Imipenem/Relebactam Plus Aztreonam: First Reported Use in MDR Klebsiella pneumoniae Sternal Infection Complicated by Bacteremia

Background: Carbapenem-resistant Klebsiella pneumoniae (CRKP) poses a significant therapeutic challenge, particularly when multiple resistance mechanisms, such as metalloβ-lactamases (MBLs) and Klebsiella pneumoniae carbapenemase (KPC), coexist. Case description: We describe a case of a 51-year-old male with a post-sternotomy surgical site infection and concurrent bacteremia caused by a CRKP. Sternal swab and mediastinal liquid culture results highlighted CRKP harboring blaNDM and blaKPC genes, while the blood isolate showed blaCTX and blaKPC, indicating phenotypic resistance to ceftazidime-avibactam. All the strains exhibited phenotypic susceptibility to meropenemvaborbactam (MEV), despite having a high minimum inhibitory concentration. Following clinical failure of MEV-based therapy, combination treatment with aztreonam (ATM) and imipenem/cilastatin/relebactam (IMI/REL), plus gentamicin, was initiated. Therapy was well tolerated and resulted in microbiological eradication and full clinical recovery. The patient completed 49 days of ATM and IMI/REL without relapse over a 3-month follow-up period. This is, to the best of our knowledge, the first reported case of IMI/REL being used in combination with ATM.