Comparative disposition of ricobendazole enantiomers after intravenous and subcutaneous administration of a racemic formulation to calves

The enantioselective disposition kinetics of the benzimidazole anthelmintic, ricobendazole (RBZ), have been characterized after its intravenous (iv) and subcutaneous (sc) administration as a racemic formulation to cattle. The (+) and (−) RBZ enantiomeric forms were recovered in plasma after iv and sc administration of the racemic RBZ formulation, using a chiral phase based HPLC method. A biexponential plasma concentration versus time curve was observed for both RBZ enantiomers following the iv treatment. Total body clearance was higher for (−) RBZ (150.4 mL/h · kg) compared with that obtained for the (+) RBZ antipode (78.1 mL/h · kg). The elimination half‐life of the (−) RBZ enantiomer was shorter (T1/2β: 2.67 h) compared with the (+) enantiomer (T1/2β: 5.41 h). The plasma availability (expressed as AUC) was significantly higher for (+) RBZ compared with that obtained for the (−) antipode following both treatments. The enantiomeric ratio in plasma at T 0 was close to unity (50% of each enantiomer); the analysis of the concentration ratios (+) RBZ/(−) RBZ, demonstrated an increase in the proportion of (+) RBZ during the time course of the kinetics after both iv and sc treatments. The results presented herein show the enantioselective disposition kinetics of RBZ in cattle and are a further contribution to the understanding of the kinetic behaviour of these sulphoxide‐containing benzimidazole anthelmintics in ruminants. Copyright © 2000 John Wiley & Sons, Ltd.