Macrobid

CLINICAL PHARMACOLOGY

Each Macrobid capsule contains two forms of nitrofurantoin. Twenty-five percent is macrocrystalline nitrofurantoin, which has slower dissolution and absorption than nitrofurantoin monohydrate. The remaining 75% is nitrofurantoin monohydrate contained in a powder blend which, upon exposure to gastric and intestinal fluids, forms a gel matrix that releases nitrofurantoin over time. Based on urinary pharmacokinetic data, the extent and rate of urinary excretion of nitrofurantoin from the 100 mg Macrobid capsule are similar to those of the 50 mg or 100 mg Macrodantin® (nitrofurantoin macrocrystals) capsule. Approximately 20-25% of a single dose of nitrofurantoin is recovered from the urine unchanged over 24 hours.

Plasma nitrofurantoin concentrations after a single oral dose of the 100 mg Macrobid capsule are low, with peak levels usually less than 1 mcg/mL. Nitrofurantoin is highly soluble in urine, to which it may impart a brown color. When Macrobid is administered with food, the bioavailability of nitrofurantoin is increased by approximately 40%.

Microbiology

Nitrofurantoin is a nitrofuran antimicrobial agent with activity against certain Gram-positive and Gram-negative bacteria.

Mechanism of Action

The mechanism of the antimicrobial action of nitrofurantoin is unusual among antibacterials. Nitrofurantoin is reduced by bacterial flavoproteins to reactive intermediates which inactivate or alter bacterial ribosomal proteins and other macromolecules. As a result of such inactivations, the vital biochemical processes of protein synthesis, aerobic energy metabolism, DNA synthesis, RNA synthesis, and cell wall synthesis are inhibited. Nitrofurantoin is bactericidal in urine at therapeutic doses. The broad-based nature of this mode of action may explain the lack of acquired bacterial resistance to nitrofurantoin, as the necessary multiple and simultaneous mutations of the target macromolecules would likely be lethal to the bacteria.

Interactions with Other Antibiotics

Antagonism has been demonstrated in vitro between nitrofurantoin and quinolone antimicrobials. The clinical significance of this finding is unknown.

Development of Resistance

Development of resistance to nitrofurantoin has not been a significant problem since its introduction in 1953. Cross-resistance with antibiotics and sulfonamides has not been observed, and transferable resistance is, at most, a very rare phenomenon.

Nitrofurantoin has been shown to be active against most strains of the following bacteria both in vitro and in clinical infections [see INDICATIONS AND USAGE):

Aerobic and facultative Gram-positive microorganisms

Staphylococcus saprophyticus

Aerobic and facultative Gram-negative microorganisms

Escherichia coli

At least 90 percent of the following microorganisms exhibit an in vitro minimum inhibitory concentration (MIC) less than or equal to the susceptible breakpoint for nitrofurantoin. However, the efficacy of nitrofurantoin in treating clinical infections due to these microorganisms has not been established in adequate and well-controlled trials.

Aerobic and facultative Gram-positive microorganisms

Coagulase-negative staphylococci (including Staphylococcus epidermidis)
Enterococcus faecalis

Staphylococcus aureus

Streptococcus aga/actiae

Group 0 streptococci

Viridans group streptococci

Aerobic and facultative Gram-negative microorganisms:

Citrobacter ama/onaticus
Citrobacter diversus

Citrobacter freundii

Klebsiella oxytoca

Klebsiella ozaenae

Nitrofurantoin is not active against most strains of Proteus species or Serratia species. It has no activity against Pseudomonas species.

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide cumulative results of the in vitro susceptibility test results for antimicrobial drugs used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting the most effective antimicrobial.

Dilution techniques: Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure. Standardized procedures are based on a dilution method (broth or agar) (1) or equivalent with standardized inoculum concentrations and standardized concentrations of nitrofurantoin powder. The MIC values should be interpreted according to the criteria provided in Table 1.

Diffusion technique: Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure (2) requires the use of standardized inoculums concentrations. This procedure uses paper disks impregnated with 300 pg of nitrofurantoin to test the susceptibility of microorganisms to nitrofurantoin. The disk diffusion interpretive criteria are provided in Table 1.

Table 1: Susceptibility Interpretive Criteria for Nitrofurantoin

Pathogen Susceptibility Interpretive Criteria
Minimum Inhibitory Concentrations (μg/mL) Disk Diffusion (zonediameter in mm)
S I R S I R
Enterobacteriaceae δ32 64 ε128 ε17 15-16 δ14
Staphylococcus spp. δ32 64 ε128 ε17 15-16 δ14

A report of Susceptible indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the urine reaches the concentrations usually achievable. A report of Intermediate indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where a high dosage of drug can be used. This category also provides a buffer zone, which prevents small, uncontrolled technical factors from causing major discrepancies in interpretation. A report of Resistant indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the urine reaches the concentrations usually achievable; other therapy should be selected.

Quality Control: Standardized susceptibility test procedures require the use of quality control microorganisms to control the technical aspects of the test procedures (3). Standard nitrofurantoin powder should provide the following range of values noted in Table 2.

Table 2: Acceptable Quality Control Ranges for Nitrofurantoin

QC Strain Acceptable Quality Control Ranges
Minimum Inhibitory Concentration (pg/mL) Disk Diffusion (zone diameter in mm)
Escherichia coli ATCC 25922 4 - 16 20 -25
Enterococcus faecalis ATCC 29212 4 - 16 NAa
Staphylococcus aureus ATCC 29213 8 - 32 NAa
Staphylococcus aureus ATCC 25923 NAa 18-22
aNot applicable

Clinical Studies

Controlled clinical trials comparing Macrobid 100 mg p.o. q12h and Macrodantin 50 mg p.o. q6h in the treatment of acute uncomplicated urinary tract infections demonstrated approximately 75% microbiologic eradication of susceptible pathogens in each treatment group.

Last reviewed on RxList: 9/20/2013
This monograph has been modified to include the generic and brand name in many instances.

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