In feed medication with fenbendazole: induction of cytochrome P450-dependent metabolism in pig liver microsomes

Abstract

Introduction: The anthelmintic fenbendazole (FBZ) undergoes to sequential S-oxidations through hepatic mixed function oxidases belonging to the cytochrome P450 (CYP) and flavin-monooxygenase (FMO) families. In addition, the in-feed medication with FBZ caused the induction of CYP1A-dependent metabolism in pig liver (1). This fact may affect the metabolic fate of FBZ itself, but also of other xenobiotics. This work evaluated the effect of the sustained administration of FBZ on its own pattern of hepatic S-oxidation, and on the metabolism of enrofloxacin (ERF) and aflatoxin B1 (AFB1).Methods: Five female Landrace piglets remained untreated (controls), and other five received a pre-mix of FBZ, combined with feed, for 9 consecutive days, as usually is recommended (2.5 g of premix, containing 4% of FBZ, per kg of feed). The average feed intake was 1.08±0.11 kg per animal/day. On day 9 th , pigs were slaughtered, and liver samples were taken for preparation of subcellular fractions. Liver microsomes were used for CYP content determination, for monitoring the CYP1A-dependent enzyme activities, 7-ethoxuresorufin O-deethylase (EROD) and methoxyresorufin O-demethylase (MROD), and for the measurement of the metabolic rates of FBZ (50 µM) S-oxidation, ERF (50 µM) conversion into ciprofloxacin (CPF) and AFB1 (16 nM) disappearance. Metabolic assays were carried out in presence of NADPH. Students t-test with Welch’s correction was used forstatistical comparisons.Results: In liver microsomes from treated animals, EROD and MROD increased 20-fold (p=0.002) and 19-fold (p=0.001), respectively. An enhanced (3-fold, p=0.0037) participation of the CYP pathway in the hepatic S-oxidation of FBZ into oxfendazole (OFZ) was observed in the liver of piglets receiving FBZ (223±69 pmol/min.nmol CYP) compared to controls (68±34 pmol/min.nmol CYP). The metabolic rate of ERF conversion into CPF increased (p=0.014) from 26.5±8.4 pmol/min.nmol CYP (controls) to 139.4±60.1 pmol/min.nmol CYP(FBZ-treated). The rate of disappearance of AFB1 in FBZ-treated pigs was 79% higher (p=0.036) compared to that measured in control animals.Conclusions: An auto-induction of the CYP1A-dependent S-oxidation of FBZ towards its active metabolite OFZ was observed. The in-feed medication with FBZ may cause potential metabolic interactions with the antimicrobial ERF and the mycotoxin AFB1. Enzyme induction caused by the anthelmintic may modify the pharmacokinetic behaviour of ERF and CPF. In addition, induction of the CYP1A-dependent metabolism of AFB1 may increase the production of a hepatotoxic AFB1-derived epoxide (2).