Pharmacokinetics of moxidectin and ivermectin following intravenous injection in pigs with different body compositions

Macrocyclic lactones (ML) are highly effective anthelmintics that provide a long protective period after administration because of their extensive dis tribution into fat. This study examined whether the body composition of the animal at the time of treatment had any influence on the pharmacokinetics of two MLs, moxidectin (MOX) and ivermectin (IVM), 'Fat' and 'lean' lines o f pigs were established using two different diets, with weekly determinatio n of liveweight and backfat thickness confirming the difference in body con dition between the groups. Blood samples were taken at regular intervals fo llowing i.v. injection of IVM or MOX at a dose of 300 mug/kg and the plasma was analysed using fluorescence high performance liquid chromatography (HP LC) to determine the concentration of IVM or MOX in the samples, Regardless of body composition IVM and MOX kinetics were very different with MOX havi ng a greater apparent volume of distribution, longer distribution and elimi nation half-lives and a slower clearance rate than IVM, which led to MOX be ing detectable in plasma for >40 days compared with only 8-10 days for IVM. Altering body composition had no detectable influence on the kinetic dispo sition of I in this study. In contrast, although there was no difference in AUC or the volume of distribution, MOX was distributed within and eliminat ed from the lean animals more rapidly than from the fat animals.