Книги фарма 2 / Bertram G. Katzung-Basic & Clinical Pharmacology(9th Edition)

1Not available in the USA.

Figure 53–1.

Katzung PHARMACOLOGY, 9e > Section VIII. Chemotherapeutic Drugs > Chapter 53. Antiprotozoal Drugs >

Treatment of Amebiasis

Amebiasis is infection with Entamoeba histolytica. This agent can cause asymptomatic intestinal infection, mild to moderate colitis, severe intestinal infection (dysentery), ameboma, liver abscess, and other extraintestinal infections. The choice of drugs for amebiasis depends on the clinical presentation (Figure 53–2; Table 53–4).

Figure 53–2.

Structural formulas of other antiprotozoal drugs.

Table 53–4. Treatment of Amebiasis.1

1Route is oral unless otherwise indicated. See text for additional details and cautions.

2Not available in the USA.

3Available in the USA only from the Drug Service, CDC, Atlanta (404-639-3670).

Treatment of Specific Forms of Amebiasis

Asymptomatic Intestinal Infection

Asymptomatic carriers generally are not treated in endemic areas but in nonendemic areas they are treated with a luminal amebicide. A tissue amebicidal drug is unnecessary. Standard luminal amebicides are diloxanide furoate, iodoquinol, and paromomycin. Each drug eradicates carriage in about 80–90% of patients with a single course of treatment. Therapy with a luminal amebicide is also required in the treatment of all other forms of amebiasis.

Amebic Colitis

Metronidazole plus a luminal amebicide is the treatment of choice for colitis and dysentery. Tetracyclines and erythromycin are alternative drugs for moderate colitis but are not effective against extraintestinal disease. Dehydroemetine or emetine can also be used, but these agents are best avoided (when possible) because of their toxicity.

Extraintestinal Infections

The treatment of choice is metronidazole plus a luminal amebicide. A 10-day course of metronidazole cures over 95% of uncomplicated liver abscesses. For unusual cases where initial therapy with metronidazole has failed, aspiration of the abscess and the addition of chloroquine to a repeat course of metronidazole should be considered. Dehydroemetine and emetine are toxic alternative drugs.

Metronidazole

Metronidazole, a nitroimidazole (Figure 53–2), is the drug of choice for the treatment of extraluminal amebiasis. It kills trophozoites but not cysts of E histolytica and effectively eradicates intestinal and extraintestinal tissue infections.

Chemistry & Pharmacokinetics

Oral metronidazole is readily absorbed and permeates all tissues by simple diffusion. Intracellular concentrations rapidly approach extracellular levels. Peak plasma concentrations are reached in 1–3 hours. Protein binding is low (< 20%), and the half-life of the unchanged drug is 7.5 hours. The drug and its metabolites are excreted mainly in the urine. Plasma clearance of metronidazole is decreased in patients with impaired liver function.

Mechanism of Action

The nitro group of metronidazole is chemically reduced in anaerobic bacteria and sensitive protozoans. Reactive reduction products appear to be responsible for antimicrobial activity.

Clinical Uses

Amebiasis

Metronidazole is the drug of choice for the treatment of all tissue infections with E histolytica. It is not reliably effective against luminal parasites and so must be used with a luminal amebicide to ensure eradication of the infection. Tinidazole, a related nitroimidazole, appears to have similar activity and a better toxicity profile than metronidazole, but it is not available in the USA.

Giardiasis

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 equally effective.

Trichomoniasis

Metronidazole is the treatment of choice. A single dose of 2 g is effective. Metronidazole-resistant organisms may lead to treatment failures. Tinidazole may be effective against some of these infections, but it is not available in the USA. Such cases may require repeat courses of metronidazole at higher doses than normally recommended—or topical therapy.

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.

Metronidazole has been reported to potentiate the anticoagulant effect of coumarin-type anticoagulants. Phenytoin and phenobarbital may accelerate elimination of the drug, while 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, though congenital abnormalities have not clearly been associated with use in humans.

Iodoquinol

Iodoquinol (diiodohydroxyquin) is a halogenated hydroxyquinoline. It is an effective luminal amebicide that is commonly used with metronidazole to treat amebic infections. Its pharmacokinetic properties are poorly understood. Ninety percent of the drug is retained in the intestine and excreted in the feces. The remainder enters the circulation, has a half-life of 11–14 hours, and is excreted in the urine as glucuronides.

The mechanism of action of iodoquinol against trophozoites is unknown. It is effective against organisms in the bowel lumen but not against trophozoites in the intestinal wall or extraintestinal tissues.

