METRONIDAZOLE & TINIDAZOLE
Metronidazole, a nitroimidazole (Figure 52–3), is the drug of choice in the treatment of extraluminal amebiasis. It kills tropho-zoites but not cysts of E histolytica and effectively eradicates intestinal and extraintestinal tissue infections.
Tinidazole, a related nitroimidazole available in the USA since 2004, appears to have similar activity and a better toxicity profile than metronidazole. It offers simpler dosing regimens and can be substituted for the indications listed below.
Pharmacokinetics & Mechanism of Action
Oral metronidazole and tinidazole are readily absorbed and permeate all tissues by simple diffusion. Intracellular concentrations rapidly approach extracellular levels. Peak plasma concentrations are reached in 1–3 hours. Protein binding of both drugs is low (10–20%); the half-life of unchanged drug is 7.5 hours for metronidazole and 12–14 hours for tinidazole. Metronidazole and its metabolites are excreted mainly in the urine. Plasma clearance of metronidazole is decreased in patients with impaired liver function.
The nitro group of metronidazole is chemically reduced in anaerobic bacteria and sensitive protozoans. Reactive reduction products appear to be responsible for antimicrobial activity. The mechanism of tinidazole is assumed to be the same.
Metronidazole or tinidazole is the drug of choice in the treatment of all tissue infections with E histolytica. Neither drug is reliably effective against luminal parasites and so must be used with a luminal amebicide to ensure eradication of the infection.
Metronidazole is the treatment of choice for giardiasis. The dosage for giardiasis is much lower—and the drug thus better tolerated— than that for amebiasis. Efficacy after a single treatment is about 90%. Tinidazole is at least equally effective.
Metronidazole is the treatment of choice. A single dose of 2 g is effective. Metronidazole-resistant organisms can lead to treatment failures. Tinidazole may be effective against some of these resistant organisms.
Adverse Effects & Cautions
Nausea, headache, dry mouth, or a metallic taste in the mouth occurs commonly. Infrequent adverse effects include vomiting, diarrhea, insomnia, weakness, dizziness, thrush, rash, dysuria, dark urine, vertigo, paresthesias, and neutropenia. Taking the drug with meals lessens gastrointestinal irritation. Pancreatitis and severe central nervous system toxicity (ataxia, encephalopathy, seizures) are rare. Metronidazole has a disulfiram-like effect, so that nausea and vomiting can occur if alcohol is ingested during therapy. The drug should be used with caution in patients with central nervous system disease. Intravenous infusions have rarely caused seizures or peripheral neuropathy. The dosage should be adjusted for patients with severe liver or renal disease. Tinidazole has a similar adverse-effect profile, although it appears to be some-what better tolerated than metronidazole.
Metronidazole has been reported to potentiate the anticoagu-lant effect of coumarin-type anticoagulants. Phenytoin and phe-nobarbital may accelerate elimination of the drug, whereas cimetidine may decrease plasma clearance. Lithium toxicity may occur when the drug is used with metronidazole.
Metronidazole and its metabolites are mutagenic in bacteria. Chronic administration of large doses led to tumorigenicity in mice. Data on teratogenicity are inconsistent. Metronidazole is thus best avoided in pregnant or nursing women, although con-genital abnormalities have not clearly been associated with use in humans.