Ex Vivo Urinary Bactericidal Activity and Urinary Pharmacodynamics of Fosfomycin after Two Repeated Dosing Regimens of Oral Fosfomycin Tromethamine in Healthy Adult Subjects

The ex vivo bactericidal activity and pharmacodynamics of fosfomycin in urine were evaluated in 18 healthy subjects. Subjects received 3 g every other day (QOD) for 3 doses and then every day (QD) for 7 doses or vice versa. Serial urine samples were collected before and up to 24 h after dosing on days 1 and 5. Eight bacterial strains with various genotypic and phenotypic susceptibilities to fosfomycin were used for all experiments (5 Escherichia coli, 2 Klebsiella pneumoniae, and 1 Proteus mirabilis). MICs were performed via agar dilution. Urinary bactericidal titers (UBTs) were performed via modified Schlichter test using participant's drug-free urine as the diluent. Urinary time-kill analyses were performed on pooled 24-h urine aliquots from days 1 and 5. All experiments were performed in triplicate with and without the addition of 25 mg/liter of glucose-6-phosphate (G6P). Mean 24-h urine concentrations of fosfomycin ranged from 324.7 to 434.6 mg/liter regardless of study day or dosing regimen. The urinary antibacterial activity of fosfomycin was also similar across study days and dosing regimens. UBT values did not correlate with MICs determined in the presence of G6P. Fosfomycin was reliably bactericidal in urine only against the 5 E. coil strains, regardless of genotype or MIC value. Together, these data do not support the use of oral fosfomycin tromethamine for pathogens other than E. coli or at a dosing frequency higher than QOD. Fosfomycin MICs determined in the presence of G6P may not accurately reflect the in vivo activity given the lack of G6P in human urine.