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Wednesday, August 20, 2025

Anthelmintics


Introduction

Anthelmintics are a diverse class of drugs designed to eliminate parasitic worms (helminths) from the human body. Helminths are broadly categorized into:

  • Nematodes (roundworms) – e.g., Ascaris lumbricoides, Enterobius vermicularis (pinworm), Trichuris trichiura (whipworm), hookworms, Strongyloides stercoralis

  • Cestodes (tapeworms) – e.g., Taenia saginata, Taenia solium, Diphyllobothrium latum

  • Trematodes (flukes) – e.g., Schistosoma species, Clonorchis sinensis, Fasciola hepatica

Helminth infections are a global health concern, particularly in tropical and subtropical regions, affecting billions and leading to malnutrition, anemia, growth retardation, and impaired cognitive development. Anthelmintic agents work through various mechanisms, including disrupting parasite metabolism, interfering with neuromuscular coordination, or damaging their tegument.

Classification of Anthelmintics

Anthelmintic drugs can be classified based on the type of helminth they target or their pharmacological mechanism:

  1. Benzimidazoles – albendazole, mebendazole, thiabendazole

  2. Tetrahydropyrimidines – pyrantel pamoate, oxantel

  3. Imidazothiazoles – levamisole

  4. Macrocyclic lactones – ivermectin, moxidectin

  5. Salicylanilides and substituted phenols – niclosamide

  6. Isoquinoline derivatives – praziquantel

  7. Quinoline derivatives – oxamniquine

  8. Other specific agents – diethylcarbamazine (DEC), piperazine, bithionol, triclabendazole

Mechanisms of Action

  • Benzimidazoles (albendazole, mebendazole): Bind β-tubulin of helminths, inhibiting microtubule polymerization. This disrupts glucose uptake and depletes glycogen stores, leading to parasite death.

  • Pyrantel pamoate: Acts as a depolarizing neuromuscular blocking agent by activating nicotinic acetylcholine receptors, causing spastic paralysis of worms.

  • Levamisole: Nicotinic receptor agonist causing sustained paralysis, leading to expulsion of worms.

  • Ivermectin / Moxidectin: Bind glutamate-gated chloride channels in nerve and muscle cells, leading to hyperpolarization, paralysis, and death of nematodes.

  • Praziquantel: Increases calcium ion influx in parasite membranes, causing tetanic contraction and vacuolization of the tegument, exposing worms to immune attack.

  • Niclosamide: Inhibits oxidative phosphorylation in cestodes, leading to energy depletion.

  • Diethylcarbamazine (DEC): Alters surface membrane of microfilariae, increasing susceptibility to host immune system.

  • Triclabendazole: Targets flukes by inhibiting microtubule formation and altering parasite motility.

Major Drugs in the Class

1. Albendazole

  • Indications: Broad-spectrum anthelmintic; effective against nematodes (ascariasis, trichuriasis, hookworm, pinworm, strongyloidiasis), cestodes (neurocysticercosis, hydatid disease), and some trematodes.

  • Doses:

    • Ascariasis, trichuriasis, hookworm, pinworm: 400 mg single dose (repeat in 2–3 weeks if needed).

    • Strongyloidiasis: 400 mg daily for 3 days.

    • Neurocysticercosis: 15 mg/kg/day in two divided doses for 8–28 days (max 800 mg/day).

    • Hydatid disease: 400 mg twice daily for 28 days, repeated in cycles with rest periods.

  • Adverse effects: GI upset, elevated liver enzymes, headache, alopecia with prolonged use.

  • Contraindications: Pregnancy (teratogenic risk), hypersensitivity, liver disease.

2. Mebendazole

  • Indications: Nematode infections, including pinworm, whipworm, ascariasis, hookworm.

  • Doses:

    • Pinworm (Enterobiasis): 100 mg single dose, repeat in 2 weeks.

    • Ascariasis / Trichuriasis / Hookworm: 100 mg twice daily for 3 days.

  • Adverse effects: GI upset, rash, rarely neutropenia.

  • Contraindications: Pregnancy, children under 1 year.

3. Thiabendazole

  • Indications: Strongyloidiasis, cutaneous larva migrans.

  • Doses: 25 mg/kg twice daily for 2 days.

  • Adverse effects: More toxic—dizziness, anorexia, hepatitis, neuropsychiatric effects.

  • Rarely used today due to toxicity.

4. Pyrantel Pamoate

  • Indications: Pinworm, hookworm, roundworm.

  • Doses: 11 mg/kg single dose (max 1 g), repeat in 2 weeks.

  • Adverse effects: GI upset, headache, dizziness.

  • Safe in children and pregnancy (Category C).

5. Levamisole

  • Indications: Ascariasis, hookworm (less used today). Also used as immunomodulator in some cancers.

  • Doses: 2.5 mg/kg single dose.

