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Monurol

Monurol

CLINICAL PHARMACOLOGY

Absorption

Fosfomycin tromethamine is rapidly absorbed following oral administration and converted to the free acid, fosfomycin. Absolute oral bioavailability under fasting conditions is 37%. After a single 3-gm dose of MONUROL, the mean (± 1 SD) maximum serum concentration (Cmax) achieved was 26.1 (± 9.1) μg/mL within 2 hours. The oral bioavailability of fosfomycin is reduced to 30% under fed conditions. Following a single 3-gm oral dose of MONUROL with a high-fat meal, the mean Cmax achieved was 17.6 (± 4.4) μg/mL within 4 hours.

Cimetidine does not affect the pharmacokinetics of fosfomycin when coadministered with MONUROL. Metoclopramide lowers the serum concentrations and urinary excretion of fosfomycin when coadministered with MONUROL. (See PRECAUTIONS: DRUG INTERACTIONS)

Distribution

The mean apparent steady-state volume of distribution (Vss) is 136.1 (± 44.1) L following oral administration of MONUROL. Fosfomycin is not bound to plasma proteins.

Fosfomycin is distributed to the kidneys, bladder wall, prostate, and seminal vesicles. Following a 50 mg/Kg dose of fosfomycin to patients undergoing urological surgery for bladder carcinoma, the mean concentration of fosfomycin in the bladder, taken at a distance from the neoplastic site, was 18.0 μg per gram of tissue at 3 hours after dosing. Fosfomycin has been shown to cross the placental barrier in animals and man.

Excretion

Fosfomycin is excreted unchanged in both urine and feces. Following oral administration of MONUROL, the mean total body clearance (CLTB) and mean renal clearance (CLR) of fosfomycin were 16.9 (± 3.5) L/hr and 6.3 (± 1.7) L/hr, respectively. Approximately 38% of a 3-gm dose of MONUROL is recovered from urine, and 18% is recovered from feces. Following intravenous administration, the mean CLTB and mean CLR of fosfomycin were 6.1 (±1.0) L/hr and 5.5 (± 1.2) L/hr, respectively.

A mean urine fosfomycin concentration of 706 (± 466) μg/mL was attained within 2-4 hours after a single oral 3-gm dose of MONUROL under fasting conditions. The mean urinary concentration of fosfomycin was 10 μg/mL in samples collected 72-84 hours following a single oral dose of MONUROL.

Following a 3-gm dose of MONUROL administered with a high fat meal, a mean urine fosfomycin concentration of 537 (± 252) μg/mL was attained within 6-8 hours. Although the rate of urinary excretion of fosfomycin was reduced under fed conditions, the cumulative amount of fosfomycin excreted in the urine was the same, 1118 (± 201) mg (fed) vs. 1140 mg (± 238) (fasting). Further, urinary concentrations equal to or greaterthan 100 μg/mL were maintained for the same duration, 26 hours, indicating that MONUROL can be taken without regard to food.

Following oral administration of MONUROL, the mean half-life for elimination (t½) is 5.7 (± 2.8) hours.

Special Populations

Geriatric: Based on limited data regarding 24-hour urinary drug concentrations, no differences in urinary excretion of fosfomycin have been observed in elderly subjects. No dosage adjustment is necessary in the elderly.

Gender: There are no gender differences in the Pharmacokinetics of fosfomycin.

Renal Insufficiency: In 5 anuric patients undergoing hemodialysis, the t½ of fosfomycin during hemodialysis was 40 hours. In patients with varying degrees of renal impairment (creatinine clearances varying from 54 mL/min to 7 mL/min), the t½ of fosfomycin increased from 11 hours to 50 hours. The percent of fosfomycin recovered in urine decreased from 32% to 11% indicating that renal impairment significantly decreases the excretion of fosfomycin.

Microbiology

Fosfomycin (the active component of fosfomycin tromethamine) has in vitro activity against a broad range of gram-positive and gram-negative aerobic microorganisms which are associated with uncomplicated urinary tract infections. Fosfomycin is bactericidal in urine at therapeutic doses. The bactericidal action of fosfomycin is due to its inactivation of the enzyme enolpyruvyl transferase, thereby irreversibly blocking the condensation of uridine diphosphate-N-acetylglucosamine with p-enolpyruvate, one of the first steps in bacterial cell wall synthesis. It also reduces adherence of bacteria to uroepithelial cells.

There is generally no cross-resistance between fosfomycin and other classes of antibacterial agents such as beta-lactams and aminoglycosides.

