Fosfomycin is indicated only for the treatment of uncomplicated urinary tract infections (acute cystitis) in women due to susceptible strains of Escherichia coli and Enterococcus faecalis. Fosfomycin is not indicated for the treatment of pyelonephritis or perinephric abscess.
If persistence or reappearance of bacteriuria occurs after treatment with Fosfomycin, other therapeutic agents should be selected.
Fosfomycin is a broad spectrum antibiotic that concentrates in kidney and bladder and is used to treat uncomplicated urinary tract infections. Fosfomycin also reduces nephrotoxicity and ototoxicity of platinum-containing anti-tumor agents.
Fosfomycin is a phosphoenolpyruvate analogue produced by Streptomyces that irreversibly inhibits enolpyruvate transferase (MurA), which prevents the formation of N-acetylmuramic acid, an essential element of the peptidoglycan cell wall.
Dosage & Administration
The recommended dosage for women 18 years of age and older for uncomplicated urinary tract infection (acute cystitis) is one sachet (3 gm) of Fosfomycin. Fosfomycin may be taken with or without food. Fosfomycin should not be taken in its dry form. Always mix Fosfomycin with water before ingesting.
Preparation: Pour the entire contents of a single-dose sachet of Fosfomycin into 3 to 4 ounces of water (½ cup) and stir to dissolve. Do not use hot water. Fosfomycin should be taken immediately after dissolving in water.
Most frequently reported adverse events occurring in more than 1%: diarrhea 10.4%, headache 10.3%, vaginitis 7.6%, nausea 5.2%, rhinitis 4.5%, back pain 3.0%, dysmenorrheal 2.6%, pharyngitis 2.5%, dizziness 2.3%, abdominal pain 2.2%, pain 2.2%, dyspepsia 1.8%, asthenia 1.7%, and rash 1.4%.
The following adverse events occurred less than 1%: abnormal stools, anorexia, constipation, dry mouth, dysuria, ear disorder, fever, flatulence, flu syndrome, hematuria, infection, insomnia, lymphadenopathy, menstrual disorder, migraine, myalgia, nervousness, paresthesia, pruritus, SGPT increased, skin disorder, somnolence, and vomiting.
Pregnancy & Lactation
General: Do not use more than one single dose of Fosfomycin to treat a single episode of acute cystitis. Repeated daily doses of Fosfomycin did not improve the clinical success or microbiological eradication rates compared to single dose therapy, but did increase the incidence of adverse events. Urine specimens for culture and susceptibility testing should be obtained before and after completion of therapy.
Carcinogenesis, Mutagenesis, Impairment of Fertility: Long term carcinogenicity studies in rodents have not been conducted because Fosfomycin is intended for single dose treatment in humans. Fosfomycin was not mutagenic or genotoxic in the in vitro Ames’ bacterial reversion test, in cultured human lymphocytes, in Chinese hamster V79 cells, and the in vivo mouse micronucleus assay. Fosfomycin did not affect fertility or reproductive performance in male and female rats.
Clostridium difficile associated diarrhea (CDAD) has been reported with 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.
Clostridium difficile produces toxins A and B which contribute to the development of CDAD. Hypertoxin producing strains of C. difficile cause increased morbidity and mortality, as these infections can be refractory to antimicrobial therapy and may require colectomy. CDAD must be considered in all patients who present with diarrhea following antibiotic use. 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.
In acute toxicology studies, oral administration of high doses of Fosfomycin up to 5 gm/kg were well-tolerated in mice and rats, produced transient and minor incidences of watery stools in rabbits, and produced diarrhea with anorexia in dogs occurring 2-3 days after single dose administration. These doses represent 50-125 times the human therapeutic dose.
The following events have been observed in patients who have taken Fosfomycin in overdose: vestibular loss, impaired hearing, metallic taste, and general decline in taste perception. In the event of overdosage, treatment should be symptomatic and supportive.
Use in Special Population
Pediatric Use: Safety and effectiveness in children age 12 years and under have not been established in adequate and well-controlled studies.
Geriatric Use: Clinical studies of Fosfomycin did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.