Pentamidine sensitizes Gram-negative pathogens to antibiotics and overcomes acquired colistin resistance

The increasing use of polymyxins(1) in addition to the dissemination of plasmid-borne colistin resistance threatens to cause a serious breach in our last line of defense against multidrug resistant Gram-negative pathogens, and heralds the emergence of truly pan-resistant infections. Colistin resistance often arises through covalent modification of lipid A with cationic residues such as phosphoethanolamine (PEtN) – as is mediated by Mcr-1(2) – which reduce the affinity of polymyxins for lipopolysaccharide (LPS)(3). Thus, new strategies are needed to address the rapidly diminishing number of treatment options for Gram-negative infections(4). The difficulty in eradicating Gram-negative bacteria is largely due to a highly impermeable outer membrane, which serves as a barrier to many otherwise effective antibiotics(5). Here, we describe an unconventional screening platform designed to enrich for non-lethal, outer membrane-active compounds with potential as adjuvants for conventional antibiotics. This approach identified the antiprotozoal drug pentamidine(6) as an effective perturbant of the Gram-negative outer membrane through its interaction with LPS. Pentamidine displayed synergy with antibiotics typically restricted to Gram-positive bacteria, yielding effective drug combinations with activity against a wide range of Gram-negative pathogens in vitro, and against systemic Acinetobacter baumannii infections in mice. Notably, the adjuvant activity of pentamidine persisted in polymyxin resistant bacteria in vitro and in vivo. Overall, pentamidine and structural analogs represent unexploited molecules for the treatment of Gram-negative infections, particularly those having acquired polymyxin resistance determinants.