Development of a nebramine-cyclam conjugate as an antibacterial adjuvant to potentiate β-lactam antibiotics against multidrug-resistant P. aeruginosa

The beta-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 beta-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 beta-lactamase expression, decreased porin production and increased efflux. Current beta-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 beta-lactam antibiotics, as well as other legacy antibiotics, against P. aeruginosa in vitro. Combination studies show that this adjuvant is able to synergize with beta-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 mu M) of the nontoxic nebramine-cyclam conjugate is able to further potentiate existing beta-lactam/beta-lactamase inhibitor combinations in beta-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.