Pharmacokinetic features of the antiparasitic macrocyclic lactones.

The macrocyclic lactones have pharmacokinetic properties which enhance their use against endo- and ectoparasites in animals and man. The most consistent physico-chemical feature of the group which contributes to their kinetic characteristics is high lipid solubility. This appears to be necessary for their pharmacodynamic action as well as common kinetic features such as large volumes of distribution and the influence of body fat composition on their disposition. They are used in all domestic animal species and are undoubtedly influenced by the anatomical and physiological differences in these species, however body fat composition also appears to exert a major influence on distribution, metabolism and persistence between species and between breeds and individuals. A myriad of formulations have been developed to enhance the convenience of administration in the different domestic animals and the macrocyclic lactones are delivered orally, subcutaneously and topically to good effect. Lipid based excipients have been developed in "depot" formulations to extend the period of effective prevention of parasite re-infection. Subtle structural changes have been made to the macrocyclic lactone molecules to reduce distribution to the central nervous system and mammary gland, thus allowing use of some compounds such as selamectin (SLM) in "toxicity sensitive" breeds of collie dog which lack P-glycoprotein efflux systems in their central nervous systems and the use of eprinomectin (EPM) in dairy cattle with a nil-milk withdrawal period. Gender differences exist in the pharmacokinetics of these compounds which may be associated with body (fat) composition or metabolism. Feeding may also reduce the availability of macrocyclic lactones which bind particulate digestive material and parasitism may impact the kinetics of the drugs because parasitized animals have altered pathophysiological processes, especially in the gastro intestinal tract but also because of the impact which parasitism may have on the body condition (and fat deposition) in animals. The pharmacokinetics of macrocyclic lactones may be affected by coadministration with compounds which interfere with P-glycoprotein transporters and these interactions have been explored as possible mechanisms for enhancing the effectiveness of these antiparasitics. The objective of this article is to provide a comprehensive review of the pharmacokinetics of macrocyclic lactones and to interpret where that information may prove clinically useful.