Fosfomycin: A Comprehensive Overview of Its Pharmacology, Clinical Applications, and Resistance Profile > 자유게시판

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Introduction

Fosfomycin is a broad-spectrum antibiotic originally discovered in 1969 from Streptomyces fradiae. It is a phosphonic acid derivative that exhibits bactericidal activity against a wide range of Gram-positive and Gram-negative pathogens, including multidrug-resistant strains. Its unique mechanism of action, favorable pharmacokinetics, and low incidence of resistance make it a valuable option for treating uncomplicated urinary tract infections (UTIs) and as a salvage therapy for complicated infections caused by carbapenem-resistant organisms (e.g., carbapenem-resistant Enterobacteriaceae, CRE, and methicillin-resistant Staphylococcus aureus, Obelit: Revisión Clínica de su Eficacia y Seguridad MRSA). This report provides a concise overview of fosfomycin’s properties, clinical uses, and evolving resistance patterns.

Mechanism of Action

Fosfomycin inhibits bacterial cell wall synthesis by irreversibly binding to the enzyme UDP‑N‑acetylglucosamine enolpyruvyl transferase (MurA). This enzyme catalyzes the first step of peptidoglycan biosynthesis (formation of N‑acetylmuramic acid). By blocking this step, fosfomycin prevents cross-linking of peptidoglycan, leading to osmotic lysis. Importantly, its action is independent of penicillin‑binding proteins, so cross-resistance with β‑lactams is rare. Fosfomycin enters bacterial cells via two transport systems: the glycerol-3-phosphate (GlpT) and hexose‑6‑phosphate (UhpT) transporters, which are expressed in many bacteria. Its activity is enhanced in acidic environments (e.g., urine, pH ~5.5) and by the presence of glucose‑6‑phosphate, which stimulates uptake.

Pharmacokinetics

Fosfomycin is available as oral fosfomycin tromethamine and intravenous fosfomycin disodium (for systemic infections). Oral bioavailability is approximately 30–40%. After a single 3 g oral dose, peak serum concentrations reach 30–40 mg/L, but the drug concentrates in urine to exceed 1000 mg/L, making it ideal for UTIs. The elimination half‑life is about 5–7 hours (oral) and 2–4 hours (IV) in normal renal function. Fosfomycin is primarily excreted unchanged by glomerular filtration and active tubular secretion, achieving high urinary levels. Tissue penetration is good (lungs, soft tissues, bone, and cerebrospinal fluid when meninges inflamed). No significant hepatic metabolism occurs, and dosage adjustment is needed only for creatinine clearance <30 mL/min.

Spectrum and Clinical Use

Uncomplicated UTIs: Fosfomycin tromethamine (3 g single dose) is approved in many countries for acute cystitis in women. It retains activity against >90% of E. coli isolates, including ESBL‑producing strains. For complicated UTIs, repeated doses (e.g., 3 g every 48–72 hours) may be used off-label.

Multidrug‑Resistant Infections: Intravenous fosfomycin is increasingly employed as combination therapy for infections caused by carbapenem‑resistant Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. It is often paired with colistin, tigecycline, or carbapenems to improve synergy and reduce resistance emergence. For example, a meta‑analysis reported clinical success rates of 60–80% in CRE infections using fosfomycin‑based combinations. It also has activity against MRSA and vancomycin‑resistant enterococci (VRE). Other indications include osteomyelitis, prostatitis, and ventilator‑associated pneumonia (IV formulation).

Resistance Mechanisms

Resistance to fosfomycin can develop via several pathways:

1. Reduced drug uptake: Mutations in glpT or uhpT transporters, or in the regulatory genes (uhpA, glpR), decrease intracellular accumulation. This is the most common resistance mechanism in Gram‑negatives.
   
2. Enzymatic inactivation: Fosfomycin-modifying enzymes (e.g., FosA, FosB, FosC) that catalyze thiol‑ or glutathione‑mediated ring opening. FosA is encoded on transposons and has been found in E. coli, K. pneumoniae, and P. aeruginosa. FosB is common in staphylococci. Plasmid‑borne fosA can spread horizontally.
   
3. Target modification: Mutations in murA reduce fosfomycin binding, but this is less frequent.
   
4. Efflux pumps: Overexpression of efflux systems (e.g., AcrAB‑TolC in E. coli) may contribute to low‑level resistance.
   
   

Despite these mechanisms, the overall resistance rate in community‑acquired E. coli remains low (<5% in most surveillance studies). However, among healthcare‑associated isolates (e.g., CRE), resistance can exceed 20–30%. Hetero‑resistance and rapid emergence of resistance during monotherapy are concerns, especially in high‑burden infections. Therefore, fosfomycin is rarely used as monotherapy for systemic infections.

Adverse Effects

Fosfomycin is generally well‑tolerated. Oral formulation may cause mild diarrhea, nausea, headache, and vaginitis. IV use may lead to phlebitis, hypokalemia (secondary to sodium load – each gram of IV fosfomycin disodium contains 14–15 mmol sodium), and elevated blood pressure. Rare but serious effects include eosinophilia, anaphylaxis, and Clostridioides difficile colitis. High‑dose IV therapy (>16 g/day) has been associated with hypernatremia and metabolic alkalosis. No significant nephrotoxicity or ototoxicity is observed. Because fosfomycin is a pregnancy category B drug, it is considered safe for UTIs during pregnancy.

Drug Interactions

Antacids or calcium‑containing compounds may reduce oral absorption (by chelation). Probenecid decreases renal excretion, raising serum levels. Metoclopramide accelerates gastric emptying, reducing absorption. In vitro synergy with β‑lactams, aminoglycosides, and colistin has been demonstrated, but antagonism with some tetracyclines (e.g., tigecycline) may occur. Clinical data on interactions from combination therapy are limited.

Global and Future Perspectives

Fosfomycin is listed on the WHO Model List of Essential Medicines for UTIs. Its role in treating CRE and other "priority pathogens" is expanding, but concerns over resistance and lack of standardized breakpoints (especially for IV formulations) remain. New oral formulations are under development. Surveillance programs to monitor resistance, especially among E. coli and K. pneumoniae, are crucial. Combination therapy and cycling strategies may help preserve its efficacy. Ongoing research into fosfomycin structural analogs and inhibitor‑based reversal of resistance (e.g., FosA inhibitors) offers promise.

Conclusion

Fosfomycin remains an effective and well‑tolerated antibiotic for uncomplicated UTIs and a vital component of combination therapy for multidrug‑resistant infections. Its unique mechanism, high urinary concentrations, and low resistance rates in the community underscore its value. However, emerging resistance in healthcare settings, particularly via plasmid‑borne fosfomycin‑modifying enzymes, necessitates prudent use and continued surveillance. With appropriate stewardship, fosfomycin will retain its role in the antibiotic armamentarium for years to come.