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Wednesday, August 20, 2025

Amebicides


Amebicides are a class of antiparasitic drugs specifically used for the treatment of infections caused by protozoa of the genus Entamoeba, particularly Entamoeba histolytica, the causative agent of amoebiasis. Amoebiasis is a major parasitic disease worldwide, particularly prevalent in regions with poor sanitation, contaminated food or water supply, and limited access to healthcare. Amebicides act by killing or inhibiting the growth of amoebas either within the intestinal lumen, in the intestinal wall, or in extraintestinal tissues such as the liver.

These drugs are broadly categorized into three types based on their site of action:

  1. Luminal amebicides – active in the intestinal lumen to eradicate cysts and trophozoites.

  2. Tissue/systemic amebicides – effective against trophozoites invading tissues such as the intestinal wall and liver.

  3. Mixed-action amebicides – effective in both intestinal lumen and systemic sites.

Mechanism of Action

The mechanism of action of amebicides varies depending on the drug class:

  • Nitroimidazoles (e.g., Metronidazole, Tinidazole, Secnidazole):
    After reduction of the nitro group inside anaerobic protozoa, toxic metabolites are generated that bind to DNA, causing strand breakage and inhibition of nucleic acid synthesis, leading to parasite death.

  • Luminal agents (e.g., Paromomycin, Diloxanide furoate, Iodoquinol):
    These act locally in the intestinal lumen. Paromomycin is an aminoglycoside that inhibits protein synthesis by binding to the 30S ribosomal subunit. Diloxanide furoate and Iodoquinol are poorly absorbed drugs that act directly on the amoeba in the lumen, though their exact mechanism is less well understood.

  • Systemic/tissue agents (e.g., Chloroquine, Emetine, Dehydroemetine):
    These accumulate in the liver and other tissues where invasive trophozoites reside, interfering with protein synthesis and cellular metabolism of the parasite.

Common Drugs and Dosages

1. Mixed Amebicides (Luminal + Tissue activity)

  • Metronidazole: 500–750 mg orally every 8 hours for 7–10 days (adults).

  • Tinidazole: 2 g orally once daily for 3–5 days.

  • Secnidazole: Single oral dose of 2 g.

  • Ornidazole: 500 mg orally twice daily for 5–10 days.

2. Luminal Amebicides (for intraluminal cyst eradication, used after nitroimidazoles to prevent recurrence)

  • Paromomycin: 25–35 mg/kg/day orally in 3 divided doses for 7 days.

  • Diloxanide furoate: 500 mg orally three times daily for 10 days.

  • Iodoquinol: 650 mg orally three times daily for 20 days.

3. Tissue/Systemic Amebicides

  • Chloroquine: 600 mg base daily for 2 days, followed by 300 mg base daily for 2–3 weeks (used mainly for hepatic amoebiasis when nitroimidazoles cannot be used).

  • Emetine/Dehydroemetine: 1 mg/kg/day intramuscularly or subcutaneously for 5 days (rarely used due to cardiotoxicity, now largely obsolete).

Clinical Uses

  • Intestinal amoebiasis (acute dysentery or chronic infection):
    Treated with a tissue-active drug (Metronidazole or Tinidazole) followed by a luminal agent (Paromomycin, Diloxanide furoate, or Iodoquinol) to eradicate cysts.

  • Hepatic amoebiasis (amoebic liver abscess):
    Nitroimidazoles are first-line, with Chloroquine as an adjunct if needed.

  • Asymptomatic cyst passers (carriers):
    Treated with a luminal agent alone (e.g., Paromomycin, Diloxanide furoate, Iodoquinol).

Contraindications

  • Metronidazole, Tinidazole, and related nitroimidazoles: Contraindicated in the first trimester of pregnancy and in patients with known hypersensitivity.

  • Iodoquinol: Contraindicated in patients with iodine hypersensitivity or optic neuropathy.

  • Paromomycin: Contraindicated in patients with intestinal obstruction.

  • Emetine/Dehydroemetine: Contraindicated in pregnancy, cardiac disease, or renal impairment.

Side Effects

  • Metronidazole/Tinidazole: Metallic taste, nausea, vomiting, diarrhea, abdominal cramps, headache, dizziness, peripheral neuropathy (with prolonged use), and disulfiram-like reaction with alcohol.

  • Paromomycin: Gastrointestinal upset, diarrhea, malabsorption, and rarely ototoxicity and nephrotoxicity (systemic absorption minimal).

  • Diloxanide furoate: Flatulence, abdominal cramps, nausea, pruritus.

  • Iodoquinol: Iodine toxicity, peripheral neuropathy, optic neuritis (rare).

  • Chloroquine: Headache, dizziness, pruritus, blurred vision, retinopathy with prolonged use.

  • Emetine/Dehydroemetine: Cardiotoxicity (arrhythmias, heart failure), neuromuscular weakness, nausea, hypotension.

Precautions

  • Nitroimidazoles should not be combined with alcohol due to disulfiram-like reactions.

  • Liver function should be monitored during prolonged Metronidazole therapy.

  • Iodoquinol should be avoided in patients with pre-existing optic neuropathy.

  • Caution with Paromomycin in patients with renal impairment.

Drug Interactions

  • Metronidazole: Interacts with alcohol (disulfiram-like reaction), warfarin (increased anticoagulant effect), lithium (increased toxicity), and phenytoin (altered metabolism).

  • Chloroquine: May interact with antacids, cimetidine (increases levels), and drugs that prolong QT interval.

  • Paromomycin: May enhance nephrotoxicity of other aminoglycosides.



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