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Mechanism of Action

Metronidazole is an antibacterial drug [see Microbiology]

Pharmacokinetics Healthy Subjects

Following a single, intravaginal 5 gram dose of metronidazole vaginal gel (equivalent to 37.5 mg of metronidazole) to 38 healthy female subjects, a mean maximum serum metronidazole concentration of 281 ng/mL was reported (range: 134 to 464 ng/mL). The average time to achieve this Cmax was 9.5 hours (range: 4 to 17 hours) after dosing with metronidazole vaginal gel. This Cmax is approximately 2% of the mean maximum serum concentration reported in healthy subjects administered a single, oral 500 mg dose of metronidazole (mean Cmax = 12,785 ng/mL).

The extent of exposure [area under the curve (AUC)] of metronidazole, when administered as a single intravaginal 5 gram dose of metronidazole vaginal gel (equivalent to 37.5 mg of metronidazole), was 5,989 ng•hr/mL (range: 2,797 to 10,515 ng•hr/mL). This AUC0-∞ is approximately 5% of the reported AUC of metronidazole following a single oral 500 mg dose of metronidazole approximately 125,000 ng•hr/mL.

Patients with Bacterial Vaginosis

Following single and multiple 5 gram doses of a similar metronidazole vaginal gel product to 4 patients with bacterial vaginosis, a mean maximum serum metronidazole concentration of 214 ng/mL on day one and 294 ng/mL (range: 228 to 349 ng/mL) on day five were reported. Steady state metronidazole serum concentrations following oral dosages of 400 to 500 mg BID have been reported to range from 6,000 to 20,000 ng/mL.


Mechanism of Action

The intracellular targets of action of metronidazole on anaerobes are largely unknown. The 5-nitro group of metronidazole is reduced by metabolically active anaerobes, and studies have demonstrated that the reduced form of the drug interacts with bacterial DNA. However, it is not clear whether interaction with DNA alone is an important component in the bactericidal action of metronidazole on anaerobic organisms.

Activity In Vitro

Metronidazole is an antibacterial agent active in vitro against most strains of the following organisms that have been reported to be associated with bacterial vaginosis:

Bacteroides spp.
Gardnerella vaginalis


Standard methodology for the susceptibility testing of the potential bacterial vaginosis pathogens, Gardnerella vaginalis and Mobiluncus spp., has not been defined. Culture and sensitivity testing of bacteria are not routinely performed to establish the diagnosis of bacterial vaginosis [see Clinical Studies].

Clinical Studies

A single, randomized, double-blind, active-controlled clinical trial was conducted to evaluate the efficacy of VANDAZOLE (metronidazole vaginal gel) for the treatment of bacterial vaginosis. A clinical diagnosis of bacterial vaginosis was defined by the presence of a homogeneous vaginal discharge that (a) has a pH of greater than 4.5, (b) emits a “fishy” amine odor when mixed with a 10% KOH solution, and (c) contains clue cells on microscopic examination. Gram's stain results consistent with a diagnosis of bacterial vaginosis include (a) markedly reduced or absent Lactobacillus morphology, (b) predominance of Gardnerella morphotype, and (c) absent or few white blood cells. Non-pregnant females at least 18 years of age were randomized to receive treatment with either VANDAZOLE or another formulation of metronidazole vaginal gel 0.75% once daily at bedtime for 5 days. The modified intent-to treat population (patients who received study medication and had a Nugent score ≥ 4) consisted of 229 VANDAZOLE (metronidazole vaginal gel) patients and 243 patients treated with another vaginal formulation of metronidazole. Therapeutic Cure defined as a clinical cure and Nugent score < 4 was assessed Day 22-31. Table 1 shows the therapeutic, clinical and Nugent score cure rates in this trial. The therapeutic cure rate was 42.8% for the VANDAZOLE (metronidazole vaginal gel) group and 30.9% for the comparator group (95% confidence interval about the 11.9% difference in therapeutic cure rate: 2.8% to 21.0%).

Table 1 : Efficacy of Vandazole (metronidazole vaginal gel) for the Treatment of Bacterial Vaginosis in a Randomized, Double-Blind Active Controlled Studya

Outcome Vandazole (metronidazole vaginal gel)
n = 229
% Cure
Active Control
n = 243
% Cure
Treatment Difference (%) [95% Confidence Interval]
Therapeutic Cure 42.8 30.9 11.9 [2.8, 21.0]
Clinical Cure 52.4 45.3 7.1 [-2.3, 16.5]
Nugent Score Cure 52.0 41.1 10.9 [1.5, 20.3]
a Modified intent-to-treat population

Last reviewed on RxList: 4/4/2011
This monograph has been modified to include the generic and brand name in many instances.

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