Fosfomycin

Introduction Overview

Generic Name

fosfomycin

Brand Name

Fosfomycin Tromethamine, Monurol

Dosage Form

oral granule for reconstitution (3 g)

Drug Class

Miscellaneous antibiotics, Urinary anti-infectives

Pharmacokinetics

Absorption:
Fosfomycin is administered orally as a powder for reconstitution, typically as a single 3 g dose. Upon oral administration, fosfomycin is rapidly absorbed from the gastrointestinal tract, with peak plasma concentrations occurring approximately 2 to 4 hours post-dose. The drug’s bioavailability is sufficient to achieve therapeutic concentrations in the urine, which is essential for its effectiveness in treating lower urinary tract infections. Absorption is not significantly affected by food intake, and patients may take it with or without meals. Clinical studies have shown that the oral absorption of fosfomycin tromethamine provides adequate and sustained urinary concentrations for at least 36 to 48 hours, supporting its single-dose regimen.

Distribution:
Following absorption, fosfomycin exhibits relatively low plasma protein binding, estimated to be less than 10%, which facilitates its distribution into extracellular fluids and tissues. It is extensively distributed to the urinary tract, bladder wall, kidneys, and perinephric tissues, which corresponds with its clinical use for urinary tract infections. Fosfomycin penetrates renal tissues efficiently but has limited penetration into cerebrospinal fluid and other privileged sites. The volume of distribution is reported to be approximately 9 to 30 liters, indicative of distribution into extracellular water compartments. Minimal accumulation occurs with single-dose use.

Metabolism:
Fosfomycin undergoes minimal or no hepatic metabolism. It remains largely unchanged as it circulates in the bloodstream, which reduces the risk of metabolic drug interactions and variability due to hepatic function. The lack of metabolism makes fosfomycin a favorable agent in patients with impaired liver function. This property also contributes to its broad spectrum of activity, as the active parent compound is maintained throughout systemic circulation and is excreted in active form.

Elimination:
Elimination of fosfomycin is predominantly renal. Approximately 93% of the administered dose is excreted unchanged in the urine via glomerular filtration within 36 to 48 hours, underscoring the importance of renal function for drug clearance. The elimination half-life ranges from 5 to 6 hours in individuals with normal renal function. Impaired renal function significantly prolongs the half-life and may necessitate dose adjustment though for uncomplicated cystitis single dosing is usually sufficient. Fosfomycin’s elimination profile supports its efficacy as a urinary antiseptic agent. No significant biliary or fecal excretion has been described. Monitoring renal function in patients with compromised kidneys is important to avoid potential accumulation.

Dosage Forms

Available Dosage Forms:
- Oral granule for reconstitution: 3 g sachet (fosfomycin tromethamine)

Dosages for Adults and Adolescents:
- The recommended dose for adults and adolescents (≥12 years) is a single oral dose of 3 g (one sachet) dissolved in approximately 3 to 4 ounces (half a cup) of cold water and taken as a single administration.
- This dosing is specifically indicated for the treatment of uncomplicated urinary tract infections (acute cystitis) caused by susceptible strains of Escherichia coli and Enterococcus faecalis in women.
- The single-dose administration is supported by pharmacokinetic studies demonstrating sustained urinary concentrations sufficient for bacterial eradication.

Specific Patient Populations:
- Pediatric Use: Fosfomycin is not recommended for children under 12 years old due to insufficient safety and efficacy data.
- Renal Impairment: Although fosfomycin is primarily renally excreted, dose adjustment is generally not necessary for single-dose therapy; however, caution is advised in severe renal impairment. Monitoring is recommended for patients with renal dysfunction.
- Pregnancy and Lactation: Fosfomycin should be used in pregnancy only if clearly needed and after careful risk-benefit assessment. Limited data suggest low risk, but consultation with healthcare providers is essential. Breastfeeding women should consult a physician before use.

Mechanism Of Action

Fosfomycin exerts its antibacterial effect by irreversibly inhibiting the initial step of bacterial cell wall synthesis. Specifically, it inactivates the enzyme UDP-N-acetylglucosamine enolpyruvyl transferase (MurA), which catalyzes the formation of N-acetylmuramic acid, an essential precursor in peptidoglycan biosynthesis. By blocking this enzyme’s activity, fosfomycin effectively halts the production of peptidoglycan, a critical component for bacterial cell wall integrity, leading to weakened cell walls and bacterial cell lysis.

At the molecular level, fosfomycin mimics phosphoenolpyruvate (PEP), the natural substrate of MurA. Its epoxide ring forms a covalent bond with an active site cysteine residue of MurA, resulting in irreversible enzyme inhibition. This mechanism is unique among antibiotics, providing activity against both Gram-positive and Gram-negative bacteria by preventing the construction of a functional cell wall early in the biosynthetic pathway.

