Nitrofurantoin Macrocrystals: Package Insert and Label Information

NITROFURANTOIN MACROCRYSTALS- nitrofurantoin capsule
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To reduce the development of drug-resistant bacteria and maintain the effectiveness of nitrofurantoin macrocrystals and other antibacterial drugs, nitrofurantoin macrocrystals should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria.

DESCRIPTION

Nitrofurantoin Capsules USP (Macrocrystals) are a synthetic chemical of controlled crystal size. It is a stable, yellow, crystalline compound. Nitrofurantoin Capsules USP (Macrocrystals) are an antibacterial agent for specific urinary tract infections.

Nitrofurantoin, USP (macrocrystals) is chemically designated as 2,4-Imidazolidinedione, 1-[[(5-nitro-2-furanyl)methylene]amino]- and has the following structural formula:

structural formula -- nitrofurantoin, USP
(click image for full-size original)

C8 H6 N4 O5 (anhydrous) M.W. 238.16

Each capsule, for oral administration, contains 50 mg or 100 mg of nitrofurantoin, USP (macrocrystals). In addition, each capsule contains the following inactive ingredients: corn starch, edible black ink (black iron oxide, D&C Yellow No. 10 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake, FD&C Blue No. 2 Aluminum Lake, FD&C Red No. 40 Aluminum Lake, propylene glycol, shellac glaze), gelatin, lactose monohydrate, talc, titanium dioxide and colorant D&C Red No. 33.

CLINICAL PHARMACOLOGY

Nitrofurantoin macrocrystals are a larger crystal form of nitrofurantoin. The absorption of nitrofurantoin macrocrystals is slower and its excretion somewhat less when compared to nitrofurantoin. Blood concentrations at therapeutic dosage are usually low. It is highly soluble in urine, to which it may impart a brown color.

Following a dose regimen of 100 mg q.i.d. for 7 days, average urinary drug recoveries (0 to 24 hours) on day 1 and day 7 were 37.9% and 35%.

Unlike many drugs, the presence of food or agents delaying gastric emptying can increase the bioavailability of nitrofurantoin macrocrystals, presumably by allowing better dissolution in gastric juices.

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 aureus

Enterococci (e.g., Enterococcus faecalis)

Aerobic and facultative Gram-negative microorganisms

Escherichia coli

NOTE: While nitrofurantoin has excellent activity against Enterococcus faecalis , the majority of Enterococcus faecium isolates are not susceptible to nitrofurantoin.

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 and Staphylococcus saprophyticus)

Streptococcus agalactiae

Group D streptococci

Viridans group streptococci

Aerobic and facultative Gram-negative microorganisms

Citrobacter amalonaticus

Citrobacter diversus

Citrobacter freundii

Klebsiella oxytoca

Klebsiella ozaenae

NOTE: Some strains of Enterobacter species and Klebsiella species are resistant to nitrofurantoin.

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 Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure2 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 300 mcg 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 (mcg/mL)

Disk Diffusion (zone diameter in mm)

S

I

R

S

I

R

Enterobacteriaceae

≤ 32

64

≥ 128

≥ 17

15 to 16

≤ 14

Staphylococcus spp.

≤ 32

64

≥ 128

≥ 17

15 to 16

≤ 14

Enterococcus spp.

≤ 32

64

≥ 128

≥ 17

15 to 16

≤ 14

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 procedures3. 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 (mcg/mL)

Disk Diffusion (zone diameter in mm)

Escherichia coli
ATCC 25922

4 to 16

20 to 25

Enterococcus faecalis
ATCC 29212

4 to 16

NAa

Staphylococcus aureus
ATCC 29213

8 to 32

NAa

Staphylococcus aureus
ATCC 25923

NAa

18 to 22

a Not applicable

INDICATIONS AND USAGE

Nitrofurantoin Capsules USP (Macrocrystals) are specifically indicated for the treatment of urinary tract infections when due to susceptible strains of Escherichia coli , enterococci, Staphylococcus aureus , and certain susceptible strains of Klebsiella and Enterobacter species.

Nitrofurantoin is not indicated for the treatment of pyelonephritis or perinephric abscesses. To reduce the development of drug-resistant bacteria and maintain the effectiveness of Nitrofurantoin Capsules USP (Macrocrystals) and other antibacterial drugs, Nitrofurantoin Capsules USP (Macrocrystals) should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.

