What it's for (Indications)
- Fosfomycin is indicated for the treatment of acute, uncomplicated urinary tract infections (UTIs) in women aged 18 years and older, caused by susceptible strains of *Escherichia coli* and *Enterococcus faecalis*.
- Its use is specifically targeted at infections confined to the bladder, offering a convenient single-dose regimen for appropriate patient populations.
- It is not indicated for the treatment of pyelonephritis, complicated UTIs, or recurrent UTIs.
- Careful consideration of microbiological susceptibility data is advised prior to administration, although empiric treatment may be initiated based on local epidemiological patterns of bacterial resistance.
- The clinical efficacy has been established for these specific pathogens and infection types, making it a valuable agent in managing uncomplicated cystitis when appropriate susceptibility is confirmed or highly probable.
- Due to its pharmacokinetic profile and spectrum of activity, it is explicitly not recommended for infections involving systemic spread or upper urinary tract involvement.
Dosage Information
| Type | Guideline |
|---|---|
| Standard | Fosfomycin tromethamine is administered as a single 3-gram dose, dissolved in 90 to 120 mL (3 to 4 ounces) of cold water (not hot water). It should be taken orally, either with or without food. The granules must be completely dissolved before consumption. This single-dose regimen simplifies patient adherence and is effective for its indicated use. It is crucial to emphasize that this medication is not intended for multiple doses for the treatment of uncomplicated UTIs, nor is it recommended for extended courses. Patients should be instructed to consume the entire dissolved solution immediately after preparation. No dosage adjustment is explicitly required for patients with mild to moderate renal impairment (creatinine clearance ≥20 mL/min), but caution is advised in severe renal dysfunction (creatinine clearance <20 mL/min), as systemic exposure may be significantly increased, potentially leading to exaggerated adverse effects. The safety and efficacy of multiple doses of fosfomycin in complicated UTIs have not been established. |
Safety & Warnings
Common Side Effects
- Common adverse reactions associated with fosfomycin treatment include diarrhea, nausea, headache, dizziness, abdominal pain, asthenia, dyspepsia, and vaginitis.
- These effects are generally mild and transient, typically resolving without intervention.
- Less common but more serious adverse events can include hypersensitivity reactions, ranging from mild dermatologic manifestations such as rash and pruritus to severe and potentially life-threatening anaphylaxis, including angioedema and anaphylactic shock.
- Patients should be advised to seek immediate medical attention if they experience symptoms of an allergic reaction.
- Furthermore, as with nearly all antibacterial agents, *Clostridioides difficile*-associated diarrhea (CDAD) has been reported with fosfomycin use.
- CDAD can range in severity from mild diarrhea to fatal pseudomembranous colitis, and its onset can occur during or several weeks after cessation of therapy.
- Other reported side effects include rhinitis, back pain, pharyngitis, and upper respiratory tract infection.
- Close monitoring for adverse reactions, especially severe hypersensitivity and gastrointestinal complications, is essential, and appropriate management should be initiated promptly if such events occur.
Serious Warnings
- Black Box Warning: Fosfomycin (e.g., Factum) does not currently carry an FDA-mandated Black Box Warning. However, it is imperative for healthcare professionals and patients to be aware of certain serious safety concerns associated with its use. **Serious Warnings:** 1. **Hypersensitivity Reactions:** Severe and occasionally fatal hypersensitivity reactions, including anaphylaxis and angioedema, have been reported with fosfomycin. These reactions can occur rapidly after the first dose. Patients should be instructed to seek immediate medical attention if they experience signs or symptoms of an allergic reaction (e.g., rash, hives, swelling of the face, lips, tongue, or throat, difficulty breathing), and fosfomycin should be discontinued permanently. 2. **_Clostridioides difficile_-Associated Diarrhea (CDAD):** _Clostridioides difficile_ associated diarrhea (CDAD) has been reported with the use of nearly all antibacterial agents, including fosfomycin, and may range in severity from mild diarrhea to fatal colitis. Treatment with antibacterial agents alters the normal flora of the colon leading to overgrowth of _C. difficile_. Therefore, it is important to consider this diagnosis in patients who present with diarrhea subsequent to the administration of any antibacterial agents. Careful medical history is necessary since CDAD has been reported to occur over two months after the administration of antibacterial agents. If CDAD is suspected or confirmed, ongoing antibiotic use not directed against _C. difficile_ may need to be discontinued. Appropriate fluid and electrolyte management, protein supplementation, antibiotic treatment of _C. difficile_, and surgical evaluation should be instituted as clinically indicated. 3. **Limitations of Use:** Fosfomycin is indicated only for the treatment of acute uncomplicated urinary tract infections (UTIs) caused by susceptible strains of *Escherichia coli* and *Enterococcus faecalis*. It is not indicated for the treatment of pyelonephritis, complicated UTIs, or recurrent UTIs, as its efficacy in these conditions has not been established and could lead to inadequate treatment and potential therapeutic failure. The safety and efficacy of multiple doses of fosfomycin in complicated UTIs have not been established, and such use is not recommended.
- Patients should be thoroughly evaluated for potential allergies to fosfomycin or its excipients prior to administration, as severe hypersensitivity reactions, including anaphylaxis and angioedema, have been reported.
- Should an allergic reaction occur, fosfomycin should be immediately discontinued, and appropriate medical management initiated.
- Treatment with antibacterial agents, including fosfomycin, alters the normal flora of the colon and may permit overgrowth of *Clostridioides difficile*, leading to *Clostridioides difficile*-associated diarrhea (CDAD).
- CDAD can occur more than two months after antibiotic administration.
- If CDAD is suspected or confirmed, ongoing antibiotic use not directed against *C.
- difficile* may need to be discontinued.
- Appropriate fluid and electrolyte management, protein supplementation, antibiotic treatment of *C.
- difficile*, and surgical evaluation should be instituted as clinically indicated.
- Fosfomycin is not indicated for infections beyond acute uncomplicated cystitis; its effectiveness in pyelonephritis or complicated UTIs has not been established and such use may lead to inadequate treatment and potential therapeutic failure.
- The development of drug-resistant bacteria is a concern with all antimicrobial agents, and fosfomycin should only be used to treat infections proven or strongly suspected to be caused by susceptible bacteria.
- Prescribing fosfomycin in the absence of a proven or strongly suspected bacterial infection is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.
How it Works (Mechanism of Action)
Fosfomycin exerts its bactericidal effect by interfering with an early step in bacterial cell wall synthesis. Specifically, it irreversibly inactivates the enzyme UDP-N-acetylglucosamine-3-enolpyruvyltransferase, commonly known as MurA (or enolpyruvyl transferase). This enzyme catalyzes the transfer of enolpyruvate from phosphoenolpyruvate (PEP) to UDP-N-acetylglucosamine (UNAG), forming UDP-N-acetylglucosamine-3-enolpyruvyl ether, a crucial precursor in peptidoglycan biosynthesis. By inhibiting MurA, fosfomycin prevents the formation of N-acetylmuramic acid, a vital component of the bacterial peptidoglycan layer, leading to compromised cell wall integrity and subsequent bacterial lysis. Its unique mechanism of action, distinct from other commonly used antibiotics such as beta-lactams or fluoroquinolones, contributes to its efficacy against a range of susceptible Gram-positive and Gram-negative bacteria, including *E. coli* and *E. faecalis*, the primary pathogens targeted for uncomplicated UTIs. This novel target also means cross-resistance with other antibiotic classes is less likely, offering a valuable alternative for certain resistant strains.