Faecal excretion profile of moxidectin and ivermectin after oral administration in horses

A study was undertaken to evaluate and compare faecal excretion of moxidect in and ivermectin in horses after oral administration of commercially avail able preparations. Ten clinically healthy adult horses, weighing 390-446 kg body weight (b.w.),were allocated to two experimental groups. Group I was treated with an oral gel formulation of moxidectin at the manufacturer's re commended therapeutic dose of 0.4 mg/kg b.w. Group II was treated with an o ral paste formulation of ivermectin at the recommended dose of 0.2 mg/kg b. w. Faecal samples were collected at different rimes between 1 and 75 days p ost-treatment. After faecal drug extraction and derivatization, samples wer e analysed by High Performance Liquid Chromatography using fluorescence det ection and computerized kinetic analysis. For both drugs the maximum concentration level was reached at 2.5 days post administration. The ivermectin treatment groups' faecal concentrations rem ained above the detectable level for 40 days (0.6 +/- 0.3 ng/g), whereas th e moxidectin treatment group remained above the detectable level for 75 day s (4.3 +/- 2.8 ng/g). Ivermectin presented a faster elimination rate than m oxidectin, reaching 90% of the total drug excreted in faeces at four days p ost-treatment, whereas moxidectin reached similar levels at eight days post treatment. No significant differences were observed for the values of maxi mum faecal concentration (Cm,) and time of C-max (T-max) between both group s of horses, demonstrating similar patterns of drug transference from plasm a to the gastrointestinal tract. The values of the area under the faecal co ncentration time curve were slightly higher in the moxidectin treatment gro up (7104 +/- 2277 ng.day/g) but were not significantly different from those obtained in the ivermectin treatment group (5642 +/- 1122 ng.day/g). The r esults demonstrate that although a 100% higher dose level of moxidectin was used, attaining higher plasma concentration levels and more prolonged excr etion and gut secretion than ivermectin, the concentration in faeces only r epresented 44.3 +/- 18.0% of the total parental drug administered compared to 74.3 +/- 20.2% for ivermectin. This suggests a higher level of metaboliz ation for moxidectin in the horse. (C) 2001 Harcourt Publishers Ltd.