Abstract
Cefiderocol/xeruborbactam is a novel β-lactam/β-lactamase inhibitor combination in which the siderophore-cephalosporin cefiderocol is paired with xeruborbactam, a broad-spectrum inhibitor that targets class A to D β-lactamases. We evaluated the contribution of Pseudomonas aeruginosa resistance mechanisms to cefiderocol/xeruborbactam susceptibility. A panel of 61 P. aeruginosa PAO1 derivatives was tested, including 20 knockout mutants representing key chromosomal resistance mechanisms (e.g., ampC overexpression, efflux upregulation, porin loss, iron uptake deficiency) and 41 transformants producing major circulating β-lactamases. Xeruborbactam was assessed in combination with cefiderocol and cefepime at 4 and 8 mg/L and compared with taniborbactam. Additionally, 99 cefiderocol-resistant clinical P. aeruginosa isolates were evaluated. Cefiderocol/xeruborbactam retained activity against most P. aeruginosa mutants with chromosomally encoded resistance mechanisms. However, the P. aeruginosa piuC-defective mutant yielded increased cefiderocol minimum inhibitory concentrations (MIC = 2 mg/L), which could not be restored by xeruborbactam. Xeruborbactam significantly increased the activity of cefiderocol against the majority of P. aeruginosa PAO1 transformants, including those producing PER-1, SHV-12, KPC Ω-loop mutants, or NDM variants. Cefiderocol/xeruborbactam was active against IMP-type MBLs (which only weakly hydrolyze cefiderocol), including xeruborbactam-resistant enzymes. Relative to taniborbactam, xeruborbactam-based combinations showed similar activity against P. aeruginosa PAO1 transformants, but with slightly higher MIC values when tested against metallo-β-lactamase producers. This MIC increase in xeruborbactam-based combinations was partly because of the constitutive MexAB-OprM efflux in the P. aeruginosa background, as confirmed with P. aeruginosa PAO1 efflux mutants. Importantly, xeruborbactam restored susceptibility in 78% of 99 cefiderocol-resistant P. aeruginosa strains, reducing the MIC(90) from 64 to 4 mg/L. Cefiderocol/xeruborbactam shows promising activity against P. aeruginosa.