Nitrofurantoins lack the broader tissue distribution of other therapeutic agents approved for urinary tract infections. Consequently, many patients who are treated with Nitrofurantoin Capsules USP (Macrocrystals) are predisposed to persistence or reappearance of bacteriuria. Urine specimens for culture and susceptibility testing should be obtained before and after completion of therapy. If persistence or reappearance of bacteriuria occurs after treatment with Nitrofurantoin Capsules USP (Macrocrystals), other therapeutic agents with broader tissue distribution should be selected. In considering the use of Nitrofurantoin Capsules USP (Macrocrystals), lower eradication rates should be balanced against the increased potential for systemic toxicity and for the development of antimicrobial resistance when agents with broader tissue distribution are utilized.

CONTRAINDICATIONS

Anuria, oliguria, or significant impairment of renal function (creatinine clearance under 60 mL per minute or clinically significant elevated serum creatinine) are contraindications. Treatment of this type of patient carries an increased risk of toxicity because of impaired excretion of the drug.

Because of the possibility of hemolytic anemia due to immature erythrocyte enzyme systems (glutathione instability), the drug is contraindicated in pregnant patients at term (38 to 42 weeks’ gestation), during labor and delivery, or when the onset of labor is imminent. For the same reason, the drug is contraindicated in neonates under one month of age.

Nitrofurantoin macrocrystals are contraindicated in patients with a previous history of cholestatic jaundice/hepatic dysfunction associated with nitrofurantoin.

Nitrofurantoin macrocrystals are also contraindicated in those patients with known hypersensitivity to nitrofurantoin.

WARNINGS

Pulmonary Reactions

ACUTE, SUBACUTE, OR CHRONIC PULMONARY REACTIONS HAVE BEEN OBSERVED IN PATIENTS TREATED WITH NITROFURANTOIN. IF THESE REACTIONS OCCUR, NITROFURANTOIN MACROCRYSTALS SHOULD BE DISCONTINUED AND APPROPRIATE MEASURES TAKEN. REPORTS HAVE CITED PULMONARY REACTIONS AS A CONTRIBUTING CAUSE OF DEATH.

CHRONIC PULMONARY REACTIONS (DIFFUSE INTERSTITIAL PNEUMONITIS OR PULMONARY FIBROSIS, OR BOTH) CAN DEVELOP INSIDIOUSLY. THESE REACTIONS OCCUR RARELY AND GENERALLY IN PATIENTS RECEIVING THERAPY FOR SIX MONTHS OR LONGER. CLOSE MONITORING OF THE PULMONARY CONDITION OF PATIENTS RECEIVING LONG-TERM THERAPY IS WARRANTED AND REQUIRES THAT THE BENEFITS OF THERAPY BE WEIGHED AGAINST POTENTIAL RISKS (SEE ADVERSE REACTIONS, Respiratory).

Hepatotoxicity

Hepatic reactions, including hepatitis, cholestatic jaundice, chronic active hepatitis, and hepatic necrosis, occur rarely. Fatalities have been reported. The onset of chronic active hepatitis may be insidious, and patients should be monitored periodically for changes in biochemical tests that would indicate liver injury. If hepatitis occurs, the drug should be withdrawn immediately and appropriate measures should be taken.

Neuropathy

Peripheral neuropathy, which may become severe or irreversible, has occurred. Fatalities have been reported. Conditions such as renal impairment (creatinine clearance under 60 mL per minute or clinically significant elevated serum creatinine), anemia, diabetes mellitus, electrolyte imbalance, vitamin B deficiency, and debilitating disease may enhance the occurrence of peripheral neuropathy. Patients receiving long-term therapy should be monitored periodically for changes in renal function.

Optic neuritis has been reported rarely in postmarketing experience with nitrofurantoin formulations.

Hemolytic Anemia

Cases of hemolytic anemia of the primaquine-sensitivity type have been induced by nitrofurantoin. Hemolysis appears to be linked to a glucose-6-phosphate dehydrogenase deficiency in the red blood cells of the affected patients. This deficiency is found in 10 percent of Blacks and a small percentage of ethnic groups of Mediterranean and Near-Eastern origin. Hemolysis is an indication for discontinuing nitrofurantoin macrocrystals; hemolysis ceases when the drug is withdrawn.

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