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Background  Carbapenem-resistant Gram-negative bacteria are a major clinical concern as they cause virtually untreatable infections since carbapenems are among the last-resort antimicrobial agents. β-Lactamases implicated in carbapenem resistance include KPC, NDM, and OXA-type carbapenemases. Antimicrobial combination therapy is the current treatment approach against carbapenem resistance in order to limit the excessive use of colistin; however, its advantages over monotherapy remain debatable. An alternative treatment strategy would be the use of carbapenem/β-lactamase inhibitor (βLI) combinations. In this study, we assessed the  in vitro  and  in vivo  phenotypic and molecular efficacies of three βLIs when combined with different carbapenems against carbapenem-resistant Gram-negative clinical isolates. The chosen βLIs were (1) Avibactam, against OXA-type carbapenemases, (2) calcium-EDTA, against NDM-1, and (3) Relebactam, against KPC-2.    Methods  Six  Acinetobacter baumannii  clinical isolates were screened for  bla  OXA-23-like ,  bla  OXA-24/40 ,  bla  OXA-51-like ,  bla  OXA-58 , and  bla  OXA-143-like , and eight  Enterobacteriaceae  clinical isolates were screened for  bla  OXA-48 ,  bla  NDM-1 , and  bla  KPC-2 . The minimal inhibitory concentrations of Imipenem (IPM), Ertapenem (ETP), and Meropenem (MEM) with corresponding βLIs for each isolate were determined. The efficacy of the most suitable  in vitro  treatment option against each of  bla  OXA-48 ,  bla  NDM-1 , and  bla  KPC-2  was assessed via survival studies in a BALB/c murine infection model. Finally, RT-qPCR was performed to assess the molecular response of the genes of resistance to the carbapenem/βLI combinations used under both  in vitro  and  in vivo  settings.    Results  Combining MEM, IPM, and ETP with the corresponding βLIs restored the isolates’ susceptibilities to those antimicrobial agents in 66.7%, 57.1%, and 30.8% of the samples, respectively. Survival studies in mice revealed 100% survival rates when MEM was combined with either Avibactam or Relebactam against  bla  OXA-48  and  bla  KPC-2 , respectively. RT-qPCR demonstrated the consistent overexpression of  bla  OXA-48  upon treatment, without hindering Avibactam’s activity, while  bla  NDM-1  and  bla  KPC-2  experienced variable expression levels upon treatment under  in vitro  and  in vivo  settings despite their effective phenotypic results.    Conclusion  New carbapenem/βLI combinations may be viable alternatives to antimicrobial combination therapy as they displayed high efficacy  in vitro  and  in vivo.  Meropenem/Avibactam and Meropenem/Relebactam should be tested on larger sample sizes with different carbapenemases before progressing further in its preclinical development.

Evaluating the Efficacies of Carbapenem/β-Lactamase Inhibitors Against Carbapenem-Resistant Gram-Negative Bacteria in vitro and in vivo