Anthelmintic and Antiprotozoal Drugs
Anthelmintic and antiprotozoal drugs target unique metabolic pathways of parasitic worms and protozoa, treating infections ranging from intestinal helminths to systemic protozoal diseases.
Overview
Parasitic infections caused by helminths (worms) and protozoa affect over a billion people globally, primarily in tropical and subtropical regions. Antiparasitic drug development exploits metabolic differences between parasites and their human hosts, targeting parasite-specific structures, energy metabolism, and neurotransmitter systems.
Benzimidazoles (Anthelmintics)
Albendazole and mebendazole bind to beta-tubulin in helminths with higher affinity than human beta-tubulin, inhibiting microtubule polymerization. Disruption of the cytoskeleton impairs glucose uptake, transport, and cellular function in worms, leading to death.
- Albendazole: Better absorbed; used for roundworm (Ascaris), hookworm, whipworm, strongyloidiasis (with ivermectin), neurocysticercosis (with dexamethasone to manage inflammation), hydatid disease, microsporidiosis
- Mebendazole: Poorly absorbed (advantage for GI helminths — high luminal concentrations); pinworm, roundworm, hookworm, whipworm
Ivermectin
Ivermectin is a macrocyclic lactone that potentiates glutamate-gated chloride channels (invertebrate-specific) and GABA-gated channels in helminths, causing hyperpolarization of nerve and muscle cells, paralysis, and death. Mammals lack glutamate-gated Cl⁻ channels; human GABA channels are protected by the blood-brain barrier.
Uses: Strongyloides stercoralis (drug of choice), onchocerciasis (river blindness), lymphatic filariasis (with albendazole), scabies, head lice.
Mazzotti reaction: Systemic inflammatory response from dying microfilariae in onchocerciasis — manage with antihistamines/steroids.
Nitroimidazoles (Antiprotozoals)
Metronidazole and tinidazole: Activated by protozoal/anaerobic bacterial nitroreductases to toxic free radical intermediates that damage DNA. Active against anaerobic protozoa and bacteria.
Uses: Giardia lamblia (intestinal giardiasis), Entamoeba histolytica (amebic dysentery + liver abscess — follow with paromomycin to clear luminal cysts), Trichomonas vaginalis, Clostridium difficile (oral, alternative to vancomycin), bacterial vaginosis, Helicobacter pylori (combination therapy). Avoid alcohol during therapy and 48 hours after (disulfiram-like reaction).
Antiprotozoals for Leishmaniasis and Trypanosomiasis
- Amphotericin B liposomal: First-line for visceral leishmaniasis (kala-azar)
- Miltefosine: Oral agent for visceral leishmaniasis; teratogenic
- Benznidazole/nifurtimox: Chagas disease (T. cruzi)
- Melarsoprol/eflornithine: African sleeping sickness (T. brucei CNS stage); melarsoprol causes fatal reactive encephalopathy in ~5%
Key Takeaways
- Benzimidazoles (albendazole, mebendazole) inhibit helminth microtubule function; used for most intestinal worm infections
- Ivermectin activates invertebrate-specific glutamate-gated chloride channels; drug of choice for Strongyloides and onchocerciasis
- Metronidazole is activated by anaerobic nitroreductases; treats Giardia, Entamoeba, Trichomonas, and anaerobic bacteria
- Alcohol must be avoided with metronidazole due to disulfiram-like (acetaldehyde accumulation) reaction