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

ABSTRACT 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. IMPORTANCE Treating 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.