  • Adverse effects: Agranulocytosis (serious), flu-like symptoms, rash.

  • Largely replaced by safer alternatives.

6. Ivermectin

  • Indications: Strongyloidiasis, onchocerciasis (river blindness), lymphatic filariasis, scabies, head lice.

  • Doses:

    • Onchocerciasis: 150 mcg/kg single dose, repeat every 6–12 months.

    • Strongyloidiasis: 200 mcg/kg single dose, repeat as necessary.

    • Scabies: 200 mcg/kg orally once, repeat in 1–2 weeks.

  • Adverse effects: Mazzotti reaction in onchocerciasis (fever, rash, lymphadenopathy due to microfilarial death), dizziness, hypotension.

  • Contraindications: Children <15 kg, pregnancy (relative).

7. Moxidectin

  • Indications: Onchocerciasis (FDA approved in 2018).

  • Dose: 8 mg single oral dose.

  • Advantage: Longer suppression of microfilariae compared to ivermectin.

8. Diethylcarbamazine (DEC)

  • Indications: Filariasis (Wuchereria bancrofti, Brugia malayi), loiasis.

  • Doses: 6 mg/kg/day in 3 divided doses for 12 days.

  • Adverse effects: Mazzotti-like reaction, headache, malaise, ocular complications in onchocerciasis (so contraindicated there).

9. Praziquantel

  • Indications: Schistosomiasis, clonorchiasis, paragonimiasis, cysticercosis, taeniasis.

  • Doses:

    • Schistosomiasis: 20 mg/kg twice daily for 1 day.

    • Cysticercosis: 50 mg/kg/day in 3 divided doses for 14 days.

    • Taeniasis: 5–10 mg/kg single dose.

  • Adverse effects: Headache, dizziness, abdominal pain, hypersensitivity due to dying parasites.

  • Contraindications: Ocular cysticercosis (risk of irreversible damage due to inflammation).

10. Niclosamide

  • Indications: Tapeworm infections (Taenia, Diphyllobothrium).

  • Doses: 2 g orally once, followed by a laxative.

  • Adverse effects: Mild GI upset.

  • Less used now since praziquantel is more effective.

11. Triclabendazole

  • Indications: Fascioliasis (Fasciola hepatica), paragonimiasis.

  • Doses: 10 mg/kg orally once; may repeat in 12–24 hours.

  • Adverse effects: Abdominal pain, headache, biliary colic due to parasite death.

12. Oxamniquine

  • Indications: Schistosoma mansoni infections.

  • Doses: 15 mg/kg single oral dose.

  • Adverse effects: CNS effects (drowsiness, seizures), reddish urine.

Adverse Effects (General Class Effects)

  • Gastrointestinal: nausea, abdominal pain, diarrhea.

  • Neurological: dizziness, headache, seizures (rare, drug-specific).

  • Hematological: rare agranulocytosis (levamisole, mebendazole).

  • Hepatic: elevated liver enzymes (albendazole, thiabendazole).

  • Immune-related: hypersensitivity, especially during parasite death (Mazzotti reaction).

Contraindications (General)

  • Pregnancy: Benzimidazoles and ivermectin generally contraindicated. Pyrantel considered safer.

  • Ocular involvement: Praziquantel contraindicated in ocular cysticercosis.

  • Severe hepatic impairment: Use with caution (benzimidazoles, praziquantel).

  • Infants <1 year: Most anthelmintics not recommended.

Precautions

  • Always ensure correct diagnosis and parasite identification before treatment.

  • Monitor liver function with prolonged therapy (albendazole, praziquantel).

  • Treat family contacts in cases of enterobiasis to prevent reinfection.

  • Consider resistance issues in endemic areas (albendazole resistance emerging in soil-transmitted helminths).

Drug Interactions

  • Albendazole / Mebendazole: Increased plasma levels with cimetidine, dexamethasone.

  • Praziquantel: Levels reduced by rifampicin, carbamazepine, phenytoin (CYP450 inducers). Increased by cimetidine.

  • Ivermectin: Increased CNS depression with other GABAergic agents (barbiturates, benzodiazepines).

  • DEC: Interaction with antifilarial drugs increases risk of severe reactions.

Public Health Role

Mass drug administration (MDA) with albendazole, mebendazole, or ivermectin is a cornerstone in global campaigns against soil-transmitted helminths, lymphatic filariasis, and onchocerciasis. Combination therapies (albendazole + ivermectin or albendazole + DEC) are standard in eradication programs.

Future Perspectives

  • Novel agents: Emodepside (targets SLO-1 potassium channels in nematodes) under development.

  • Combination therapies: Improve efficacy and delay resistance.

  • Vaccine research: For hookworm and schistosomiasis.

  • Nanotechnology drug delivery: Sustained-release oral formulations under study.




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