Differences in plasma and abomasal kinetics of albendazole and its metabolites in calves grazed on pasture or fed a grain-based diet

We evaluated the comparative plasma and abomasal fluid disposition kinetics of albendazole (ABZ) and its metabolites in calves either grazing on pastu re or fed a grain-based concentrate diet. Six male Holstein calves (weight 180 to 200 kg) were allowed to graze on lush pasture for three weeks before intraruminal administration of ABZ at 10 mg kg(-1) (pasture group). After a three-week wash-out period, the same animals were housed and fed on a gra in-based concentrate diet for three weeks prior to receiving the same ABZ t reatment (concentrate group). Jugular blood and abomasal fluid samples were collected over 120 hours post-treatment. Plasma and abomasal fluid samples were analysed by high performance liquid chromatography (HPLC). The digest s transit time was measured using cobalt (Co) as a fluid marker; abomasal f luid and faecal samples were collected and Co concentrations measured by at omic absorption spectrophotometry. Complementary studies of the in vitro di ssolution of ABZ particles at different pH values were also conducted. The pH of abomasal fluid collected from animals kept under both feeding conditi ons was registered. Increased concentrations of ABZ sulphoxide (ABZSO) and sulphone (ABZSO(2)) in plasma, resulting in significantly higher Cmax and a rea under the curve (AUC) values for both metabolites, were obtained in cal ves fed on the concentrate diet compared to those grazing on pasture. Enhan ced abomasal fluid levels of ABZ and ABZSO were observed in concentrate-fed calves. The mean retention time of the digestive fluid marker in the gastr ointestinal (GI) tract was significantly longer in the animals fed the grai n-based diet. The in vitro dissolution of ABZ at a pH value equivalent to t hat obtained in the abomasum of the concentrate-fed calves (1.75) was signi ficantly greater than that obtained at the pH registered in pasture-fed ani mals (2.00). The characterisation of the kinetic/metabolic behaviours and t he resultant efficacy of antiparasitic drugs in animals reared under differ ent management conditions may be relevant in increasing parasite control in livestock.