METRONIDAZOLE

METRONIDAZOLE- metronidazole injection, solution
General Injectables and Vaccines, Inc.

BOXED WARNING

WARNING

Metronidazole has been shown to be carcinogenic in mice and rats (see PRECAUTIONS). Unnecessary use of the drug should be avoided. Its use should be reserved for the conditions described in the INDICATIONS AND USAGE section below.

DESCRIPTION

Metronidazole Injection, USP is a sterile, nonpyrogenic, isotonic, buffered parenteral dosage form of metronidazole in water for injection.

Each 100 mL contains metronidazole 500 mg (5 mg/mL) and sodium chloride 790 mg in water for injection; with dibasic sodium phosphate (anhydrous) 48 mg and citric acid (anhydrous) 23 mg added as buffers. The osmolarity of this solution is 314 mOsmol/liter (calc.). Each 100 mL contains 14 mEq sodium, pH 5.8 (4.5 – 7.0).

Metronidazole is classified as a synthetic antibacterial and antiprotozoal agent and is administered by the intravenous route.

Metronidazole, USP is chemically designated 2-methyl-5-nitroimidazole-1-ethanol (C6 H9 N3 O3 ), a crystalline powder sparingly soluble in water. It has the following structural formula:

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Sodium Chloride, USP is chemically designated NaCl, a white crystalline powder freely soluble in water.

Water for Injection, USP is chemically designated H2 O.

The flexible plastic container is fabricated from a specially formulated polyvinylchloride. Water can permeate from inside the container into the overwrap but not in amounts sufficient to affect the solution significantly. Solutions inside the plastic container also can leach out certain of its chemical components in very small amounts before the expiration period is attained. However, the safety of the plastic has been confirmed by tests in animals according to USP biological standards for plastic containers.

CLINICAL PHARMACOLOGY

In patients treated with metronidazole injection using a dosage regimen of 15 mg/kg loading dose followed six hours later by 7.5 mg/kg every six hours, the average peak steady-state concentrations (Cmax ) and trough (Cmin ) were 25 mcg/mL and 18 mcg/mL, respectively. Plasma concentrations of metronidazole are proportional to the administered dose. An eight-hour intravenous infusion of 100 mg to 4,000 mg of metronidazole in normal subjects showed a linear relationship between dose and peak plasma concentration. The average elimination half-life of metronidazole in healthy subjects is eight hours.

Distribution

Metronidazole is the major component appearing in the plasma, with lesser quantities of metabolites also being present. Less than 20% of the circulating metronidazole is bound to plasma proteins. Metronidazole appears in cerebrospinal fluid, saliva and breast milk in concentrations similar to those found in plasma. Bactericidal concentrations of metronidazole have also been detected in pus from hepatic abscesses.

Following a single intravenous dose of metronidazole 500 mg, 4 healthy subjects who underwent gastrointestinal endoscopy had peak gastric juice metronidazole concentrations of 5 to 6 mcg/mL at one hour post-dose. In patients receiving intravenous metronidazole in whom gastric secretions are continuously removed by nasogastric aspiration, sufficient metronidazole may be removed in the aspirate to cause a reduction in serum levels.

Metabolism

The metabolites of metronidazole result primarily from side-chain oxidation [1-(βhydroxyethyl)-2-hydroxymethyl-5-nitroimidazole and 2-methyl-5-nitroimidazole-1-ylacetic acid] and glucuronide conjugation. Both the parent compound and the hydroxyl metabolite possess in vitro antimicrobial activity.

Excretion

The major route of elimination of metronidazole and its metabolites is via the urine (60 to 80% of the dose), with approximately 20% of the amount excreted appearing as unchanged metronidazole. Renal clearance of metronidazole is approximately 10 mL/min/1.73 m2. Fecal excretion accounts for 6 to 15% of the dose.

Renal Impairment

Decreased renal function does not alter the single-dose pharmacokinetics of metronidazole. Subjects with end-stage renal disease (ESRD; CLCR =8.1±9.1 mL/min) and who received a single intravenous infusion of metronidazole 500 mg had no significant change in metronidazole pharmacokinetics but had a 2-fold higher Cmax of hydroxy- metronidazole and 5-fold higher Cmax of metronidazole acetate, compared to healthy subjects with normal renal function (CLCR =126±16 mL/min). Thus, on account of the potential accumulation of metronidazole metabolites in ESRD patients, monitoring for metronidazole associated adverse events is recommended (see PRECAUTIONS).

Effect on Dialysis

Following a single intravenous infusion or oral dose of metronidazole 500 mg, the clearance of metronidazole was investigated in ESRD subjects undergoing hemodialysis or continuous ambulatory peritoneal dialysis (CAPD). A hemodialysis session lasting for 4 to 8 hours removed 40% to 65% of the administered metronidazole dose, depending on the type of the dialyzer membrane used and the duration of the dialysis session. If the administration of metronidazole cannot be separated from the dialysis session, supplementation of metronidazole dose following hemodialysis should be considered (see DOSAGE AND ADMINISTRATION). A peritoneal dialysis session lasting for 7.5 hours removed approximately 10% of the administered metronidazole dose. No adjustment in metronidazole dose is needed in ESRD patients undergoing CAPD.

