Entamoeba Drug Treatment and Resistance
Emetine was first demonstrated to be an effective amoebicide in 1912 97. A miscellany of drugs in addition to emetine have since been reported to be effective to various degrees. Metronidazole has become the drug of choice following recognition of its amoebicidal properties in the mid-1960s.
Emetine (or dehydroemetine) hydrochloride is a potent tissue amoebicide. Although it continues to be a life-saving drug in appropriate situations, this drug does have serious side effects 97. The drug is slowly excreted into the gut and urine, allowing high concentrations to build up in the liver, heart, and other viscera. To illustrate, emetine may be detectable in urine 40 days after the termination of treatment. Emetine is administered by daily intramuscular or deep subcutaneous injections for a maximum of 10 days, with local pain and tenderness at the sites of injection being common. If given orally, the drug usually causes vomiting, and very little is absorbed. Given intravenously, it is dangerously cardiotoxic 97.
There is evidence that multidrug resistance transporters may be involved in emetine and colchicine accumulation in a drug-resistant selected line, because uptake was lower in the resistant line and verapamil increased drug accumulation to that seen in the parental line9, 20. Six P-glycoprotein-like genes have been cloned: four are expressed in a drug-resistant line, and there is differential expression 72.
The synthetic 5-nitroimidazole metronidazole, on the other hand, is remarkably safe in comparison to emetine and is now the recommended drug for the treatment of amoebiasis. It is also effective against a wide range of anaerobic bacteria 64 because of similar low-redox-activating enzymes. Oral doses of metronidazole are readily absorbed and can be found in most body fluids with few side effects. It is now so widely used both therapeutically and prophylactically for numerous major and minor ailments that exposure of Entamoeba to this drug worldwide is guaranteed. Inappropriate short-term exposure and exposure to sublethal levels of metronidazole are normally prescribed for prophylaxis and are precisely the conditions under which drug resistance is induced 208.
Indeed, using stepwise incremental increases in drug dose, we have induced metronidazole resistance in two axenic lines of E. histolytica 167. Parasites of strains HM-I:IMSS (isolated from a rectal ulcer from an adult male in Mexico City in 1967) and HTH-56:MUTM (established in culture from an amoebic abscess from an adult in Bangkok in 1992) currently grow in our laboratory in 10 μM metronidazole, a concentration of drug which is normally lethal to parasites in vitro 68. A minimum lethal dose for this strain has been reported as 11.6 μM over 72 h, and a 50% inhibitory dose of 29.7 μM has been reported, while the reported susceptibility of the HK-9 strain ranges from a 50% effective dose of 2 μM to a lethal dose of 11.6 μM over 100 h 167. Expression of the iron-containing SOD was increased three- to five-fold in metronidazole-resistant parasites, and a second SOD activity disappeared 167. Unlike in Giardia and Trichomonas, PFOR activity did not decrease significantly. The work with the iron-containing SOD has been confirmed at the gene expression level by Wassmann et al. 227, with parasites resistant to 40 μM metronidazole; peroxiredoxin expression was also increased, and ferredoxin 1 expression was decreased.
Unique in that it is effective both in the bowel lumen and in tissues, metronidazole has been reported to eradicate only up to 50% of luminal infections 191. This statement has support from a study of 36 patients with amoebic liver abscess for whom the hepatic lesions were cleared; but 20 were recolonized in the intestine, 16 asymptomatically. This was ascribed to the pharmacokinetics of metronidazole cycling in the liver and the action of metronidazole against trophozoites but not invariable eradication of cysts, creating E. histolytica carrier states 90, 93. These figures are considerably higher than generally published, although no single treatment has claims of 100% efficacy; 90% success after 5 to 10 days of metronidazole treatment is regarded as adequate by some workers. Alternatively, other workers have regarded that this level of efficacy suggests resistance and the need for multiple drug treatment 157. Because the organism is difficult to culture axenically from patients, there are insufficient reliable data regarding resistance levels, although a recent study indicates decreased susceptibility in clinical isolates from Chandigarh (R. Sehgal, unpublished data). Therefore, current recommendations suggest the use of metronidazole or tinidazole plus the luminal amoebicide diloxanide furoate or iodoquinol, with other combinations (including paromomycin, tetracycline, and chloroquine) depending on the severity of the infection and site, i.e., whether it is intraluminal, invasive, or abscessed 157, 191. Iodoquinol and diloxanide furoate have unknown mechanisms of action 93. Like diloxanide furoate, paromomycin is poorly absorbed. There are no reports of the very high levels of resistance to metronidazole found in Trichomonas or Giardia.
Similar to the interpretation of pathogenic versus nonpathogenic strains or species ofEntamoeba, coupled with treatment failure due to resistance, there is disagreement over which cases should receive treatment. Clearly, anyone with serious disease requires adequate treatment with the available drugs. It has been strongly argued that metronidazole should not be used prophylactically to preserve the usefulness and potency of the drug 76, 208, 218. Sargeaunt 170 is quite adamant that only E. histolytica is pathogenic, obviating the need to treat E. dispar carriers, while Diamond and Clark 48caution against witholding treatment from asymptomatic individuals because E. histolytica has been found in these carriers. Burchard 28 advises treatment of E. histolytica only, but that all cyst passers be treated in the absence of differentiation. Results of a treatment and placebo trial in Mexico were interpreted to mean that money spent treating carriers in this country could be better spent on treating cases of amoebic disease 66. All these opinions appear to be correct in their context, in the absence of adequate funds and new, effective antiprotozoal drugs, and in lieu of eradication.