Objectives MK-7655 is a novel inhibitor of class A and C ß-lactamases. We investigated its potential to protect imipenem.
Methods Chequerboard MICs were determined by CLSI agar dilution: (i) for Enterobacteriaceae with carbapenemases; (ii) for Enterobacteriaceae with carbapenem resistance contingent on combinations of impermeability together with an extended-spectrum ß-lactamase or AmpC enzyme; and (iii) for Pseudomonas aeruginosa and other non-fermenters.
Results At a concentration of 4 mg/L, MK-7655 reduced imipenem MICs for Enterobacteriaceae with KPC carbapenemases from 16–64 mg/L to 0.12–1 mg/L. Synergy also was seen for Enterobacteriaceae with impermeability-mediated carbapenem resistance, with weaker synergy seen for isolates with the OXA-48 enzyme. On the other hand, MK-7655 failed to potentiate imipenem against Enterobacteriaceae with metallo-carbapenemases. In the case of P. aeruginosa, where endogenous AmpC confers slight protection versus imipenem, 4 mg/L MK-7655 reduced the MIC of imipenem for all isolates, except those with metallo-carbapenemases: the MICs of imipenem fell from 1–2 mg/L to 0.25–0.5 mg/L for imipenem-susceptible P. aeruginosa and from 16–64 mg/L to 1–4 mg/L for OprD-deficient strains. No potentiation was seen for chryseobacteria or for Stenotrophomonas maltophilia.
Conclusions MK-7655 potentiated imipenem against Enterobacteriaceae with KPC carbapenemases or combinations of ß-lactamase and impermeability, but not those with metallo-carbapenemases. It augmented the activity of imipenem against P. aeruginosa in general and OprD mutants in particular.