Hepatic Impairment

Following a single intravenous infusion of 500 mg metronidazole, the mean AUC24 of metronidazole was higher by 114% in patients with severe (Child-Pugh C) hepatic impairment, and by 54% and 53% in patients with a mild (Child-Pugh A) and moderate (Child-Pugh B) hepatic impairment, respectively, compared to healthy control subject. There were no significant changes in the AUC24 of hydroxyl-metronidazole in these hepatically impaired patients. A reduction in metronidazole dosage by 50% is recommended in patients with severe (Child-Pugh C) hepatic impairment (see DOSAGE AND ADMINISTRATION). No dosage adjustment is needed for patients with mild to moderate hepatic impairment. Patients with mild to moderate hepatic impairment should be monitored for metronidazole associated adverse events (see PRECAUTIONS and DOSAGE AND ADMINISTRATION).

Geriatric Patients

Following a single 500 mg oral or IV dose of metronidazole, subjects >70 years old with no apparent renal or hepatic dysfunction had a 40% to 80% higher mean AUC of hydroxy-metronidazole (active metabolite), with no apparent increase in the mean AUC of metronidazole (parent compound), compared to young healthy controls <40 years old. In geriatric patients, monitoring for metronidazole associated adverse events is recommended (see PRECAUTIONS).

Pediatric Patients

In one study newborn infants appears to demonstrate diminished capacity to eliminate metronidazole. The elimination half-life, measured during the first three days of life, was inversely related to gestational age. In infants whose gestational ages were between 28 and 40 weeks, the corresponding elimination half-lives ranged from 109 to 22.5 hours.

Microbiology

Mechanism of Action

Metronidazole, a nitroimidazole, exerts antibacterial effects in an anaerobic environment against most obligate anaerobes. Once metronidazole enters the organism by passive diffusion and activated in the cytoplasm of susceptible anaerobic bacteria, it is reduced; this process includes intra-cellular electron transport proteins such as ferredoxin, transfer of an electron to the nitro group of the metronidazole, and formation of a short-lived nitroso free radical. Because of this alteration of the metronidazole molecule, a concentration gradient is created and maintained which promotes the drug’s intracellular transport. The reduced form of metronidazole and free radicals can interact with DNA leading to inhibition of DNA synthesis and DNA degradation leading to death of bacteria. The precise mechanism of action of metronidazole is unclear.

Drug Resistance

A potential for development of resistance exists against metronidazole.

Resistance may be due to multiple mechanisms that include decreased uptake of the drug, altered reduction efficiency, overexpression of the efflux pumps, inactivation of the drug, and/or increased DNA damage repair.

Metronidazole does not possess any clinically relevant activity against facultative anaerobes or obligate aerobes.

Activity In Vitro and in Clinical Infections

Metronidazole has been shown to be active against most isolates of the following bacteria both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.

Gram-positive anaerobes

Clostridium species

Eubacterium species

Peptococcus species

Peptostreptococcus species

Gram-negative anaerobes

Bacteroides fragilis group (B. fragilis, B. distasonis, B. ovatus, B. thetaiotaomicron, B. vulgatus)

Fusobacterium species

The following in vitro data are available, but their clinical significance is unknown.

Metronidazole exhibits in vitro minimal inhibitory concentrations (MIC’s) of 8 mcg/mL or less against most (≥90%) isolates of the following bacteria; however, the safety and effectiveness of metronidazole in treating clinical infections due to these bacteria have not been established in adequate and well-controlled clinical trials.

Gram-negative anaerobes

Bacteroides fragilis group (B. caccae, B. uniformis)

Prevotella species (P. bivia, P. buccae, P. disiens)

Susceptibility Tests

When available, the clinical microbiology laboratory should provide results of in vitro susceptibility tests results for antimicrobial drug products used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial or community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug product for treatment.

For Anaerobes

Quantitative methods are used to determine antimicrobial inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. For anaerobic bacteria, the susceptibility of metronidazole can be determined by the reference broth and/or agar method.1,2

The MIC values should be interpreted according to the criteria provided in the following table.

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A report of “Susceptible” (S) indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations at the infection site necessary to inhibit growth of the pathogen.

A report of “Intermediate” (I) implies that an infection due to the isolate may be appropriately treated in the body sites where the drugs are physiologically concentrated or when a high dosage of drug is used.

A report of “Resistant” (R) indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentration usually achievable at the infection site; other therapy should be selected.

Quality Control

Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test.1,2 Standard metronidazole powder should provide a value within the MIC ranges noted in the following table:

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