The β-lactams are the most widely used class of antibiotics due to their safety, effectiveness, and spectrum of activity. As a result of their ubiquitous usage, there has been a steady rise in β-lactam resistant Gram-negative bacteria, especially Pseudomonas aeruginosa, resulting in limited treatment options. P. aeruginosa can develop multidrug-resistant phenotypes using a multifaceted approach of β-lactamase expression, decreased porin production and increased efflux. Current β-lactamase inhibitors address drug hydrolyzing enzymes but may not be as effective in phenotypes with reduced permeability and/or overexpressed efflux pumps. Herein, we present the synthesis and biological evaluation of a nebramine-cyclam conjugate molecule that is able to potentiate β-lactam antibiotics, as well as other legacy antibiotics, against P. aeruginosa in vitro. Combination studies show that this adjuvant is able to synergize with β-lactams such as aztreonam and ceftazidime against multidrug-resistant and extremely drug-resistant clinical isolates through a hypothesized mechanism of outer membrane permeabilization. Importantly, the addition of low concentrations (8 µM) of the nontoxic nebramine-cyclam conjugate is able to further potentiate existing β-lactam/β-lactamase inhibitor combinations in β-lactamase-harboring P. aeruginosa strains. These data support a potential application of the nebramine-cyclam conjugate as an adjuvant for treating infections caused by P. aeruginosa strains that utilize multiple mechanisms of resistance.
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The authors would like to thank the University of Manitoba and the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support in the form of a discovery grant (2018-06047). We would also like to thank Prof. Ayush Kumar (University of Manitoba) for providing us with the efflux deficient strains.
Conflict of interest
The authors declare that they have no conflict of interest.
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