In vitro activity of the two principal oxidative metabolites of metronidazole against Bacteroides fragilis and related species

Abstract

Metronidazole and its two principal oxidative metabolites were tested in vitro against 20 clinical isolates of the Bacteroides fragilis group. Both metabolites were bactericidal, and they exhibited 65 and 5%, respectively, of the inhibitory effect of metronidazole. Additive or weak synergistic effects resulted in combination with the parent compound.Metronidazole is metabolized in humans by oxidative mechanisms or by conjugation of the parent compound (6). The major metabolic product, 1-(2-hydroxyethyl)-2-hydroxymethyl-5-nitroimidazole (hydroxy metabolite), has been shown to occur in considerable amounts in the plasma after oral or intravenous administration of metronidazole (2,3,7). A second oxidative metabolite, 1-acetic acid-2-methyl-5-nitroimidazole (acid metabolite), was found predominantly in the urine, but was also identified in the plasma (3, 7). Little is known about the activity of these two metabolites against anaerobic bacteria. In bioassays performed with two strains of Clostridium perfringens and C. sporogenes, they exhibited 30% (hydroxy metabolite) and 5% (acid metabolite) of the activity of the parent compound (4).In this study the minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) of metronidazole and its two principal oxidative metabolites were compared, using 20 clinical isolates of Bacteroides fragilis (10 strains), B. fragilis subsp. thetaiotaomicron (5 strains), and B. fragilis subsp. vulgatus (5 strains) as test organisms. The MICs were measured on Wilkins-Chalgren agar (8) by the reference dilution method proposed by Sutter et al. (5); results are summarized in Table 1. As indicated by the geometric means of the MICs, the hydroxy metabolite and the acid metabolite exhibited about 65 and 5% of the activity of the parent compound, respectively. The concentration required to inhibit 90% of the isolates was 2 ,g/ml for metronidazole and for the hydroxy metabolite and was 32 utg/ml for the acid metabolite. All of the test strains except one isolate of B. fragilis subsp. thetaiotaomicron showed a similar pattern of susceptibility.Since agar dilution does not permit MBC determination, the bactericidal action of the two metabolites was measured by microdilution technique in Schaedler broth, using an inoculum of about 106 bacteria per ml. Viable cell counts were performed at time zero and after 48 h of incubation, and the MBC was read as the lowest concentration to yield a 99.9% kill rate. Inhibition of growth was read from the same experiments; MICs differed by no more than two dilutions from the reference agar dilution meth-

In vitro activities of metronidazole and its hydroxy metabolite against Bacteroides spp

“…In these studies, the MICs for 90% of organisms tested ranged from 0.5 to 2.0 ,ug/ml, with all organisms inhibited by a metronidazole concentration of 4 to 8 ,ug/ml. Although the data are limited, similar results have been reported with the hydroxy metabolite (15,23). In these studies, the MICs for 90% of organisms tested for the active metabolite ranged from 0.5 to 2.0 ,ug/ml.…”

“…While the activity of the acid metabolite against anaerobes is negligible, the activity of the hydroxy metabolite is approximately 65% of the activity of the parent drug (15). In studies with clinical isolates, the MICs for 90% of organisms tested were 2 ,ug/ml for both metronidazole and its hydroxy metabolite, with the MICs of both agents consistently reported within 1 or 2 dilutions of each other (15,23).…”

“…In studies with clinical isolates, the MICs for 90% of organisms tested were 2 ,ug/ml for both metronidazole and its hydroxy metabolite, with the MICs of both agents consistently reported within 1 or 2 dilutions of each other (15,23). Pharmacokinetic studies have shown that the concentrations of both compounds in serum remain above the MICs for most anaerobic organisms over an 8-h dosing interval (18).…”

Pharmacokinetics and Pharmacodynamics of Intravenous Levofloxacin at 750 Milligrams and Various Doses of Metronidazole in Healthy Adult Subjects

“…The in vitro activity of the hydroxy and acid metabolites against anaerobic bacteria is approximately 30 to 65 and 5%, respectively, of that seen with the parent compound (13,26,30). In addition, the combination of the metabolite and metronidazole has also demonstrated additive or partial synergistic effects (13,27). Based on these pharmacokinetic and pharmacodynamic characteristics of metronidazole, the optimal dosing strategy may involve using higher doses with an extended dosing interval (11,27,30).…”

In vitro susceptibility of Helicobacter pylori to several antimicrobial combinations

“…So far, attention has not been focused on the role of metronidazole's hydroxymetabolite or its interactions with metronidazole, amoxicillin, and tetracycline-HCl against H. pylon in vitro. Metronidazole and its hydroxymetabolite can act synergistically against both anaerobic and facultative anaerobic bacteria, such as Bacteroides fragilis and Actinobacillus actinomycetemcomitans (3,9). Moreover, for the latter species, the MICs of the hydroxymetabolite are 2 to 4 times lower than those of metronidazole (4,9).…”