Plasma disposition and faecal excretion of oxfendazole, fenbendazole and albendazole following oral administration to donkeys

Fenbendazole (FBZ), oxfendazole (fenbendazole sulphoxide, FBZSO), and albendazole (ABZ) were administered orally to donkeys at 10 mg/kg bodyweight. Blood and faecal samples were collected from 1 to 120 h post-treatment. The plasma and faecal samples were analysed by high performance liquid chromatography (HPLC). The parent molecule and its sulphoxide and sulphone (FBZSO₂) metabolites did not reach detectable concentrations in any plasma samples following FBZ administration. ABZ was also not detected in any plasma samples, but its sulphoxide and sulphone metabolites were detected, demonstrating that ABZ was completely metabolised by first-pass mechanisms in donkeys. Maximum plasma concentrations (C_max) of FBZSO (0.49 μg/mL) and FBZSO₂ (0.60 μg/mL) were detected at (t_max) 5.67 and 8.00 h, respectively, following administration of FBZSO. The area under the curve (AUC) of the sulphone metabolite (10.33 μg h/mL) was significantly higher than that of the parent drug FBZSO (5.17 μg h/mL). Cmax of albendazole sulphoxide (ABZSO) (0.08 g/mL) and albendazole sulphone (ABZSO₂) (0.04 μg/mL) were obtained at 5.71 and 8.00 h, respectively, following ABZ administration. The AUC of the sulphoxide metabolite (0.84 μg h/mL) of ABZ was significantly higher than that of the sulphone metabolite (0.50 μg h/mL).