Mechanism of Action

Metronidazole possesses bactericidal activity against obligate anaerobes, although the mechanism of action has not yet been thoroughly elucidated. Metronidazole is a prodrug, requiring intracellular nitroreduction to become active; thus metronidazole in the unchanged form is not pharmacologically active.¹ During the process of reduction, cytotoxic intermediates are formed, and these intermediates are thought to be responsible for killing the cells. The reduction process depends on ongoing energy metabolism but not on ongoing cell multiplication, which translates into activity against both dividing and nondividing cells.^1,4

Spectrum of Activity and Clinical Uses

Anaerobic bacteria of the Bacteroides fragilis group are known to be the most clinically important anaerobic pathogens, owing to their multidrug-resistant nature and the frequency with which they are involved in infectious diseases including polymicrobial infections such as intraabdominal infections, obstetric-gynecologic infections, and diabetic foot infections.^5,6 Nosocomial diarrhea and/or pseudomembranous colitis associated with antibiotic use are frequently caused by Clostridium difficile, another clinically important anaerobe.⁷ Metronidazole is highly effective against both of these medically relevant anaerobes (Table 125-1). Although metronidazole possesses significant antimicrobial activity against several obligate anaerobes, it is not considered to be clinically active versus aerobic bacteria.¹ Tinidazole has similar spectrum of activity to metronidazole against most anaerobic bacteria, including B. fragilis and microaerophilic bacteria such as Helicobacter pylori and Campylobacter spp., but is only currently approved for the treatment of trichomoniasis, giardiasis, amebiasis, and amebic liver abscess. Other uses outside of these indications are considered experimental.⁸

TABLE 125-1 Metronidazole Minimal Inhibitory Concentration and Percent Susceptibility for Various Anaerobes

MIC90, minimum inhibitory concentration that inhibits 90% of organisms.

* Includes Bacteroides fragilis, B. distasonis, B. thetaiotaomicron, B. ovatus, B. vulgatus, B. uniformis.

Adapted and modified from Drummond LJ, McCoubrey J, Smith DG et al. Changes in sensitivity patterns to selected antibiotics in Clostridium difficile in geriatric in-patients over an 18-month period. J Med Microbiol 2003;52:259-63; and from Aldridge KE, Ashcraft D, Cambre K et al. Multicenter survey of the changing in vitro antimicrobial susceptibilities of clinical isolates of Bacteroides fragilis group, Prevotella, Fusobacterium, Porphyromonas, and Peptostreptococcus species. Antimicrob Agents Chemother 2001;45:1238-43.

Clinically, metronidazole has been used successfully to treat anaerobic bacteremia, endocarditis, meningitis, brain abscesses, intraabdominal infections, and mixed aerobic-anaerobic infections, although the addition of an antibiotic effective against aerobic bacteria is necessary for the latter.^2,9–11 Additionally, although without formal FDA approval, metronidazole remains the drug of choice for mild and moderate CDI due to historical and epidemic BI/NAP1/027 strains, owing to excellent oral bioavailability, low potential for selecting for vancomycin-resistant Enterococcus (VRE), lack of detectable resistance among BI/NAP/027 strains, and low cost.¹² However, in patients with severe CDI, treatment with metronidazole resulted in a less than optimal response compared to oral vancomycin therapy.¹³ Consequently, vancomycin is the preferred agent for severe CDI.¹²

Pharmacokinetics

Given orally, metronidazole is almost completely absorbed, with a bioavailability of greater than 90%.¹⁴ In patients with CDI, absorption of oral metronidazole is reduced due to increased bowel emptying causing fecal concentrations to be high, coupled with secretion from plasma into the colon.¹⁵ However, levels decrease rapidly after treatment of CDI is initiated, from 9.3 mg/g in watery stools to 1.2 mg/g in formed stools to an undetectable level once diarrhea has resolved.¹⁵ Intravenous metronidazole is able to maintain high fecal levels in patients with CDI with toxic megacolon or ileus; otherwise, oral metronidazole is recommended.^12,15 Metronidazole is a relatively small molecular entity (molecular weight = 171.16 D) with low protein binding (<20%) and is widely distributed throughout the body.¹ The steady-state volume of distribution in adults is 0.51 to 1.1 L/kg.¹³ The elimination half-life of metronidazole is 6 to 8 hours for patients with normal liver function.^1,14 Metronidazole undergoes metabolism in the liver to form five known metabolites, two of which are 1-(2-hydroxyethyl)-2-hydroxymethyl-5-nitroimidazole (the hydroxy metabolite) and 2-methyl-nitroimidazole-1-acetic acid (the acid metabolite). The hydroxy metabolite exhibits 30% to 65% of the anaerobic activity of the parent compound.¹⁴

