Pharmacokinetic/pharmacodynamic analysis of oral fosfomycin against Enterobacterales, Pseudomonas aeruginosa and Enterococcus spp. In an in vitro bladder infection model: Impact on clinical breakpoints

Objectives: Fosfomycin is an established treatment for uncomplicated urinary tract infections (UTIs), yet evidence supporting susceptibility breakpoints is limited. We examine the UTI susceptibility criteria. Methods: Fosfomycin susceptibility, heteroresistance and in vitro growth in a bladder infection model, after a single 3 g dose of oral fosfomycin, were bridged to human pharmacokinetics with pharmacokinetic/pharmacodynamic and Monte Carlo analyses. Data from common uropathogens (24 Escherichia coli, 20 Klebsiella pneumoniae, 4 Enterobacter cloacae, 14 Pseudomonas aeruginosa, 8 Enterococcus faecalis and 8 Enterococcus faecium) were compared and analysed to ascertain species-specific PTA. Results: Glucose-6-phosphate (G6P) increased MICs of E. coli, K. pneumoniae and E. cloacae (median 2-fold dilutions 3-5), but not of P. aeruginosa and Enterococcus. Atypical E. coli lacking G6P potentiation were killed in the bladder infection model despite high MICs (32-128 mg/L). Fosfomycin heteroresistance was uncommon in E. coli (MIC>2 mg/L) but was detected in the majority of K. pneumoniae (MIC>1 mg/L) and P. aeruginosa (MIC>8 mg/L). For these species, baseline heteroresistance was a strong predictor for treatment failure in the model. No heteroresistance was found in Enterococcus. The fAUC/MIC targets for stasis were 1935, 3393, 9968, 2738 and 283 for typical E. coli, K. pneumoniae, E. cloacae, P. aeruginosa and E. faecalis, respectively (synthetic human urine medium alone promoted a 1 log10 kill in E. faecium). A >95% PTA for stasis was only found at MIC≤epidemiological cut-off (ECOFF) for E. coli (4 mg/L). For other species, PTAs were low for WT populations. Conclusions: With the exception of E. coli, fosfomycin is a poor target for other uropathogen species. A reduction in oral fosfomycin UTI breakpoints is supported.