Selection of the appropriate avibactam concentration for use with ceftibuten in broth microdilution susceptibility testing

Ceftibuten is an oral cephalosporin approved by the US Food and Drug Administration in 1995 that is in early clinical development to be combined with an oral prodrug of avibactam. We evaluated the activity of ceftibuten-avibactam against molecularly characterized Enterobacterales that produced clinically relevant β-lactamases and assessed the best avibactam concentration to be combined with ceftibuten for susceptibility testing. Resistance mechanisms were evaluated by whole genome sequencing. MIC values were determined by broth microdilution of ceftibuten, avibactam, and ceftibuten combined with fixed concentrations (2, 4, and 8 mg/L) and ratios (1:1 and 2:1) of avibactam. The organism collection (n = 71) included Enterobacterales producing ESBLs, KPC, metallo-β-lactamases, AmpC, K-1, OXA-48, and SME, as well as isolates with porin alterations. The ceftibuten-avibactam combination that best separated isolates with β-lactamases inhibited by avibactam from isolates with resistance mechanisms that are not affected by avibactam was the combination with avibactam at a fixed concentration of 4 mg/L.