Pyrantel pamoate is a broad-spectrum antihelminthic highly effective for the treatment of pinworm, ascaris, and Trichostrongylus orientalis infections. It is moderately effective against both species of hookworm. It is not effective in trichuriasis or strongyloidiasis. Oxantel pamoate, an analog of pyrantel not available in the USA, has shown better efficacy against trichuriasis than any other single agent and promising activity in combination with albendazole or ivermectin for this indication. Pyrantel/oxantel pamoate combinations are widely used in veterinary medicine and have been studied for some human indications.
Pyrantel pamoate is a tetrahydropyrimidine derivative. It is poorly absorbed from the gastrointestinal tract and active mainly against luminal organisms. Peak plasma levels are reached in 1–3 hours. Over half of the administered dose is recovered unchanged in the feces. Pyrantel is effective against mature and immature forms of susceptible helminths within the intestinal tract but not against migratory stages in the tissues or against ova. The drug is a neuromuscular blocking agent that causes release of acetylcholine and inhibition of cholinesterase; this results in paralysis of worms, followed by expulsion.
The standard dose is 11 mg (base)/kg (maximum, 1 g), given orally once with or without food. For pinworm, the dose is repeated in 2 weeks, and cure rates are greater than 95%. The drug is available in the USA without prescription for this indication.
For ascariasis, a single dose yields cure rates of 85–100%. Treatment should be repeated if eggs are found 2 weeks after treatment. For hookworm infections, a single dose is effective against light infections; but for heavy infections, especially with Necator americanus, a 3-day course is necessary to achieve 90% cure rates. A course of treatment can be repeated in 2 weeks.
Adverse Reactions, Contraindications, & Cautions
Pyrantel’s adverse effects are infrequent, mild, and transient. They may include nausea, vomiting, diarrhea, abdominal cramps, dizziness, drowsiness, headache, insomnia, rash, fever, and weakness. Pyrantel should be used with caution in patients with liver dysfunction, as transient aminotransferase elevations have been noted. Experience with the drug in pregnant women and children younger than 2 years is limited.
Thiabendazole is an alternative to ivermectin or albendazole for the treatment of strongyloidiasis and cutaneous larva migrans.
Thiabendazole is a benzimidazole compound. Although it is a chelating agent that forms stable complexes with a number of metals, including iron, it does not bind calcium. Thiabendazole is rapidly absorbed after ingestion. With a standard dose, drug concentrations in plasma peak within 1–2 hours; the half-life is 1.2 hours. The drug is almost completely metabolized in the liver to the 5-hydroxy form; 90% is excreted in the urine in 48 hours, largely as the glucuronide or sulfonate conjugate. Thiabendazole can also be absorbed from the skin. The mechanism of action of thiabendazole is probably the same as that of other benzimidazoles (inhibition of microtubule synthesis). The drug has ovicidal effects against some parasites.