PHARMACOKINETICS AND METABOLISM OF AMITRAZ IN PONIES AND SHEEP

Amitraz and its active metabolite BTS27271 were given intravenously to ponies and sheep at equimolar doses of I mg/kg and 0.68 mg/kg, respec tively and the plasma concentrations of amitraz and BTS27271 estimated at various times thereafter. Amitraz was hydrolysed to BTS27271 in bo th species. Amitraz was undetectable in sheep plasma after approximate ly 5 min but persisted in the plasma of ponies for at least 90 min. Th e persistence of unmetabolized amitraz in ponies may have implications for the toxicity of amitraz in that species. The primary and secondar y disposition half-lives of amitraz in ponies were 2 and 39 min, respe ctively. BTS27271 was distributed rapidly outside the plasma in both s pecies with a primary disposition half-life of 4.4 min In sheep and 5. 9 min in ponies. The secondary disposition half-lives were 51 and 55 m in, respectively. The secondary phase of the disposition of BTS27271 w as similar whether BTS27271 was given directly or derived by hydrolysi s from amitraz. However, significant differences were evident in the p rimary phase of the disposition of BTS27271. Sheep demonstrated a larg er apparent volume of distribution of BTS27271 than ponies and more ra pid body clearance.