Pharmacokinetics of doramectin and ivermectin after oral administration in horses

A study was undertaken in order to compare plasma disposition kinetic parameters of doramectin (DRM) and ivermectin (IVM) in horses after oral administration. Ten crossbreed adult horses, clinically healthy, weighing 380-470 kg body weight (bw) were selected for study. Faecal examinations were performed to determine faecal parasite egg counts. Horses were allocated to two groups of five animals to provide an even distribution considering the variables sex, body weight and faecal egg count. Group I, were treated with an oral paste formulation of IVM at 0.2 mg/kg b/w and Group II, were treated with an oral dose of 0.2 mg/kg bw of DRM prepared as paste from the injectable formulation for oral administration. Blood samples were collected by jugular puncture between 0 h and 75 days post-treatment. Plasma was separated and later solid phase extraction and derivatization samples were analysed by high performance liquid chromatography (HPLC); a computerised kinetic analysis was carried out. Data were compared using the Mann-Whitney U-test. The mean plasma concentrations of DRM and IVM after oral administration in horses were detected until 30 and 20 days, respectively. Both drugs showed similar patterns of absorption and no significant differences were found for peak concentration, the time to peak concentration, or for absorptive half-life. The terminal elimination half-life was significantly (P<0.05) longer in the DRM treated group than for the IVM treated group. The differences observed in the elimination half-life explain the longer mean residence time and high values of area under the concentration time curve for the group treated with DRM, which are 30% higher than those of the IVM group. Considering its pharmacokinetics, tolerance and anthelmintic efficacy, the oral administration of DRM, could be an alternative to IVM for the control of parasitic diseases of horses.