Emerging Cross-Resistance to Cefiderocol and Ceftazidime-Avibactam in KPC-Producing Klebsiella pneumoniae During Ceftazidime-Avibactam Therapy

Background/Objectives: Ceftazidime-avibactam (CAZ/AVI) and cefiderocol (FDC) retain activity against most clinical isolates of KPC-producing Klebsiella pneumoniae (KPC-Kp). Nevertheless, the emergence of cross-resistance between both agents has become an increasing clinical concern. In this study, the development of resistance to both CAZ/AVI and FDC was retrospectively investigated in KPC-Kp isolates recovered from patients treated with CAZ/AVI between 2014 and 2023. Methods: Twenty-two isolates (pre- and post-therapy) obtained from nine patients were included. Demographic and clinical data were collected. FDC susceptibility testing was determined for both clinical isolates and E. coli TOP10 transformants by reference broth microdilution using iron-depleted medium, and MICs were interpreted according to EUCAST breakpoints. Resistance mechanisms were characterized using whole genome sequencing. Results: Cross-resistance to both CAZ/AVI and FDC emerged in six patients (66.7%) during CAZ/AVI therapy, after a median treatment duration of 12.8 ± 7.6 days. The presence of different KPC-3 variants in the CAZ/AVI-FDC resistant isolates was revealed by genomic analysis. Furthermore, the role of KPC-28, KPC-31, KPC-47, KPC-94, KPC-95, and KPC-148 in mediating reduced susceptibility to CAZ/AVI and FDC was confirmed by AST of transformants carrying blaKPC variants. However, differences in MIC values compared with those obtained for the corresponding clinical isolates supported the relevance of additional mechanisms of co-resistance. The presence of ferric citrate transport (FEC) system was also associated with higher FDC MICs. Mutations in genes coding siderophore-iron transporter, PBPs, sensor histidine kinase or Tol-Pal system were also found in some of the isolates. Conclusions: These findings highlight the ability of KPC-Kp to develop co-resistance to both CAZ/AVI and FDC during CAZ/AVI therapy.