Anthelmintics, benzimidazoles and related substances.

Fenbendazole is an anthelmintic belonging to the benzimidazole carbamates group. It acts by interfering in the energy metabolism of the nematode. The anthelmintic efficacy is based on inhibition of the polymerisation of tubulin to microtubuli. The anthelmintic affects both adult and immature stages of gastro-intestinal and respiratory nematodes.

Fenbendazole is only partly absorbed from the intestine and reaches maximum plasma concentration in dogs 6 - 24 hours after oral administration. In cats the mean maximum serum concentration of fenbendazole was reached about 4 hours after treatment. Administration of fenbendazole in food did significantly increase its bioavailability compared to the administration on an empty stomach.

Fenbendazole is metabolised mainly by enzymes of the cytochrome P -450 system in the liver. The major oxidative metabolite is fenbendazole sulfoxide which is further metabolised to fenbendazole sulfone.

Fenbendazole and its metabolites are distributed throughout the body but highest concentrations are found in the liver.

The elimination half-life from plasma is about 15 hours after oral administration.

Fenbendazole and its metabolites are predominantly excreted via the faeces.