Cellular uptake of fosfomycin is facilitated by bacterial transport systems, primarily the glycerophosphate transporter (GlpT) and the hexose phosphate transporter (UhpT). These uptake pathways contribute to selective drug entry into bacterial cells. Resistance may arise through mutations in these transporter genes, reducing fosfomycin uptake.

Clinically, fosfomycin's mechanism confers broad-spectrum activity effective against common uropathogens such as Escherichia coli and Enterococcus faecalis. Compared to other cell wall synthesis inhibitors like beta-lactams, fosfomycin acts earlier in the biosynthesis pathway, offering a complementary mechanism that can overcome resistance to penicillins or cephalosporins. Research supports this unique mechanism as valuable in multidrug-resistant urinary tract infections, reinforcing fosfomycin's role in empiric therapy and combination treatments.

Pharmacological Actions

Bactericidal antibiotic activity through inhibition of bacterial cell wall synthesis.
Selective inhibition of MurA enzyme, halting the production of peptidoglycan precursors.
Broad-spectrum antibacterial activity against both Gram-negative and Gram-positive uropathogens.
High urinary concentration achievable with single-dose oral administration.
Minimal metabolism enabling preserved active drug excretion intact via the kidneys.
Low protein binding allowing extensive tissue distribution in urinary tract sites.
Ability to penetrate renal and bladder tissues effectively.
Limited impact on gut flora due to rapid urinary excretion and single-dose regimen.
Useful against multidrug-resistant bacteria including some beta-lactam-resistant strains.
Unique uptake mechanism via specific bacterial transport systems, reducing cross-resistance with other antibiotics.
Relatively low potential for systemic toxicity due to minimal systemic exposure.
Demonstrated efficacy in rapid bacterial eradication in uncomplicated urinary tract infections.
Compatible with single-dose therapeutic regimen improving patient compliance.
Observed low incidence of bacterial resistance developing during monotherapy.
Minimal drug-drug interactions given non-metabolized pharmacokinetics.

Therapeutic Or Clinical Uses

Treatment of uncomplicated urinary tract infections (acute cystitis) caused by susceptible strains of Escherichia coli.
Effective management of lower urinary tract infections in women.
Alternative treatment option for urinary infections caused by Enterococcus faecalis.
Utilized as a single-dose oral therapy to improve adherence and reduce treatment burden.
Treatment of community-acquired urinary infections where resistance to other agents is a concern.
Part of empiric regimens in women with acute cystitis symptoms pending culture results.
Has utility in outpatient antibiotic therapy for urinary infections.
May be used in patients allergic to beta-lactam antibiotics as an alternative.
Useful in preserving use of broad-spectrum antibiotics by providing effective narrow-spectrum coverage.
Potential off-label use in combination therapies for multidrug-resistant pathogens in urinary tract infections.
Not indicated for treatment of pyelonephritis or complicated urinary tract infections.
Considered a first-line agent in some clinical guidelines for uncomplicated cystitis.
Utilized in cases to reduce adverse effects associated with longer antibiotic courses.
Appropriate for women who require a rapid and convenient treatment option for bladder infections.
Supports antimicrobial stewardship by limiting exposure duration and reducing resistance pressure.

Side Or Adverse Effects

Gastrointestinal disturbances including nausea, upset stomach, and diarrhea.
Headache and dizziness have been reported as common neurological effects.
Localized hypersensitivity reactions such as rash and pruritus.
Sore throat and upper respiratory symptoms like runny nose.
Weakness or malaise reported in some patients.
Menstrual abnormalities including menstrual pain and vaginal itching or discharge.
Serious allergic reactions are rare but may include hives, facial swelling, and difficulty breathing.
Potential to cause antibiotic-associated diarrhea; warning for development of Clostridioides difficile infection.
Possible transient alterations in taste perception.
Rare ototoxic symptoms such as hearing problems have been noted in overdose scenarios.
No significant hematological side effects have been commonly reported.
Side effects are generally mild and self-limited with single-dose administration.
Adverse reactions typically resolve without intervention upon cessation.
Patients with preexisting allergies may exhibit higher risk of hypersensitivity.
Incidence of adverse effects does not significantly increase with single-dose therapy.

Precautions

Do not use fosfomycin in patients with known hypersensitivity to the drug or related compounds.
Use with caution in patients with impaired renal function as elimination is chiefly renal.
Avoid administration to children under 12 years due to insufficient safety data.
Pregnant or breastfeeding women should consult healthcare providers before use.
Avoid sharing medication with others, even if symptoms appear similar.
Maintain adequate hydration during therapy to support renal clearance and urinary drug concentration.
Monitor for signs of allergic reactions and discontinue if they occur.
Be vigilant for symptoms of antibiotic-associated diarrhea; report prolonged or bloody diarrhea immediately.
Fosfomycin is not appropriate for treatment of complicated urinary infections like pyelonephritis.
Assess urinary pathogen susceptibility before use where feasible to ensure effectiveness.
Use in patients with glucose-6-phosphate dehydrogenase deficiency should be cautious.
Inform patients about proper preparation and ingestion of oral granules to ensure full dosing.
Avoid mixing fosfomycin powder with hot water to preserve drug stability.
Use promptly after solution preparation to prevent reduced efficacy.
Patients with underlying gastrointestinal disorders should report any unusual symptoms during treatment.

