Designer broad-spectrum polyimidazolium antibiotics

For a myriad of different reasons most antimicrobial peptides (AMPs) have failed to reach clinical application. Different AMPs have different shortcomings including but not limited to toxicity issues, potency, limited spectrum of activity, or reduced activity in situ. We synthesized several cationic peptide mimics, main-chain cationic polyimidazoliums (PIMs), and discovered that, although select PIMs show little acute mammalian cell toxicity, they are potent broad-spectrum antibiotics with activity against even pan-antibiotic-resistant gram-positive and gram-negative bacteria, and mycobacteria. We selected PIM1, a particularly potent PIM, for mechanistic studies. Our experiments indicate PIM1 binds bacterial cell membranes by hydrophobic and electrostatic interactions, enters cells, and ultimately kills bacteria. Unlike cationic AMPs, such as colistin (CST), PIM1 does not permeabilize cell membranes. We show that a membrane electric potential is required for PIM1 activity. In laboratory evolution experiments with the gram-positive Staphylococcus aureus we obtained PIM1-resistant isolates most of which had menaquinone mutations, and we found that a site-directed menaquinone mutation also conferred PIM1 resistance. In similar experiments with the gram-negative pathogen Pseudomonas aeruginosa, PIM1-resistant mutants did not emerge. Although PIM1 was efficacious as a topical agent, intraperitoneal administration of PIM1 in mice showed some toxicity. We synthesized a PIM1 derivative, PIM1D, which is less hydrophobic than PIM1. PIM1D did not show evidence of toxicity but retained antibacterial activity and showed efficacy in murine sepsis infections. Our evidence indicates the PIMs have potential as candidates for development of new drugs for treatment of pan-resistant bacterial infections.

AMPs and AMP mimics have attracted considerable attention as candidates for therapeutic development (1). The basic design elements include a region of charged residues, generally cationic residues, enabling interaction with bacterial cell surfaces, combined with a hydrophobic nature in AMPs (2). Unfortunately, AMPs and related polymers, in general, have one or more issues that limit their use as broad-spectrum antibiotics. Some are quite toxic to human cells, the potency of some is not adequate for human administration, others are sensitive to salt at levels present in human fluids, and some are too difficult and expensive to synthesize (3, 4). One broad-spectrum antimicrobial peptide, CST has seen increased recent use as a last resort antibiotic. CST is believed to kill bacteria by virtue of its ability to disrupt membrane integrity (5). This antibiotic requires intravenous administration and is nephrotoxic (6). The emergence of CST-resistant pathogens has also become a significant problem (7). We are unaware of any new broad-spectrum AMPs that have advanced to clinical trials.Imidazolium (IM) salts are antimicrobials (8), and there is an emerging literature on antimicrobial activity of side-chain and main-chain polyimidazolium (PIM) salts with chemical structures that are in some ways similar to those we describe. Although PIMs are potent antimicrobials, there are biocompatibility problems hindering their development, and some have somewhat limited activity spectra. As with other AMPs, there have been toxicity issues, potency issues, and delivery issues as many have large molecular masses, and there is little known about mammalian cell toxicity or mechanism of action (9–12).Here we show that members of a series of PIMs we designed and synthesized are potent broad-spectrum antibacterial compounds. We selected two for further analysis and showed they retain activity even against pan-antibiotic-resistant bacteria. Unlike CST and many other AMPs, which disrupt bacterial membranes, our model PIM is bactericidal without disrupting bacterial membranes. Our experiments provide insights about mechanism of action, the potential for the emergence of PIM resistance, and indicate PIMs are effective against a model gram-negative and a model gram-positive pathogen in murine infection models.