ABSORPTION, METABOLISM AND ELIMINATION OF THIABENDAZOLE IN FARM ANIMALS AND A METHOD FOR ITS ESTIMATION IN BIOLOGICAL MATERIALS

¹ Merck Institute for Therapeutic Research, Rahway, New Jersey and Merck Sharp & Dohme Research Laboratories, Rahway, New Jersey
² Merck Institute for Therapeutic Research, Rahway, New Jersey and Merck Sharp & Dohme Research Laboratories, Rahway, New Jersey

Radioactive thiabendazole administered orally to farm animals (goats, cattle and swine) as single doses of 50 to 200 mg/kg was rapidly absorbed and metabolized, and effectively excreted. In these animals the only known metabolites are 5-hydroxythiabendazole and its glucuronide or sulfate ester. A spectrophoto-fluorometric procedure for the estimation of these compounds in biological fluids and tissues is presented. The analytical procedure accounted nearly quantitatively for the radioactivity present in the urine.

Peak concentrations in plasma occurred 4 to 7 hours after dosage after which the concentrations decreased rapidly. Approximately 85% of the radioactivity was eliminated in the urine (65%) and feces (20%) in 3 days. No residual thiabendazole or metabolites were detectable in tissue 30 days after administration of 50 mg/kg. Traces of nonthiabendazole radioactivity were barely demonstrable in calves 2 months following doses of 150 and 200 mg/kg.

Small quantities of the drug appeared in the milk of lactating animals. In goats less than 1%, and in dairy cows about 0.1% of the dose was secreted in the milk and peak drug concentrations were found within 24 hours after dosage. Nearly all of the drug was metabolized thiabendazole—only trace quantities of the unchanged compound were present. Thiabendazole and its metabolites were not detectable in the milk 4 days after goats were given a dose of 150 mg/kg or 60 hours after cows were given 220 mg/kg.