Benzimidazoles

This class of drugs are known by farmers as white drenches. The product code in the UK is 1-BZ. The most commonly used drugs in this category are; albendazole, fenbendazole and oxfendazole.

Spectrum of Activity

• Wide spectrum, they are active against adult and immature nematodes, though have limited action against hypobiotic forms.
• Some have activity against the cestodes.
• Albendazole has activity against adult liver flukes.
• Albendazole and fenbendazole are active against Giardia.
• Triclabendazole is active against all stages of Fasciola hepatica, but inactive against nematodes and cestodes.
• They work by binding to tubulin. This caps the ends of microtubules in the parasites, resulting in disruption in their intestinal and integumental cells. Thus they can't utilise glucose efficiently and they starve to death.

Pharmacokinetics

• They kill the parasites slowly and so potency is determined by the duration of exposure not peak plasma concentration.
• So the more soluble the drug the less potent it is.
• Multiple low doses are better than one large dose.
• They work better in ruminants and horses than carnivores. This is because the rumen and caecum act as reservoirs for the drug.
• In ruminants, the potency is increased by starving them for 12-24 hours, as the transit time through the gut will be increased. This is the opposite in carnivores.
• These drugs are often given in a pro-drug form which is metabolised to the sulphoxide form of the drug (the most potent form) and then to the inactive sulphone form. Netobimin is the prodrug of albendazole, whilst febantel is the prodrug fenbendazole.

Safety

• Some members of this group are teratogenic. Thus they should not be used in pregnant animals.