Medication with fenbendazole in feed: plasma concentrations and effects on hepatic xenobiotic metabolizing enzymes in swine.

Fenbendazole (FBZ), a benzymidazole (BZD) anthelmintic drug, is used for in-feed medication in pigs. BZD-containing drugs may induce cytochrome P450 isozymes (CYPs), particularly those members of the CYP1A subfamily. The current research evaluated the plasma and liver availability and metabolism of FBZ and its metabolites, oxfendazole (OFZ) and fenbendazole sulphone (FBZSO), after the administration of the parent drug in feed, and characterized the effect of the sustained administration of the anthelmintic on the catalytic activities of xenobiotic metabolizing enzymes in pig liver. Five female Landrace piglets remained untreated (controls), and other six were treated with a pre-mix of FBZ, combined with feed, for 9 consecutive days as usually is recommended. Blood samples were collected from each treated animal up to day 9 and analyzed by HPLC; all animals were slaughtered for preparation of liver microsomes. Plasma concentration ratios OFZ/FBZ and FBZSO/OFZ increased significantly (p < 0.05) from the beginning to the end of drug exposure, which may indicate an enhanced conversion of FBZ into its metabolites. FBZ represented 45.8 ± 3.4% of the total anthelmintic molecules in liver tissue. Increased CYP1A-dependent 7-ethoxy (24.5-fold, p = 0.0032) and 7-methoxyresorufin (17.2-fold, p = 0.0006) O-dealkylase activities was observed in liver microsomes from FBZ-treated animals. In addition, a 64% increase (p = 0.042) in the rate of FBZ S-oxidation was observed in pigs treated with the anthelmintic drug compared to that measured in untreated animals. Thus, the continuous FBZ administration may accelerate its own in vivo hepatic metabolism through the CYP1A pathway.