Breed differences on the plasma availability of moxidectin administered pour-on to calves.

The pharmacokinetic profile of avermectin and milbemycin compounds is affected by different drug- and host-related factors. This work reports the influence of cattle breeds on the plasma kinetics of moxidectin (MXD) after topical (pour-on) administration. Parasite-free Aberdeen Angus and Holstein calves were treated with a commercial MXD pour-on formulation at 500 microg/kg. Blood samples were collected over a period of 35 days post-treatment and the recovered plasma was analysed by high performance liquid chromatography using fluorescence detection. MXD was detected in plasma from two hours up to 35 days post-treatment in animals from both breeds. A slow MXD absorption and delayed peak plasma concentration were observed in Aberdeen Angus compared to Holstein calves. Significant lower systemic availability (expressed as AUC) (P<0.01) and peak plasma concentration (C(max)) (P<0.05) were also observed in Aberdeen Angus calves, although the plasma mean residence time (MRT) and elimination half-lives (T(1/2el)) of MXD in both breeds were similar. The pharmacokinetic differences observed between cattle breeds contribute to explain the variability in the pattern of clinical efficacy for pour-on administered endectocide compounds reported in different field trials.