The disposition of radioactivity after administration of the anthelminthic methyl-14C-5-cyclopropylcarbonyl-2-benzimidazole carbamate (ciclobendazole) to rats and dogs.

1. The disposition of radioactivity has been studied in rats and dogs after administration of a new anthelminthic agent, 14C-labelled methyl-5-cyclopropylcarbonyl-2-benzimidazole carbamate (14C-ciclobendazole). 2. An oral dose of 14C-ciclobendazole (4 mg/kg) to rats was rapidly absorbed and about 70% and 20% of the dose was excreted in the faeces and urine, respectively, during 2 days. Bile duct cannulated rats excreted about 80% of the dose in 48-h bile, about 2% in the faeces and about 10% in the urine showing that an oral dose was well-absorbed and that some enterohepatic circulation probably occurred. The excretion of radioactivity in the bile was less after i.v. administration. 3. An oral dose of 14C-ciclobendazole (4 mg/kg) to dogs was mainly eliminated during 2 days with about 80% of the dose in the faeces and only about 10% in the urine. Anaesthetised bile duct-cannulated dogs, excreted between 26% and 35% of an oral dose in the bile during 24 h and up to 58% of an oral dose was absorbed at this time. 4. The tissue distribution of radioactivity in rats and dogs after single or multiple oral doses of 14C-ciclobendazole (4 mg/kg) showed that there was no unusual accumulation or localisation of radioactivity in the measured tissues. Highest concentrations were present in the intestinal tract, liver and kidneys, organs associated with biotransformation and excretion and also in the lungs and adrenals. 5. After oral administration of 14C-ciclobendazole to rate at three different dose levels (4, 40 and 400 mg/kg), peak plasma levels occurred at 15-30 min and declined with similar half-lives (about 20 h). A comparison of peak concentrations and areas under the plasma concentration-time relationships showed that the absorption of ciclobendazole was probably dose-dependent, a lower proportion probably being absorbed at higher doses. After repeated daily oral dosing with 14C-ciclobendazole (4 mg/kg), there were no significant changes in either the daily plasma concentrations or the biological half-life measured after the last dose, indicating that ciclobendazole probably did not induce or inhibit its own metabolism when dosed repeatedly at 4 mg/kg. 6. A comparison of the areas under the plasma concentration-time relationships after oral, i.p. and i.v. administration of 14C-ciclobendazole to rates indicated that there was no signigicant uptake by the liver during first pass and that an oral dose was well absorbed by rats. 7. The peak plasma concentration in the dog, after an oral dose of 14C-ciclobendazole (4 mg/kg) was reached at about 30 min and declined with a half-life of about 3 h. 8. Ciclobendazole was probably well-absorbed by rats and dogs and excreted more rapidly by the latter species than by the former Relatively higher plasma concentrations of drug and/or metabolites were thus achieved in rats than in dogs.