Pharmacodynamic activity of fosfomycin simulating urinary concentrations achieved after a single 3-g oral dose versus Escherichia coli using an in vitro model.

We assessed the activity of fosfomycin simulating urinary concentrations achieved after a single 3-g oral dose against Escherichia coli using an in vitro pharmacodynamic model. Eleven urinary isolates of E. coli were studied. Isolates were ESBL-producing or carbapenemase-producing. The in vitro pharmacodynamic model was inoculated with an inoculum of (~1×106cfu/mL). Fosfomycin was administered to simulate maximum free (ƒ) urine (U) concentrations and a t1/2 obtained after a standard single 3-g oral dose in healthy volunteers (ƒUmax, 4000mg/L; t1/2, 6h). Sampling was performed over 48h to assess the rate and extent of bacterial reduction as well as resistance selection. Complete bacterial eradication from the model was defined by no regrowth over the 48h study period. Fosfomycin MICs ranged from 1 to 4μg/mL for ESBL producers, while all 3 carbapenemase-producing E. coli demonstrated a fosfomycin MIC of 2μg/mL. Fosfomycin ƒT>MIC of 100% (ƒAUC0-24/MIC, ≥~7250) resulted in bacterial killing (reductions in log10 CFU assessed relative to the starting inoculum at 2, 4, 6, 12, 24, and 48h of ≥3.0) at each time-point versus all isolates of ESBL-producing and carbapenemase-producing E. coli. We conclude that fosfomycin urinary concentrations obtained after a single 3-g oral dose were bactericidal as early as 1h after dosing with complete bacterial eradication at all time-points over the 48h testing period against urinary isolates of E. coli (including MDR ESBL- and/or carbapenemase-producing strains). Our data help to explain the high (>90%) microbiological and clinical cure rates achieved with fosfomycin when used as a single 3-g oral dose to treat patients with acute uncomplicated cystitis.