Infrequent adverse effects include diarrhea—which usually stops after several days—anorexia, nausea, vomiting, abdominal pain, headache, rash, and pruritus. The drug may increase proteinbound serum iodine, leading to a decrease in measured 131I uptake that persists for months. Some halogenated hydroxyquinolines can produce severe neurotoxicity with prolonged use at greater than recommended doses. Iodoquinol is not known to produce these effects at its recommended dosage, and this dosage should never be exceeded.

Iodoquinol should be taken with meals to limit gastrointestinal toxicity. It should be used with caution in patients with optic neuropathy, renal or thyroid disease, or nonamebic hepatic disease. The drug should be discontinued if it produces persistent diarrhea or signs of iodine toxicity (dermatitis, urticaria, pruritus, fever). It is contraindicated in patients with intolerance to iodine.

Diloxanide Furoate

Diloxanide furoate is a dichloroacetamide derivative. It is an effective luminal amebicide but is not active against tissue trophozoites. In the gut, diloxanide furoate is split into diloxanide and furoic acid; about 90% of the diloxanide is rapidly absorbed and then conjugated to form the glucuronide, which is promptly excreted in the urine. The unabsorbed diloxanide is the active antiamebic substance. The mechanism of action of diloxanide furoate is unknown.

Diloxanide furoate is considered by many the drug of choice for asymptomatic luminal infections, but it is no longer available in the USA. It is used with a tissue amebicide, usually metronidazole, to treat serious intestinal and extraintestinal infections. Diloxanide furoate does not produce serious adverse effects. Flatulence is common, but nausea and abdominal cramps are infrequent and rashes are rare. The drug is not recommended in pregnancy.

Paromomycin Sulfate

Paromomycin sulfate is an aminoglycoside antibiotic (see also Chapter 45: Aminoglycosides & Spectinomycin) that is not significantly absorbed from the gastrointestinal tract. It is used only as a luminal amebicide and has no effect against extraintestinal amebic infections. The small amount absorbed is slowly excreted unchanged, mainly by glomerular filtration. However, the drug may accumulate with renal insufficiency and contribute to renal toxicity. Paromomycin is an effective luminal amebicide that appears to have similar efficacy and probably less toxicity than other agents; in a recent study, it was superior to diloxanide furoate in clearing asymptomatic infections. Adverse effects include occasional abdominal distress and diarrhea. Paromomycin should be avoided in patients with significant renal disease and used with caution in persons with gastrointestinal ulcerations. Parenteral paromomycin is under investigation for the treatment of visceral leishmaniasis.

Emetine & Dehydroemetine

Emetine, an alkaloid derived from ipecac, and dehydroemetine, a synthetic analog, are effective against tissue trophozoites of E histolytica, but because of major toxicity concerns they have been almost completely replaced by metronidazole. The drugs are administered parenterally because oral preparations are absorbed erratically. They accumulate in tissues and are eliminated slowly via the kidneys.

The use of emetine and dehydroemetine is limited to unusual circumstances in which severe amebiasis warrants effective therapy and metronidazole cannot be used. Dehydroemetine is preferred over emetine because of its somewhat better toxicity profile. The drugs should be used to treat amebic dysentery or amebic liver abscess for the minimum period needed to relieve severe symptoms (usually 3–5 days).

Emetine and dehydroemetine should be administered subcutaneously (preferred) or intramuscularly (but never intravenously) in a supervised setting. Adverse effects are generally mild when the drugs are used for 3–5 days but increase with prolonged use. They should not be used for more than 10 days. Pain and tenderness in the area of injection are frequent, and sterile abscesses may develop. Diarrhea is common. Other adverse effects are nausea, vomiting, muscle weakness and discomfort, and minor electrocardiographic changes. Serious toxicities include cardiac arrhythmias, heart failure, and hypotension. The drugs should not be used in patients with cardiac or renal disease, in young children, or in pregnancy unless absolutely necessary.

Katzung PHARMACOLOGY, 9e > Section VIII. Chemotherapeutic Drugs > Chapter 53. Antiprotozoal Drugs >

Other Antiprotozoal Drugs

See Table 53–5 for a list of drugs used in the treatment of other protozoal infections. Important drugs that are not covered elsewhere in this or other chapters are discussed below.

Table 53–5. Treatment of Other Protozoal Infections.

1Established, relatively simple dosing regimens are provided. Route is oral unless otherwise indicated. See text for additional information, toxicities, cautions, and discussions of dosing for the more rarely used drugs, many of which are highly toxic.

2Nitazoxanide is also available for pediatric use.

3Not available in the USA.

4Available in the USA only from the Drug Service, CDC, Atlanta (404-639-3670).

5P jiroveci (carinii in animals) has traditionally been considered a protozoan because of its morphology and drug sensitivity, but recent molecular analyses have shown it to be most closely related to fungi.

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