LAUSR

Background: Amidst the era of widespread resistance, there has been a renewed interest in older antibiotics such as fosfomycin, owing to its activity against certain resistant Gram-negative pathogens, including multidrug-resistant variants expressing extended spectrum β-lactamases or carbapenemases. The goal of the study was to investigate pharmacokinetic/pharmacodynamic (PK/PD) index and PK/PD targets of fosfomycin in murine thigh and kidney infection models, employing clinical isolates of Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae). Methods: Seven isolates of E. coli (one wild-type and six clinical isolates) and five isolates of K. pneumoniae (one wild-type and four clinical isolates) were utilized for in vivo PK/PD studies. Single-dose plasma PK studies were conducted in infected mice by subcutaneous route. PD index was determined from exposure-response analysis employing 24-hr dose fractionation studies in neutropenic murine thigh infection model, while pharmacodynamic targets (PDTs) were derived from both thigh and kidney infection models. Results: Dose fractionation studies demonstrated that in vivo efficacy of fosfomycin best correlated with AUC/MIC for E. coli (R2 = 0.9227) and K. pneumoniae (R2 = 0.8693). The median AUC/MIC linked to 1 log10 kill effects were 346.2 and 745.2 in thigh infection model and 244.1 and 425.4 in kidney infection model for E. coli and K. pneumoniae, respectively. The mice plasma protein binding of fosfomycin was estimated to be 5.4%. Conclusions: The in vivo efficacy of fosfomycin against Enterobacterales was best described by AUC/MIC. The PDTs derived from this study may help define the coverage potential of fosfomycin at the clinical doses approved.