Class: Urinary Anti-infectives
VA Class: AM900
Chemical Name: (1R,2S)-(1,2-Epoxypropyl)phosphonic acid, compound with 2-amino-2-(hydroxymethyl)-1,3-propanediol (1:1)
Molecular Formula: C3H7O4P•C4H11N O13
CAS Number: 78964-85-9
Antibacterial; phosphonic acid derivative.
Uses for Fosfomycin
Urinary Tract Infections (UTIs)
Treatment of uncomplicated UTIs (acute cystitis) in women caused by susceptible Escherichia coli or Enterococcus faecalis (formerly Streptococcus faecalis).
Oral fosfomycin (single 3-g dose) is one of several preferred anti-infectives for empiric treatment of uncomplicated cystitis in women. Considered a good choice for such infections since incidence of fosfomycin resistance among uropathogens (e.g., E. coli) is low to date and the single-dose regimen is well tolerated and has not been associated with selection of drug-resistant bacteria or colonization/infection with multidrug-resistant strains.
Has been used (single dose or multiple doses†) for treatment of UTIs caused by multidrug-resistant bacteria and some clinicians suggest it may be a viable option for treatment of such infections; however, additional study needed.
Do not use for treatment of pyelonephritis or perinephric abscesses. Avoid if diagnostic uncertainty exists regarding cystitis versus early pyelonephritis (e.g., cystitis symptoms accompanied by subjective fever [not verified at time of examination] and/or vague flank pain or tenderness, cystitis symptoms present for >5–7 days).
Has been effective when used in a limited number of men for treatment of chronic prostatitis† caused by multidrug-resistant Enterobacteriaceae (e.g., E. coli). However, treatment failures also reported and further study needed to evaluate efficacy and safety of oral fosfomycin for treatment of chronic prostatitis and to identify optimal dosage and duration of fosfomycin treatment for such infections.
Has been effective when used for perioperative prophylaxis in men undergoing transrectal biopsy of the prostate†.
Has been effective when used for perioperative prophylaxis in men undergoing transurethral resection of the prostate† for benign prostatic hyperplasia.
Additional study needed to evaluate efficacy and safety and optimal timing for perioperative prophylaxis in men undergoing transrectal biopsy of the prostate or transurethral resection of the prostate†.
Fosfomycin Dosage and Administration
Administer orally as fosfomycin tromethamine.
Has been administered IV† as disodium salt; parenteral formulation not available in US.
Granules for oral solution must be dissolved in water prior to administration; do not ingest as dry granules.
Prepare oral solution by adding entire contents of single-dose packet (sachet) containing 3 g of the drug to 90–120 mL (3–4 ounces) of water; do not use hot water. After stirring to dissolve granules, immediately ingest entire oral solution.
May be taken with or without food.
Available as fosfomycin tromethamine; dosage expressed in terms of fosfomycin.
Urinary Tract Infections (UTIs)
Acute cystitis in women ≥18 years of age: 3 g as a single dose.
Contact clinician if cystitis symptoms do not improve within 2–3 days.
Do not give additional doses; repeated doses do not improve clinical success or microbiologic eradication rates and may increase incidence of adverse effects.
Acute cystitis in women: Single dose.
No specific dosage recommendations.
Dosage adjustment not necessary. (See Pharmacokinetics.)
Dosage adjustment not necessary. Select dosage with caution because of age-related decreases in hepatic, renal, and/or cardiac function and concomitant disease and drug therapy. (See Geriatric Use under Cautions.)
Cautions for Fosfomycin
Known hypersensitivity to fosfomycin.
Superinfection/Clostridium difficile-associated Diarrhea and Colitis (CDAD)
Treatment with anti-infectives alters normal colon flora and may permit overgrowth of Clostridium difficile. C. difficile infection (CDI) and C. difficile-associated diarrhea and colitis (CDAD; also known as antibiotic-associated diarrhea and colitis or pseudomembranous colitis) reported with nearly all anti-infectives, including fosfomycin, and may range in severity from mild diarrhea to fatal colitis. C. difficile produces toxins A and B which contribute to development of CDAD; hypertoxin-producing strains of C. difficile are associated with increased morbidity and mortality since they may be refractory to anti-infectives and colectomy may be required.
Consider CDAD if diarrhea develops during or after anti-infective treatment and manage accordingly. Obtain careful medical history since CDAD may occur as late as ≥2 months after anti-infective therapy is discontinued.
If CDAD is suspected or confirmed, anti-infectives not directed against C. difficile should be discontinued. Manage moderate to severe cases with fluid, electrolyte, and protein supplementation, anti-infective therapy active against C. difficile (e.g., oral metronidazole or vancomycin), and surgical evaluation when clinically indicated.
Anaphylaxis, rash, and angioedema reported; causal relationship to the drug not established.
Other Warnings and Precautions
Selection and Use of Anti-infectives
Use results of culture and in vitro susceptibility testing when selecting or modifying anti-infective therapy.
Obtain urine specimens for culture and in vitro susceptibility testing before and after completion of therapy.
If persistence or recurrence of bacteriuria occurs after fosfomycin treatment, use other antibacterial agents.
No adequate and well-controlled studies in pregnant women; use during pregnancy only if clearly needed.
Not known whether distributed into milk.
Because of potential for serious adverse reactions in nursing infants, discontinue nursing or do not administer drug, taking into account importance of the drug to the woman.
