Antiparasitic and Antimicrobial Effects. Metronidazole and related nitroimidazoles are active in vitro against a wide variety of anaerobic protozoal parasites and anaerobic bacteria (Freeman et al., 1997). The compound is directly trichomonacidal. Sensitive isolates of T. vaginalis are killed by <0.05 μg/mL of the drug under anaerobic conditions; higher concentrations are required when 1% oxygen is present or to affect isolates from patients who display poor therapeutic responses to metronidazole. The drug also has potent amebicidal activity against E. histolytica. Trophozoites of G. lamblia are affected by metronidazole at concentrations of 1-50 μg/mL in vitro. In vitro studies on drug-sensitive and drug-resistant protozoan parasites indicate that the nitro group on C5 of metronidazole is essential for activity and that substitutions at the 2 position of the imidazole ring that enhance the resonance conjugation of the chemical structure increase antiprotozoal activity. In contrast, substitution of an acyl group at the 2 position ablates such conjugation and reduces antiprotozoal activity (Upcroft et al., 1999).
Metronidazole manifests antibacterial activity against all anaerobic cocci and both anaerobic gram-negative bacilli, including Bacteroides spp., and anaerobic spore-forming gram-positive bacilli. Nonsporulating gram-positive bacilli often are resistant, as are aerobic and facultatively anaerobic bacteria.
Metronidazole is clinically effective in trichomoniasis, amebiasis, and giardiasis, as well as in a variety of infections caused by obligate anaerobic bacteria, including Bacteroides, Clostridium, and microaerophilic bacteria such as Helicobacter and Campylobacter spp. Metronidazole may facilitate extraction of adult guinea worms in dracunculiasis even though it has no direct effect on the parasite (Chapter 51).
Mechanism of Action and Resistance. Metronidazole is a prodrug; it requires reductive activation of the nitro group by susceptible organisms. Its selective toxicity toward anaerobic and microaerophilic pathogens such as the amitochondriate protozoa T. vaginalis, E. histolytica, and G. lamblia and various anaerobic bacteria derives from their energy metabolism, which differs from that of aerobic cells (Land and Johnson, 1997; Samuelson, 1999; Upcroft and Upcroft, 1999). These organisms, unlike their aerobic counterparts, contain electron transport components such as ferredoxins, small Fe–S proteins that have a sufficiently negative redox potential to donate electrons to metronidazole. The single electron transfer forms a highly reactive nitro radical anion that kills susceptible organisms by radical-mediated mechanisms that target DNA and possibly other vital biomolecules. Metronidazole is catalytically recycled; loss of the active metabolite's electron regenerates the parent compound. Increasing levels of O2 inhibit metronidazole-induced cytotoxicity because O2 competes with metronidazole for electrons generated by energy metabolism. Thus, O2 can both decrease reductive activation of metronidazole and increase recycling of the activated drug. Anaerobic or microaerophilic organisms susceptible to metronidazole derive energy from the oxidative fermentation of ketoacids such as pyruvate. Pyruvate decarboxylation, catalyzed by pyruvate:ferredoxin oxidoreductase (PFOR), produces electrons that reduce ferredoxin, which, in turn, catalytically donates its electrons to biological electron acceptors or to metronidazole.
Clinical resistance to metronidazole is well documented for T. vaginalis, G. lamblia, and a variety of anaerobic and microaerophilic bacteria but has yet to be shown for E. histolytica. Resistant strains of T. vaginalis derived from nonresponsive patients have shown two major types of abnormalities when tested under aerobic conditions. The first correlates with impaired oxygen-scavenging capabilities, leading to higher local O2 concentrations, decreased activation of metronidazole, and futile recycling of the activated drug. The second type is associated with lowered levels of PFOR and ferredoxin, the latter owing to reduced transcription of the ferredoxin gene. That PFOR and ferredoxin are not completely absent may explain why infections with such strains usually respond to higher doses of metronidazole or more prolonged therapy. Studies on metronidazole-resistant isolates of G. intestinalis indicate that similar mechanisms may be operating, with PFOR levels reduced 5-fold compared with susceptible strains (Upcroft and Upcroft, 2001). Metronidazole resistance has not been found in clinical isolates of E. histolytica but has been induced in vitro by culturing trophozoites in gradually increasing concentrations of the drug. Interestingly, although some decrease in PFOR levels was reported, metronidazole resistance was mediated primarily by increased expression of superoxide dismutase and peroxiredoxin in amebic trophozoites (Wassmann et al., 1999). Resistance of anaerobic bacteria to metronidazole is being recognized increasingly and has important clinical consequences. In the case of Bacteroides spp., metronidazole resistance has been linked to a family of nitroimidazole (nim) resistance genes, nimA, -B, -C, -D, -E, and -F, that can be encoded chromosomally or episomally (Gal and Brazier, 2004). The exact mechanisms underlying resistance are not known, but nim genes appear to encode a nitroimidazole reductase capable of converting a 5-nitroimidazole to a 5-aminoimidazole, thus stopping the formation of the reactive nitroso group responsible for microbial killing. Metronidazole has been used widely for the treatment of the microaerophilic organism Helicobacter pylori, the major cause of ulcer disease and gastritis worldwide. However, Helicobacter can develop resistance to metronidazole rapidly. Multiple mechanisms probably are operating, but there are data associating loss-of-function mutations in an oxygen-independent NADPH nitroreductase (rdxA gene) with resistance to metronidazole (Mendz and Mégraud, 2002).
