Pharmacokinetics of [3H]levamisole in pigs after oral and intramuscular administration

A single oral (10 mg/kg of body weight) or IM (7.5 mg/kg) dose of [3H]levamisole was administered to pigs. Liquid scintillation counting and high performance liquid chromatography were used to determine total radioactivity and drug levels in plasma, duodenal and cecal contents, bile, and urine for 24 and 72 hours after dosing. Pharmacokinetic analysis indicated a 1-compartment open model with higher plasma bioavailability of levamisole after IM injection. Biological half-lives for elimination of the drug were 9.3 and 6.9 hours after oral and IM administration, respectively. Anthelmintic concentrations were higher in intestinal contents after oral gavage than after IM injection. The drug appeared extensively metabolized in all body fluids and particularly in bile, regardless of the route of administration. Biliary excretion of radioactivity and unchanged levamisole represented only slight percentages of the administered dose (approx 0.4% and 4.2%, respectively, in 72 hours). In contrast, about 60% and 20% of the dose were eliminated via urine as tritiated materials and unchanged drug. The choice of the most efficacious route of administration is discussed in regard to localization of helminthic disease.