Safety and efficacy not established in children ≤12 years of age.
Insufficient experience in those ≥65 years of age to determine whether they respond differently than younger adults. No evidence of substantial differences in safety and efficacy relative to younger adults.
Although dosage adjustment not usually necessary, select dosage with caution because of age-related decreases in hepatic, renal, and/or cardiac function and potential for concomitant disease and drug therapy.
Common Adverse Effects
Diarrhea, nausea, dyspepsia, headache, dizziness, asthenia, vaginitis.
Interactions for Fosfomycin
Drugs that Increase GI Motility
May decrease serum and urine concentrations of fosfomycin.
Pharmacokinetics of fosfomycin not affected
In vitro evidence of synergistic antibacterial activity against E. coli
Decreased serum concentrations and decreased urinary excretion of fosfomycin
Fosfomycin tromethamine is rapidly absorbed following oral administration and converted to fosfomycin (the free acid).
Following single 3-g oral dose of fosfomycin under fasting conditions, peak serum concentrations of 26 mcg/mL were attained within 2 hours.
High interindividual variability in pharmacokinetics reported in healthy women following single 3-g oral dose.
Food decreases oral bioavailability; bioavailability is 37% or 30% under fasting or fed conditions, respectively.
Cumulative amount of drug excreted in urine is similar whether given with or without food.
Distributed to kidneys, bladder wall, prostate, and seminal vesicles.
Crosses placenta in animals and humans.
Not known whether distributed into milk.
Plasma Protein Binding
Not bound to plasma proteins.
Converted in vivo to fosfomycin, the microbiologically active form of the drug.
Eliminated as unchanged fosfomycin in urine (38% of an oral dose) and feces (18% of an oral dose).
Following 3-g oral dose given with a high-fat meal, mean fosfomycin concentrations in urine were 537 mcg/mL 6–8 hours after the dose.
Fosfomycin urinary concentrations ≥100 mcg/mL have been maintained for approximately 26 hours in patients who received a single oral dose of fosfomycin with or without food.
Removed by hemodialysis.
Renal impairment: Decreased elimination and prolonged half-life. Half-life increased from 11 to 50 hours in patients with varying degrees of renal impairment (Clcr ranging from 54 to 7 mL/minute).
Geriatric adults: Pharmacokinetics similar to that reported in younger adults (based on 24-hour urinary drug concentrations).
25°C (may be exposed to 15–30°C).
Actions and Spectrum
Phosphonic acid derivative, synthetic antibacterial agent.
Usually bactericidal in urine when administered in therapeutic doses.
Antibacterial activity results from interference with bacterial cell wall synthesis by inhibiting the enzyme enolpyruvyl transferase, which catalyzes the formation of uridine diphosphate-N-acetylmuramic acid in the first step of bacterial cell wall synthesis. Also reduces adherence of bacteria to uroepithelial cells.
Active in vitro against a broad spectrum of gram-negative and gram-positive bacteria commonly associated with uncomplicated UTIs.
Has been active in vitro against Enterobacteriaceae resistant to fluoroquinolones and generally has activity against extended-spectrum β-lactamase (ESBL)-producing strains resistant to extended-spectrum cephalosporins.
Gram-negative bacteria: Active in vitro and in vivo in clinical infections against Escherichia coli. Although clinical importance unknown, has in vitro activity against Citrobacter diversus, C. freundii, Enterobacter aerogenes, Klebsiella oxytoca, K. pneumoniae, Proteus mirabilis, P. vulgaris, Serratia marcescens, and Pseudomonas aeruginosa.
Gram-positive bacteria: Active in vitro and in vivo in clinical infections against Enterococcus faecalis. Has been active in vitro against some vancomycin-resistant E. faecalis. Although clinical importance unknown, has in vitro activity against some other gram-positive cocci, including E. faecium.
E. coli resistant to fosfomycin have been produced in vitro and clinical isolates of E. coli, K. pneumoniae, P. mirabilis, and Ps. aeruginosa resistant to fosfomycin reported rarely. Fosfomycin-resistant E. cloacae and E. faecalis reported.
Fosfomycin resistance in Enterobacteriaceae usually related to presence of the fosA gene, which can be chromosome-mediated (e.g., K. pneumoniae, Ps. aeruginosa) or plasmid-mediated (e.g., E. coli).
Cross-resistance does not usually occur between fosfomycin and other antibacterials (e.g., β-lactams, aminoglycosides).
Advice to Patients
Advise patients that fosfomycin may be taken with or without food.
Importance of diluting the granules with water (not hot water) just prior to administration.
Importance of contacting a clinician if UTI symptoms do not improve within 2–3 days after treatment.
Advise patients that diarrhea is a common problem caused by anti-infectives and usually ends when the drug is discontinued. Importance of contacting a clinician if watery and bloody stools (with or without stomach cramps and fever) occur during or as late as ≥2 months after the last dose.
Importance of informing clinicians of existing or contemplated concomitant therapy, including prescription and OTC drugs, as well as any concomitant illnesses.
Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.
Importance of informing patients of other important precautionary information. (See Cautions.)
Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.
Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.
Granules, for solution
3 g (of fosfomycin) per packet
AHFS DI Essentials™. © Copyright 2022, Selected Revisions December 17, 2018. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, Maryland 20814.
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