Warnings

Use fosfomycin only as directed by a healthcare provider to prevent treatment failure.
Discontinue and seek immediate medical attention if signs of anaphylaxis or severe allergic reaction appear.
Risk of Clostridioides difficile-associated diarrhea exists; report watery or bloody diarrhea promptly.
Not recommended for treatment of systemic or upper urinary tract infections like pyelonephritis.
Do not exceed single-dose regimen unless specifically instructed by a physician.
Avoid unprescribed or repeated use to minimize resistance development.
Inform healthcare providers of all concurrent medications to avoid unidentified interactions.
Consult a physician before use in patients with significant hepatic or renal impairment.
Pregnancy risk has not been fully established; balance potential benefits against unknown risks.
Avoid administration in pediatric patients under 12 years old due to lack of evidence supporting safety.
Do not use expired medication or powders that have been compromised by moisture or heat.
Ensure accurate urine culture and sensitivity testing prior to and after treatment to confirm eradication.
Monitor for any new or worsening bladder symptoms during or after treatment.
Patients should avoid sharing medication to prevent inappropriate usage.
Storage conditions must be maintained to preserve drug efficacy and prevent degradation.

Contraindication

Known hypersensitivity or allergy to fosfomycin or any component of the formulation.
Children younger than 12 years of age due to lack of established safety and efficacy.
Not indicated for treatment of complicated urinary tract infections such as pyelonephritis or renal abscesses.
Avoid use in patients with a history of severe allergic reactions to structurally related antibiotics.
Contraindicated in individuals with previous severe hypersensitivity reactions to fosfomycin tromethamine.
Use cautiously or avoid if the patient has significant renal impairment without medical supervision.
Not suitable for intravenous administration; only oral formulation is approved for use.
Not to be used to treat infections caused by pathogens known to be resistant to fosfomycin.
Contraindicated in patients who have experienced antibiotic-associated colitis while on fosfomycin.

Drug Interactions

Potential interaction with metoclopramide, which may alter gastrointestinal motility and affect drug absorption.
Minimal metabolism reduces the likelihood of cytochrome P450-mediated drug interactions.
No significant interactions reported with common vitamins or herbal supplements, but clinical vigilance is warranted.
Concurrent use with nephrotoxic agents should be approached cautiously because of renal elimination.
No notable interference with oral contraceptives or anticoagulants documented.
No documented interaction with other urinary antiseptics or antibiotics used for UTIs.
Use with other agents that alter urinary pH may affect fosfomycin solubility and excretion.
Absence of significant protein binding reduces displacement risks with highly protein-bound drugs.
No known effect on hepatic enzyme induction or inhibition.
Combination therapy with fosfomycin and other antimicrobials may be synergistic but requires clinical guidance.
No documented interactions with common over-the-counter analgesics or antipyretics.
No reported interactions requiring dose adjustments of fosfomycin or concomitantly administered drugs.
Monitor for unexpected side effects when fosfomycin is administered with multiple concomitant medications.
Patients should inform healthcare providers about all medications they are taking, including non-prescription drugs.
Comprehensive drug interaction data remains limited; cautious use advised in polypharmacy settings.

Preparations Of The Drug

Fosfomycin tromethamine oral granule powder supplied in single-dose sachets (3 g).
Each sachet designed for reconstitution with water to create an oral suspension.
Powder must be dissolved in 3 to 4 ounces (approximately half a cup) of cold water before ingestion.
Granules should not be taken in dry form or mixed with hot water as this may degrade the drug.
Manufactured in moisture-proof packaging to maintain drug stability until use.
Stored at room temperature away from heat and moisture to prevent degradation.
Each dose intended for single administration with no division or partial use recommended.
Original packaging should be retained until time of dose preparation to ensure integrity.
No intravenous, intramuscular, or other formulations are currently approved for fosfomycin tromethamine.
Commercial preparations designed to maximize patient compliance with simple single-dose regimens.
Packages include instructions for proper mixing and administration to ensure full dosing.
No known extemporaneous compounding beyond manufactured granule form is recommended.
Shelf-life and stability data support storage guidelines and safe use within expiry dates.
No flavoring additives typically included, although the granule formulation provides acceptable palatability.
Manufacturing standards conform to regulatory guidelines ensuring consistent quality and efficacy.