Adverse Reactions

The most common side effects of metronidazole treatment (at standard doses) are gastrointestinal disturbances including mild nausea, a bad/metallic taste in the mouth, and furring of the tongue. More rare adverse reactions to metronidazole include vaginal and/or urethral burning, dark/discolored urine, and neurologic toxicity such as headache, ataxia, vertigo, somnolence, depression, and peripheral neuropathy.¹ Metronidazole is recognized for causing a disulfiram-like reaction with the concurrent ingestion of alcohol. However, a study conducted by Visapää and coworkers found no evidence of disulfiram-like properties of metronidazole when it was given concomitantly with ethanol,¹⁶ and this reaction has also been disputed by others.¹⁷

Pharmacodynamics

The standard dosing regimen for metronidazole (500-1000 mg q 6-8h) was determined long before pharmacodynamics emerged as a science. Metronidazole exhibits concentration-dependent bactericidal activity along with a significant post-antibiotic effect (>3 h).^14,18–20 These factors, in combination with a long half-life and a favorable safety profile, provide a wide corridor to manipulate the metronidazole dose and dosage interval. Much more convenient regimens of larger doses (e.g., 1000-1500 mg) given every 12 hours or once daily are plausible because of the pharmacokinetic and pharmacodynamic (PK/PD) characteristics of this antibiotic.^18,19 PK analyses show that similar and adequate drug exposure is achievable with metronidazole doses of 500 mg every 8 hours, 1000 mg daily, or 1500 mg daily.^19,21 Therefore, from a convenience and cost standpoint, once-daily doses of metronidazole may be adequate when the organism minimum inhibitory concentration (MIC) is less than 2 mg/L.^19,21 Knowledge of pharmacodynamic parameters and utilization of such parameters to appropriately dose patients is of utmost importance in the current era of antimicrobial resistance.

Resistance

With more than 40 years of clinical use, worldwide resistance of anaerobes to metronidazole is estimated to be less than 5%.²² A recent multicenter study conducted in the United States reported the first confirmed metronidazole-resistant B. fragilis isolate (MIC = 64 mg/mL) in 2002.²³ Following this report, additional data collected revealed two more isolates that were metronidazole resistant, one of which was also a B. fragilis isolate.²² This report is the first to document metronidazole resistance among Bacteroides spp. and, although negligible, still raises concern, since susceptibility testing is not typically performed on anaerobic cultures. The concern with increasing resistance is not limited to metronidazole alone but includes agents such as ampicillin/sulbactam, clindamycin, and moxifloxacin. Susceptibility of carbapenems, cefoxitin, and piperacillin/tazobactam appears stable.²²

Four genes (chromosomally borne nimB and plasmid-borne nimA, nimC, and nimD) of Bacteroides spp. are commonly associated with metronidazole resistance.^1,11 The suggested mechanism of resistance mediated by these genes is the conversion of the nitro group of metronidazole to an amino group, foregoing the formation of the toxic nitroradicals.¹¹ Evidence of gene transfer has also been found within different Bacteroides spp. and between Bacteroides and Prevotella.¹¹

Diniz and associates exposed B. fragilis group species to 4 mg/L of metronidazole and found that exposure to low levels of metronidazole increased both the virulence and the viability of the isolates.⁶ Another factor to consider is the supposed protective effect of Enterococcus faecalis on B. fragilis when exposed to metronidazole.²⁴ The investigators found that E. faecalis was able to negate the bactericidal effect of metronidazole on B. fragilis. However, a more recent study could not confirm these findings.²⁵

Using a resistance breakpoint of 32 mg/L or higher for metronidazole, Peláez and coworkers, when studying 415 C. difficile isolates, found that 6.3% of the isolates were resistant.²⁶ Another study evaluated the susceptibility patterns of 186 C. difficile isolates from a geriatric population.⁷ Contrary to the findings of Peláez and associates, no resistance to metronidazole was documented.

Susceptibility testing of anaerobes is usually either not performed or not used to make clinical decisions because of several limiting factors: the slow growth of anaerobes, convolution of the testing method, questions surrounding the appropriate testing media, involvement of multiple organisms in anaerobic infections, and the generally held belief that susceptibility patterns of anaerobes have not changed over the years and remain forseeable.²⁷ Studies have proven the value and importance of susceptibility testing, showing that appropriate initial therapy is critical to a positive patient outcome²⁸ and that in vitro susceptibility results reliably predict the clinical outcome of patients.²⁷ Therefore, clinicians must realize that susceptibility testing of anaerobes is necessary and that the susceptibility patterns have changed over the years.