Resensitizing the Untreatable: Zidovudine and Polymyxin Combinations to Combat Pan-Drug-Resistant Klebsiella pneumoniae

<b>Background:</b> The emergence of pan-drug-resistant (PDR) <i>Klebsiella pneumoniae</i> has compromised the efficacy of last-line agents, leaving few therapeutic options. Repurposing zidovudine, an FDA-approved thymidine analog with antibacterial activity, may enhance existing therapies, but pharmacodynamic data under clinically relevant conditions are scarce. This study addresses this gap using static and dynamic in vitro models. <b>Materials/methods:</b> A PDR strain of <i>Klebsiella pneumoniae</i> harboring <i>bla</i>_NDM-1, <i>bla</i>_CMY-6, <i>bla</i>_CTX-M-15, <i>bla</i>_SHV-2, and disrupted <i>mgrB</i> was used in this study. Minimum inhibitory concentrations (MICs) followed by static time-kills were performed to investigate the synergistic interplay between zidovudine and last-line antibiotics (ceftazidime/avibactam, polymyxin B). To simulate human pharmacokinetics, a hollow-fiber infection model (HFIM) was employed using steady-state concentrations of zidovudine (4 mg/L), polymyxin B (4 mg/L), and avibactam (22 mg/L). Structural and morphological effects on bacterial cells were examined via fluorescence microscopy following glutaraldehyde fixation. <b>Results:</b> In this study, the PDR <i>K. pneumoniae</i> showed a ~5-fold reduction in polymyxin MIC when combined with zidovudine (from >4 µg/mL to 0.25 µg/mL). Time-kill assays demonstrated ≥2.5 log₁₀ CFU/mL bacterial reduction with zidovudine-based combinations, whereas monotherapies failed to inhibit bacterial growth. In the HFIM, the triple combination achieved rapid bactericidal activity (>3 log₁₀ CFU/mL reduction within 4 h) and sustained killing (>5-6 log₁₀ reduction maintained through 216 h), with bacterial counts remaining below 1 CFU/mL. In contrast, dual combinations initially reduced bacterial burden (1-3 log₁₀ reduction) but failed to maintain suppression, with significant regrowth (>10¹⁰ CFU/mL) observed by 168 h. Microscopy corroborated these findings, revealing extensive cellular damage in the zidovudine-containing treatment arms. These HFIM results underscore the potential of zidovudine-based triple therapy in overcoming resistance to last-line antibiotics in <i>K. pneumoniae</i>. <b>Conclusions:</b> Our results provide promising unprecedented insight into novel zidovudine-based combination therapies against difficult-to-treat MBL Gram-negatives. The observed synergy in MIC reduction, rapid killing in time-kill assays, and near-complete eradication in the HFIM underscore the therapeutic potential of this triple combination. Future studies will focus on broadening the application of these novel combinations to other 'superbugs', such as highly resistant strains of <i>Acinetobacter baumannii</i> and <i>Pseudomonas aeruginosa</i>.