Fosfomycin has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section:

Aerobic gram-positive microorganisms

Enterococcus faecalis

Aerobic gram-negative microorganisms

Escherichia coli

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

Fosfomycin exhibits in vitro minimum inhibitory concentrations (MIC's) of 64 μg/mL or less against most ( ≥ 90%) strains of the following microorganisms; however, the safety and effectiveness of fosfomycin in treating clinical infections due to these microorganisms has not been established in adequate and well-controlled clinical trials:

Aerobic gram-positive microorganisms

Enterococcus faecium

Aerobic gram-negative microorganisms

Citrobacter diversus
Citrobacter freundii

Enterobacter aerogenes

Klebsiella oxytoca

Klebsiella pneuomoniae

Proteus mirabilis

Proteus vulgaris

Serratia marcescens

Susceptibility Testing

Dilution Techniques

Quantitative methods are used to determine minimum inhibitory concentrations (MIC's). These MIC's provide estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure uses a standardized agar dilution method1 or equivalent with standardized inoculum concentrations and standardized concentrations of fosfomycin tromethamine (in terms of fosfomycin base content) powder supplemented with 25 μg/mL of glucose-6-phosphate. BROTH DILUTION METHODS SHOULD NOT BE USED TO TEST SUSCEPTIBILITY TO FOSFOMYCIN. The MIC values obtained should be Interpreted according to the following criteria:

MIC (μ g/mL) Interpretation
≤ 64 Susceptible (S)
128 Intermediate (I)
≥ 256 Resistant (R)

A report of “susceptible” indicates that the pathogen is likely to be inhibited by usually achievable concentrations of the antimicrobial compound in the urine. 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 provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “resistant” indicates that usually achievable concentrations of the antimicrobial compound in the urine are unlikely to be inhibitory and that other therapy should be selected.

Standardized susceptibility test procedures require the use of laboratory control microorganisms. Standard fosfomycin tromethamine powder should provide the following MIC values for agar dilution testing in media containing 25 μg/mL of glucose154 6- phosphate. [Broth dilution testing should not be performed].

Microorganism MIC (μ g/mL)
Enterococcus faecalis ATCC 29212 32-128
Escherichia coli ATCC 25922 0.5-2
Pseudomonas aeruginosa ATCC 27853 2-8
Staphylococcus aureus ATCC 29213 0.5-4

Diffusion Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial agents. One such standardized procedure2 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 200-μg fosfomycin and 50-μg of glucose-6168 phosphate to test the susceptibility of microorganisms to fosfomycin.

Reports from the laboratory providing results of the standard single-disk susceptibility tests with disks containing 200 μg of fosfomycin and 50 μg of glucose-6-phosphate should be interpreted according to the following criteria:

Zone Diameter (mm) Interpretation
≥ 16 Susceptible (S)
13-15 Intermediate (I)
≤ 12 Resistant (R)

Interpretation should be stated as above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for fosfomycin.

As with standardized dilution techniques, diffusion methods require use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 200-μg fosfomycin disk with the 50-μg of glucose-6-phosphate should provide the following zone diameters in these laboratory quality control strains:

Microorganism Zone Diameter (mm)
Escherichia coli ATCC 25922 22-30
Staphylococcus aureus ATCC 25923 25-33

Clinical Studies

In controlled, double-blind studies of acute cystitis performed in the United States, a single-dose of MONUROL was compared to three other oral antibiotics (See table below). The study population consisted of patients with symptoms and signs of acute cystitis of less than 4 days duration, no manifestations of upper tract infection (e.g., flank pain, chills, fever), no history of recurrent urinary tract infections (20% of patients in the clinical studies had a prior episode of acute cystitis within the preceding year), no known structural abnormalities, no clinical or laboratory evidence of hepatic dysfunction, and no known or suspected CNS disorders, such as epilepsy, or other factors which would predispose to seizures. In these studies, the following clinical success (resolution of symptoms) and microbiologic eradication rates were obtained

Treatment Arm Treatment Duration (days) Microbiologic Eradication Rate Clinical Success Rate Outcome (based on difference in microbiologic eradication rates 5-11 days post therapy)
5-11 days post therapy Study day 12-21
Fosfomycin 1 630/771 (32%) 591/771 (77%) 542/771 (70%)  
Ciprofloxacin 7 219/222 (98%) 219/222 (93%) 213/222 (96%) Fosfomycin inferior to ciprofloxacin
Trimethoprim/ sulfamethox-azole 10 194/197 (98%) 194/197 (93%) 186/197 (94%) Fosfomycin inferior to trimethoprim/ sulfamethoxazole
Nitrofurantoin 7 130/233 (76%) 180/233 (76%) 133/238 (77%) Fosfomycin equivalent to nitrofurantoin

Pathogen Fosfomycin 3 gm single dose Ciprofloxacin 250 mg bid x 7d Trimethoprim sulfamethoxazole 160 mg/ 800 mg bid x10d Nitrofurantoin 100mg bid x 7d
E coli 509/644 134/137 171/174 146/137
(79%) (93%) (93%) (73%)
E. faecalis 10/10 0/0 4/4 ½
(100%)   (100%) (50%)

REFERENCES

1. National Committee for Clinical Laboratory Standards, Methods for Dilution. Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically – Third Edition; Approved Standard NCCLS Document M7-A3, Vol. 13, No. 25 NCCLS, Villanova, PA, December, 1993.

2. National Committee for Clinical Laboratory Standards, Performance Standard for Antimicrobial Disk Susceptibility Tests – Fifth Edition; Approved Standard NCCLS Document M2-A5, Vol. 13, No. 24 NCCLS, Villanova, PA, December, 1993.

Last reviewed on RxList: 5/31/2011
This monograph has been modified to include the generic and brand name in many instances.

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