Pharmacokinetic studies of ivermectin: Effects of formulation

Studies are reported which describe the effects of formulation, animal species, and route of administration on the pharmacokinetics of ivermectin. Biological half-life t_1/2 increases in the order: swine (0.5 day) > dogs (1.8 day) > cattle sheep (2.8 day). Formulation modifications, based upon the solubility properties of the drug, have been directed towards the development of a nonaqueous injectable formulation for cattle and an aqueous vehicle for horses. Bioavailability following subcutaneous injection in cattle can be regulated by control of injection solvent composition: a vehicle composed of a mixed aqueous-organic solvent exhibits pharmacokinetic properties (i.e., C_p, t_1/2, AUC, and F) intermediate between those furnished by an aqueous formulation and via a purely nonaqueous solvent. The longer apparent biological half-life from this latter vehicle (t_1/2=8.3 days) confirms that a slow absorption process dominates the pharmacokinetics in the nonaqueous injectable product to produce an effective controlled-release formulation. These bioavailability results illustrate the increase in the concentration of an organic solvent and a concomitant decrease in surfactant concentration in a micellar aqueous system for prolonged drug delivery via injection.