Absorption, Fate, and Excretion. The pharmacokinetic properties of metronidazole and its two major metabolites have been investigated intensively (Lamp et al., 1999). Preparations of metronidazole are available for oral, intravenous, intravaginal, and topical administration. The drug usually is absorbed completely and promptly after oral intake, reaching concentrations in plasma of 8-13 μg/mL within 0.25-4 hours after a single 500-mg dose. (Mean effective concentrations of the compound are ≤8 μg/mL for most susceptible protozoa and bacteria.) A linear relationship between dose and plasma concentration pertains for doses of 200-2000 mg. Repeated doses every 6-8 hours result in some accumulation of the drug; systemic clearance exhibits dose dependence. The t1/2 of metronidazole in plasma is ~8 hours; its volume of distribution approximates total body water. Less than 20% of the drug is bound to plasma proteins. With the exception of the placenta, metronidazole penetrates well into body tissues and fluids, including vaginal secretions, seminal fluid, saliva, breast milk, and CSF.
After an oral dose, >75% of labeled metronidazole is eliminated in the urine largely as metabolites; ~10% is recovered as unchanged drug. The liver is the main site of metabolism, and this accounts for >50% of the systemic clearance of metronidazole. The two principal metabolites result from oxidation of side chains, a hydroxy derivative and an acid. The hydroxy metabolite has a longer t1/2 (~12 hours) and has ~50% of the antitrichomonal activity of metronidazole. Formation of glucuronides also is observed. Small quantities of reduced metabolites, including ring-cleavage products, are formed by the gut flora. The urine of some patients may be reddish brown owing to the presence of unidentified pigments derived from the drug. Oxidative metabolism of metronidazole is induced by phenobarbital, prednisone, rifampin, and possibly ethanol. Cimetidine appears to inhibit hepatic metabolism of the drug.
Therapeutic Uses. The uses of metronidazole for anti-protozoal therapy have been reviewed extensively (Freeman et al., 1997; Nash, 2001; Stanley, 2003). Metronidazole cures genital infections with T. vaginalis in both females and males in >90% of cases. The preferred treatment regimen is 2 g metronidazole as a single oral dose for both males and females. Tinidazole, which has a longer t1/2 than metronidazole, is also used at a 2-g single dose and appears to provide equivalent or better responses than metronidazole. For patients who cannot tolerate a single 2-g dose of metronidazole (or 1 g twice daily in the same day), an alternative regimen is a 250-mg dose given three times daily or a 375-mg dose given twice daily for 7 days. When repeated courses or higher doses of the drug are required for uncured or recurrent infections, it is recommended that intervals of 4-6 weeks elapse between courses. In such cases, leukocyte counts should be carried out before, during, and after each course of treatment.
Treatment failures owing to the presence of metronidazole-resistant strains of T. vaginalis are becoming increasingly common. Most of these cases can be treated successfully by giving a second 2-g dose to both patient and sexual partner. In addition to oral therapy, the use of a topical gel containing 0.75% metronidazole or a 500- to 1000-mg vaginal suppository will increase the local concentration of drug and may be beneficial in refractory cases. Metronidazole is an effective amebicide and is the agent of choice for the treatment of all symptomatic forms of amebiasis, including amebic colitis and amebic liver abscess. The recommended dose is 500-750 mg metronidazole taken orally three times daily for 7-10 days, or for children, 35-50 mg/kg/day given in three divided doses for 7-10 days.
Although standard recommendations are for 7-10 days' duration of therapy, amebic liver abscess has been treated successfully by short courses (2.4 g daily as a single oral dose for 2 days) of metronidazole or tinidazole (Stanley, 2003). E. histolytica persist in most patients who recover from acute amebiasis after metronidazole therapy, so it is recommended that all such individuals also be treated with a luminal amebicide.
