Plasma and faecal disposition kinetic of moxidectin after oral admnistration in horses.

A study was undertaken with the aim to study the plasma disposition kinetic and the faecal excretion profile of moxidectin after oral administration in horses. Five mixedbred saddle horses of 416.8 +/- 49.5 kg body weight and positives to the nematodes faecal eggs count were treated with an oral gel formulation of moxidectin (Equest((R))) at doses of 0.4 mg/kg. Blood and faecal samples were collected before and at different rimes after treatment for a period of 75 days. After solid phase extraction and derivatization samples were analysed by high performance liquid chromatography (HPLC). Data were analysed by a computerised kinetic analysis. The limit of quantification of the method was 0.5 ng/mL and the drug was detected in the plasma between 30 min (33.9 +/- 17.8 ng/mL) to 75 days post administration (0.3 +/- 0.2 ng/mL). The analysis of pharmacokinetic parameters showed a short half life of absorption ( t _ ab 0.84 +/- 0.5 h) and values of maximum concentration (Cmax) of 68.7 +/- 24.1 ng/mL obtained at 0.117 +/- 0.07 d (Tmax). The terminal half life of elimination (t _beta) was 18.3 +/- 3,2 d, which produces a delayed mean residence time (MRT of 19.6 +/- 4.6 d). The area under the concentration time curve was: 172.3 +/- 51.2 ng.d / mt. The mean concentration of moxidectin in faeces ranged between 423.5 +/- 570.7 ng/g at 24 h and 0.8 +/- 0.8 ng/g 75 d after treatment. The mean maximum faecal concentration (Cmax) 5204.5 +/- 916.5 ng/g was observed at 2.5 d post-treatment. It is concluded that moxidectin has a delayed period of permanency in the organism when it is administered by oral route in horses.