Outer Membrane Interaction Kinetics of New Polymyxin B Analogs in Gram-Negative Bacilli

Infections caused by drug-resistant Gram-negative bacilli are a severe global health threat, limiting effective drug choices for treatment. In this study, polymyxin analogs designed to have reduced nephrotoxicity, direct activity, and potentiating activity were assessed for inhibition and outer membrane interaction kinetics against wild-type (WT) and polymyxin or multidrug-resistant (MDR) Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. In MIC assays, two polymyxin B (PMB) analogs (SPR1205 and SPR206) and a polymyxin E analog (SPR946), with shortened peptide side chains and branched aminobutyryl N termini, exhibited promising activity compared with PMB and previously tested control polymyxin analogs SPR741 and polymyxin B nonapeptide (PMBN). Using dansyl-polymyxin (DPX) binding to assess the affinity of interaction with lipopolysaccharide (LPS), purified or in the context of intact cells, SPR206 exhibited similar affinities to PMB but higher affinities than the other SPR analogs. Outer membrane permeabilization measured by the 1-N-phenyl-napthylamine (NPN) assay did not differ significantly between the polymyxin analogs. Moreover, Hill numbers were greater than 1 for most of the compounds tested on E. coli and P. aeruginosa strains which indicates that the disruption of the outer membrane by one molecule of compound cooperatively enhances the subsequent interactions of other molecules against WT and MDR strains. The high activity demonstrated by SPR206 as well as its ability to displace LPS and permeabilize the outer membrane of multiple strains of Gram-negative bacilli while showing cooperative potential with other membrane disrupting compounds supports further research with this polymyxin analog.