Although effective for the therapy of giardiasis, metronidazole has yet to be approved for treatment of this infection in the U.S. However, tinidazole is approved for the treatment of giardiasis as a single 2-g dose and is appropriate first-line therapy. Metronidazole is a relatively inexpensive, highly versatile drug with clinical efficacy against a broad spectrum of anaerobic and microaerophilic bacteria. It is used for the treatment of serious infections owing to susceptible anaerobic bacteria, including Bacteroides, Clostridium, Fusobacterium, Peptococcus, Peptostreptococcus, Eubacterium, and Helicobacter. The drug is also given in combination with other antimicrobial agents to treat polymicrobial infections with aerobic and anaerobic bacteria. Metronidazole achieves clinically effective levels in bones, joints, and the CNS. Metronidazole can be given intravenously when oral administration is not possible. A loading dose of 15 mg/kg is followed 6 hours later by a maintenance dose of 7.5 mg/kg every 6 hours, usually for 7-10 days. Metronidazole is used as a component of prophylaxis for colorectal surgery and is employed as a single agent to treat bacterial vaginosis. It is used in combination with other antibiotics and a proton pump inhibitor in regimens to treat infection with H. pylori (Chapter 45).
Metronidazole is used as primary therapy for Clostridium difficile infection, the major cause of pseudomembranous colitis. Given at doses of 250-500 mg orally three times daily for 7-14 days (or even longer), metronidazole is an effective and cost-saving alternative to oral vancomycin therapy. However, a reported increase in treatment failures and higher rates of disease recurrence with metronidazole (as compared to oral vancomycin) are raising questions about the role of metronidazole as first-line therapy (McFarland, 2008). Finally, metronidazole is also used in the treatment of patients with Crohn's disease who have perianal fistulas, and it can help control colonic (but not small bowel) Crohn's disease. However, high doses (750 mg three times daily) for prolonged periods may be necessary, and neurotoxicity may be limiting (Podolsky, 2002). Metronidazole and other nitroimidazoles can sensitize hypoxic tumor cells to the effects of ionizing radiation, but these drugs are not used clinically for this purpose.
Toxicity, Contraindications, and Drug Interactions. The toxicity of metronidazole has been reviewed (Raether and Hanel, 2003). Side effects only rarely are severe enough to discontinue therapy. The most common are headache, nausea, dry mouth, and a metallic taste. Vomiting, diarrhea, and abdominal distress are experienced occasionally. Furry tongue, glossitis, and stomatitis occurring during therapy may be associated with an exacerbation of candidiasis. Dizziness, vertigo, and very rarely, encephalopathy, convulsions, incoordination, and ataxia are neurotoxic effects that warrant discontinuation of metronidazole. The drug also should be withdrawn if numbness or paresthesias of the extremities occur. Reversal of serious sensory neuropathies may be slow or incomplete. Urticaria, flushing, and pruritus are indicative of drug sensitivity that can require withdrawal of metronidazole. Metronidazole is a rare cause of Stevens-Johnson syndrome (toxic epidermal necrolysis); one report described a high rate of this syndrome among individuals receiving high doses of metronidazole and concurrent therapy with the antihelminthic mebendazole (Chen et al., 2003).
Dysuria, cystitis, and a sense of pelvic pressure have been reported. Metronidazole has a well-documented disulfiram-like effect, and some patients experience abdominal distress, vomiting, flushing, or headache if they drink alcoholic beverages during or within 3 days of therapy with this drug. Patients should be cautioned to avoid consuming alcohol during metronidazole treatment even though the risk of a severe reaction is low. By the same token, metronidazole and disulfiram or any disulfiram-like drug should not be taken together because confusional and psychotic states may occur. Although related chemicals have caused blood dyscrasias, only a temporary neutropenia, reversible after discontinuation of therapy, occurs with metronidazole.
Metronidazole should be used with caution in patients with active disease of the CNS because of its potential neurotoxicity. The drug also may precipitate CNS signs of lithium toxicity in patients receiving high doses of lithium. Plasma levels of metronidazole can be elevated by drugs such as cimetidine that inhibit hepatic microsomal metabolism. Moreover, metronidazole can prolong the prothrombin time of patients receiving therapy with coumadin anticoagulants. The dosage of metronidazole should be reduced in patients with severe hepatic disease.
Given in high doses for prolonged periods, metronidazole is carcinogenic in rodents; it also is mutagenic in bacteria (Raether and Hanel, 2003). Mutagenic activity is associated with metronidazole and several of its metabolites found in the urine of patients treated with therapeutic doses of the drug. However, there is no evidence that therapeutic doses of metronidazole pose any significant increased risk of cancer to human patients. There is conflicting evidence about the teratogenicity of metronidazole in animals. Although metronidazole has been taken during all stages of pregnancy with no apparent adverse effects, its use during the first trimester generally is not advised.