Biapenem-fosfomycin shows enhanced in vitro activity compared with meropenem-fosfomycin against NDM-producing Enterobacterales.

Treatment options for New Delhi metallo-β-lactamase-producing Enterobacterales (NDME) remain limited, and the clinical relevance of fosfomycin-carbapenem combinations has not been fully established. We evaluated whether biapenem-fosfomycin provides greater in vitro activity than meropenem-fosfomycin. A total of 54 clinical NDME isolates (Escherichia coli, n = 22; Klebsiella pneumoniae, n = 21; Enterobacter cloacae complex, n = 11) collected in Bangladesh and Japan were analyzed. Antimicrobial susceptibility testing, checkerboard assays, checkerboard-derived cumulative inhibition analyses, time-kill assays, and assessment of fosfomycin-resistant subpopulations were performed. Biapenem exhibited lower MICs than meropenem across all species. Biapenem-fosfomycin demonstrated higher rates of synergy than meropenem-fosfomycin, particularly in E. coli (73% vs 45%). At concentration pairs corresponding to median minimum Fractional Inhibitory Concentration Index (mFICI) values, a greater proportion of isolates was inhibited at low biapenem concentrations in the presence of fosfomycin, whereas this effect was more limited with meropenem. At a fixed carbapenem concentration of 1 μg/mL, ≥90% inhibition was achieved at lower fosfomycin concentrations with biapenem-fosfomycin than with meropenem-fosfomycin. Time-kill analyses demonstrated that mFICI-based conditions resulted in regrowth, whereas MIC/MIC combinations of biapenem-fosfomycin maintained bacterial suppression over 24 h. Fosfomycin alone was associated with frequent recovery of fosfomycin-resistant subpopulations, whereas biapenem-fosfomycin reduced their recovery under the tested conditions. Biapenem-fosfomycin demonstrated favorable in vitro activity compared with meropenem-fosfomycin against NDME. These findings indicate that checkerboard-defined synergy should be interpreted together with cumulative inhibition profiles, killing kinetics, and resistant-subpopulation data.IMPORTANCETreating infections caused by New Delhi metallo-β-lactamase (NDM)-producing Enterobacterales remains a major global challenge because therapeutic options are limited. Fosfomycin-carbapenem combinations are frequently assessed using checkerboard-based Fractional Inhibitory Concentration Index (FICI), but this screening approach does not fully describe time-dependent killing, regrowth, or the recovery of resistant subpopulations. In this study, we compared biapenem-fosfomycin with meropenem-fosfomycin using checkerboard assays and checkerboard-derived cumulative inhibition analysis and further characterized biapenem-fosfomycin using time-kill assays and fosfomycin-resistant subpopulation assessment. Biapenem-fosfomycin showed more favorable in vitro activity than meropenem-fosfomycin under the tested conditions. These findings support further evaluation of biapenem-fosfomycin using dynamic pharmacokinetic models that better reproduce human drug exposure profiles.