Moxidectin: a review of chemistry, pharmacokinetics and use in horses

This article reviews the current knowledge of the use of moxidectin (MOX) in horses, including its mode of action, pharmacokinetic and pharmacodynamic properties, efficacy, safety and resistance profile. Moxidectin is a second generation macrocyclic lactone (ML) with potent endectocide activity. It is used for parasite control in horses in an oral gel formulation. The principal mode of action of MOX and of other MLs is binding to gamma-aminobutyric (GABA) and glutamate-gated chloride channels. Moxidectin is different from other MLs in that it is a poor substrate for P-glycoproteins (P-gps) and therefore less susceptible to elimination from parasite cells through this mechanism. Due to its unique physicochemical and pharmacokinetic characteristics, MOX provides broad distribution into tissues, long half-life, significant residual antiparasitic activity, and high efficacy against encysted cyathostomin larvae. These characteristics allow for high efficacy and longer treatment interval against all important nematodes, when compared to other equine anthelmintics. A combination of MOX with praziquantel provides expanded spectrum of activity by adding activity against cestodes. Appropriate use of MOX allows for the development of strategic anthelmintic programmes that are different from those with conventional anthelmintics. Fewer treatments are required over a period of time, and therefore impose less frequent selection pressure for resistance.