We demonstrated the in vitro efficacy of MEM in combination with CMZ against IMP-producing CRE isolates. This investigation was novel and was intended to corroborate the potency of CMZ, which may function as an antagonist of P-type carbapenemases. In addition to the double-carbapenem therapy  and ceftazidime/avibactam , newer β-lactamase inhibitors including relebactam (MK-7655), nacubactam (OP0595), zidebactam (WCK 5107), and vaborbactam (RPX7009) have been developed to treat CPE that produce serine-carbapenemases . There are still no treatment options for infections caused by MBL-producing CRE.
CMZ is one of the established antimicrobials for which clinical experience of its use is being increasingly accumulated. It is a cephamycin antibiotic that is stable against extended-spectrum β-lactamases (ESBLs) , but is hydrolyzed by carbapenemases, including IMPs . Presently, CRE isolates that produced IMPs were highly resistant to CMZ, with MICs ranging 64 to 2048 µg/mL. These findings may indicate a high affinity of CMZ to IMPs, and is consistent with our previous hypothesis that CMZ binds avidly to IMP-type carbapenemases and helps MEM exert its bactericidal activity, as seen in double-carbapenem therapy . ESBL producing Enterobacteriaceae have globally disseminated  and CRE isolates co-harboring genes encoding ESBLs and carbapenemases have been previously described [13, 14]. Particularly, blaIMP-6 was reported to disseminate mainly through the horizontal transmission of the prevalent plasmid, pKPI-6, which simultaneously carries blaIMP-6 and blaCTX-M2 . The CMZ combination therapy would be a ready-to-apply, cost-effective strategy for IMP-producing CRE, compared to the newer β-lactamase inhibitors.
However, several limitations remain to be overcome prior to clinical use. First, the CMZ concentrations tested in this study exceed the serum levels attainable in humans. Second, as shown in our previous study , an inoculum effect may exist in this combination therapy, possibly resulting in treatment failure using a high bacterial inoculum. Third, various antimicrobial resistance mechanisms, such as AmpC-type beta-lactamases, efflux pumps, and porin loss, may influence the inhibitory activity of CMZ.
Despite these limitations, the collective data from this study demonstrate a preferential effect of MEM and CMZ when used in combination against IMP-producing CRE in vitro. Faced with the limited availability of new antimicrobials, the revived use of an existing antimicrobial agent could provide effective treatment. In vivo experiments as well as pharmacokinetic and pharmacodynamic studies are required before the clinical application of the new combination therapy.