Introduction: Antimicrobial resistance (AMR) is a rapidly evolving threat to human health, predicted to cause 10 million deaths annually by 2050 [1]. Novel therapeutic strategies are urgently needed. The polymyxin lipopeptide antibiotics are membrane-targeting agents used clinically as last resort antibiotics against multi-drug-resistant Gram-negative pathogens. However, resistance to polymyxins is rapidly growing. A closely related novel class of lipopeptides, the octapeptins (OPX) [2-4], are active against polymyxin-resistant strains.
We performed a screening approach to evaluate whether different types of octapeptins can act synergistically when combined with currently approved antibiotics. A range of antibiotics were tested, with rifampicin selected as a preferred partner.
Methods: Cyclic octapeptin lipopeptides were synthesised by solid phase peptide synthesis, using orthogonal protection and off-resin cyclisation for the tail-to-side chain linkage. Synergy was evaluated through the calculation of fractional inhibitory concentration index using minimum inhibitory concentration of the compounds alone and in combination. FICI values ≤0.5 were interpreted as synergistic. Multiple strain of susceptible species were tested. Infection efficacy models were conducted in BALB/c mice inoculated in the thigh with MDR Acinetobacter baumannii (CDC AR Bank 0313) or the lung with MDR Klebsiella pneumoniae (CDC AR Bank 0098). Vehicle and/or test substances were administered 2 and 14 hours later. At 26 hours after inoculation, animals were euthanized, thigh or lung tissue was harvested, homogenized and plated onto NB agar for colony counts.
Results: The ansamycin antibiotic rifampin, used to treat tuberculosis, consistently showed in vitro synergy with OPX against multiple strains of A. baumannii and K. pneumoniae. The combination showed efficacy in both thigh and lung infection murine models, with more than 2 log reduction in bacterial count when compared with the 2h timepoint vehicle control, and over 5 log reduction compared to the 24h vehicle control.
Conclusion: Improving the spectrum of activity of already-approved drugs offers a practical strategy to overcome antimicrobial resistance. By systematically exploring combinations of repurposed drugs with modified octapeptins, we have uncovered potential new treatments to combat highly resistant Gram-negative infections.