Comparison of the pharmacokinetics of moxidectin (Equest (R)) and ivermectin (Eqvalan (R)) in horses

A study was undertaken in order to evaluate and compare plasma disposition kinetic parameters of moxidectin and ivermectin after oral administration o f their commercially available preparations in horses, Ten clinically healt hy adult horses, weighing 390-446 kg body weight (b.w.), were allocated to two experimental groups of five horses. Group I was treated with an oral ge l formulation of moxidectin (MXD) at the manufacturers recommended therapeu tic dose of 0.4 mg/kg bw. Group II was treated with an oral paste formulati on of ivermectin (IVM) at the manufacturers recommended dose of 0.2 mg/kg b .w. Blood samples were collected by jugular puncture at different times bet ween 0.5 h and 75 days post-treatment. After plasma extraction and derivati zation, samples were analysed by HPLC with fluorescence detection. Computer ized kinetic analysis was carried out. The parent molecules were detected i n plasma between 30 min and either 30 (IVM) or 75 (MXD) days posttreatment. Both drugs showed similar patterns of absorption and no significant differ ence was found for the time corresponding to peak plasma concentrations or for absorption half-life. Peak plasma concentrations (C-max) of 70.3 +/- 10 .7 ng/mL (mean +/- SD) were obtained for MXD and 44.0 +/- 23.1 ng/mL for IV M. Moreover, the values for area under concentration-time curve (AUC) were 363.6 +/- 66.0 ng.d/ml for the MXD treated group, and 132.7 +/- 47.3 ng.d/m l for the IVM treated group. The mean plasma residence times (MRT) were 18. 4 +/- 4.4 and 4.8 +/- 0.6 days for MXD and IVM treated groups, respectively . The results showed a more prolonged residence of MXD in horses as demonst rated by a four-fold longer MRT than for IVM. The longer residence and the higher concentrations found for MXD in comparison to IVM could possibly exp lain a more prolonged anthelmintic effect. It is concluded that in horses t he commercial preparation of MXD presents a pharmacokinetic profile which d iffers significantly from that found for a commercial preparation of IVM. T o some extent these results likely reflect differences in formulation and d oses.