Novel synergistic combinations of last-line antibiotics and FDA-approved drugs against Klebsiella pneumoniae revealed by in vitro synergy screenings

Treatment of infections caused by multi-drug resistant (MDR) enterobacteria remains challenging due to the limited therapeutic options. Drug repurposing could accelerate the development of urgently needed successful interventions. This work aimed to identify and characterize novel drug combinations against Klebsiella pneumoniae based on the concepts of synergy and drug repurposing. We performed a semi-qualitative high-throughput synergy screening (sHTSS) with tigecycline, colistin and fosfomycin (last-line antibiotics against MDR Enterobacteriaceae) combined with an FDA-library containing 1,430 clinically approved drugs. Selected hits were further validated by secondary checkerboard (CBA) and time-kill (TKA) assays. Our sHTSS results yielded 37, 31 and 41 hits showing synergy with tigecycline, colistin and fosfomycin, respectively. Most hits (75%) were known antibiotics. Non-antibiotic compounds included other anti-infective agents (7%), antineoplastics (7%) or antipsychotics (3%). Overall, 15.09% and 65.85% of hits were further confirmed by CBA and TKA, respectively, indicating that TKA is more useful than CBA for the validation of synergistic combinations. Accordingly, TKA were used for synergy classification based on determination of the bactericidal activities at 8, 24 and 48 hours. Twenty-seven combinations were validated with effective synergistic activity against K. pneumoniae by TKA, six of them novel non-antibiotic combinations. Based on our observations we conclude that repurposing approaches allowed to enhance the activity of last-line antibiotics in the treatment of MDR K. pneumoniae. sHTSS paired to TKA was a powerful tool for the identification of novel synergistic drug combinations against K. pneumoniae. Further pre-clinical studies might support the translational potential of these novel combinations.