and the N-dealkylation of the side chain to 3-methylquinoxaline 2-carboxamide (O12). Other metabolic pathways were also proposed which involved the direct methyl oxidation and N-oxide on the quinoxaline ring in the three species as well as N-hydroxylation only in rat. The intrinsic clearance values in the liver microsomes for O10 and O12 were ranked in the order of chicken>pig≫rat and rat>pig≫chicken, respectively. Inhibition studies indicated that 8-methoxypsoralen, 4-methylpyrazole and α-naphthoflavone could inhibit the formations of O10 and O12 in all species. Quinidine, troleandomycin, diethyldithiocarbamate, and disulfiram showed an interspecies difference in the inhibition of the formation of two metabolites. In rat and pig liver cytosol, 4-methylpyrazole, menadione and chlorpromazine strongly inhibited the formation of O10. Both diethyldithiocarbamate and disulfiram were found to inhibit O10 formation in rat cytosol but not in pig cytosol. These results indicated the following: In rat liver microsomes, CYP2A might be involved in the formation of O10, and CYP1A, CYP2A and CYP2E would be involved in the O12 formation. In pig liver microsomes, CYP1A and CYP2E might catalyze the formations of O10 and O12. In rat cytosol, alcohol dehydrogenase, aldehyde oxidase and aldehyde dehydrogenase should catalyze the O10 formation. In pig cytosol, alcohol dehydrogenase and aldehyde oxidase might be involved in the formation of O10. In chicken, it was found that various CYP isoenzymes were capable of catalyzing the two reactions; none of the inhibitors of cytosol enzymes inhibited O10 formation in chicken cytosol.