In vitro synergistic activity of minocycline and biapenem against cefiderocol-resistant New Delhi metallo-β-lactamase-producing Enterobacterales

OBJECTIVES: Cefiderocol is an important treatment option for infections caused by multidrug-resistant Gram-negative bacteria; however, resistance among New Delhi metallo-β-lactamase-producing Enterobacterales (NDME) is increasingly reported, leaving limited therapeutic alternatives. This study evaluated the in vitro activity of the minocycline-biapenem combination against cefiderocol-resistant NDME. METHODS: Twenty-three non-duplicate clinical NDME isolates with cefiderocol MIC ≥ 16 mg/L determined by reference broth microdilution in iron-depleted cation-adjusted Mueller-Hinton broth were analysed. Checkerboard assays were performed to assess combination activity using fractional inhibitory concentration indices (FICIs), sub-MIC interaction analysis and cumulative inhibition heatmaps. Time-kill assays were conducted using three representative isolates producing NDM-1 or NDM-5. RESULTS: Susceptibility rates for minocycline and biapenem tested alone were 52% and 13%, respectively. The minocycline-biapenem combination demonstrated synergistic activity (FICI ≤ 0.5) in 22 of 23 isolates (96%), with a median FICI of 0.063. Synergy was consistently observed across Escherichia coli, Enterobacter cloacae and Klebsiella pneumoniae. Sub-MIC analyses revealed asymmetric pharmacodynamic interactions, with sustained reductions in biapenem MIC at minocycline exposures as low as 1/512× baseline MIC. Cumulative inhibition heatmaps showed growth inhibition in 100% of isolates at the susceptible breakpoint concentrations of minocycline (4 mg/L) and biapenem (1 mg/L). Time-kill assays confirmed rapid and exposure-dependent bactericidal activity at clinically achievable concentration pairs. CONCLUSIONS: The minocycline-biapenem combination showed consistent in vitro synergistic and bactericidal activity against cefiderocol-resistant NDME across multiple species and NDM variants. These findings support further pharmacodynamic and translational evaluation of this combination as a potential therapeutic option for cefiderocol-refractory infections.