Anthelmintics Overview and Mechanisms

Anthelmintics Overview

• Anthelmintics are drugs designed to kill and expel parasitic worms from the body.
• Vermifuges are agents that specifically expel parasites from the intestinal tract.
• Vermicides not only expel but also kill intestinal parasites.

Ideal Anthelmintic Properties

• Should have a broad spectrum of activity against both mature and immature parasites.
• Easy to administer with a wide margin of safety.
• Should have minimal withholding periods and be economical.
• Must be selectively toxic to helminth parasites.

Mechanism of Selectivity

• The action of anthelmintics relies on drug receptors being unique or more sensitive in parasites.
• They work by inhibiting essential metabolic processes in the parasite that are either unimportant or absent in the host.
• Pharmacokinetic properties ensure higher concentrations in parasites than in host cells.

Exceptions in Anthelmintics Usage

• Organophosphorus compounds and levamisole are exceptions, having pharmacological effects through specific mechanisms.
• Most anthelmintics can be used across various domestic animal species, with exceptions for specific drugs like levamisole and praziquantel.

Species Specificity of Anthelmintics

• Levamisole is not suitable for horses and cats.
• Praziquantel is only approved for use in dogs, cats, and horses.
• Piperazine salts are restricted to monogastrics.
• Salicylanilides, nitroxynil, and clorsulon are only for ruminant species.

Classes of Anthelmintics

• Benzimidazoles (carbamates) include Albendazole, Fenbendazole, Mebendazole, Oxfendazole, Oxibendazole, Parbendazole.
• Benzimidazoles (thiazoles) consist of Thiabendazole and Cambendazole.
• Probenzimidazoles are inactive until converted to active forms by ruminal microbes or the liver, including Netobimin and Febantel.
• Imidazothiazoles consist of Levamisole and Tetramisole.
• Tetrahydropyrimidines include Pyrantel, Morantel, and Oxantel.
• Organophosphate compounds include Dichlorvos, Trichlorfon, Haloxon, Coumaphos, and Crufomate.
• Macrocyclic lactones (avermectins) include Ivermectin, Abamectin, Doramectin, Eprinomectin, and Selamectin.
• Macrocyclic lactones (Milbemycin) consist of Milbemycin oxime and Moxidectin.
• Piperazine compounds include Piperazine and Diethylcarbamazine citrate (DEC).
• Benzenedisulphonamide refers to Clorsulon.
• Isoquinoline class has Praziquantel and Epsiprantel.
• Salicylanilide class includes Closantel, Oxyclozanide, and Rafoxanide.
• Nitrobenzonitrile class is represented by Nitroxynil.
• Arsenical compounds consist of Thiacetarsamide sodium and Melarsomine.

Benzimidazoles Efficacy

• Generally effective against nematodes; specific substituted benzimidazoles target certain tapeworms and flukes.
• Exhibit broad-spectrum efficacy against gastrointestinal and lung nematodes with a wide safety margin.
• Mechanism of action involves irreversible binding to β-tubulin in nematodes, preventing microtubule formation.
• Resistance development in parasites is often due to variations in β-tubulin.

Pharmacokinetics of Benzimidazoles

• Lower aqueous solubility leads to poor gastrointestinal absorption.
• Clinical application demands consideration of pharmacokinetics for effective treatment outcomes.