Interaction between fenbendazole and piperonyl butoxide: Pharmacokinetic and pharmacodynamic implications

The effect of the cytochrome P450 inhibitor, piperonyl butoxide on the pharmacokinetics and anthelmintic efficacy of the benzimidazole compound fenbendazole was studied in sheep and goats. Pretreatment of goats with the inhibitor caused a greater than three-fold increase in the relative bioavailability of fenbendazole and fenbendazole sulphoxide. A pharmacokinetic dose titration study was carried out in sheep with fenbendazole (5 mg kg) and piperonyl butoxide administered orally at 0, 15, 31, 63, 125 and 250 mg kg. The AUC of fenbendazole and the sulphoxide were significantly increased when fenbendazole was co-administered with piperonyl butoxide at dose rates equal to or higher than 31 mg kg. Peak plasma concentrations (C(max)) and mean residence time (MRT) were also significantly increased. The efficacy of the combination was assessed in sheep against two species of benzimidazole-resistant abomasal nematodes; Ostertagia circumcincta and Haemonchus contortus. The percentage reduction in the total number of O. circumcincta worms was 7.9% (fenbendazole) and 97.8% (fenbendazole-piperonyl butoxide). For H. contortus, the percentage reduction was 84.8% (fenbendazole) and 99.0% (fenbendazole-piperonyl butoxide). The in-vitro S-oxidation of fenbendazole and fenbendazole sulphoxide was studied using microsomal preparations from rat liver. Piperonyl butoxide inhibited significantly the sulphoxidation and sulphonation of fenbendazole. It was concluded that piperonyl butoxide inhibited the oxidative conversion of fenbendazole into inactive metabolites and this resulted in a potentiated anthelmintic action.