Targeting Drug-Resistant Bacteria with Metal-Binding Compounds for Antibacterial Activity and Metallo-β-Lactamase Inhibition

Abstract

New strategies are urgently needed to overcome the growing threat of antibacterial drug resistance. One mechanism of drug resistance commonly employed by bacterial pathogens is expression of β-lactamases, which confer resistance through hydrolysis of β-lactam antibiotics, an important class of antibacterial compounds that target cell wall biosynthesis. We have developed multiple new strategies to take advantage of β-lactamase expression to deliver antibacterial metal chelators to drug-resistant bacteria. First, we describe the copper-dependent activity of an antibacterial prodrug named PcephPT, which is based on the structure of a cephalosporin, a type of β-lactam antibiotic. Using antibacterial susceptibility assays and analytical techniques, we demonstrate that PcephPT disrupts the homeostasis of copper, an essential but toxic metal, by releasing the antimicrobial chelator pyrithione (PT) in response to β-lactamase-mediated hydrolysis. We found that, in addition to inhibiting bacterial growth in a copper-dependent manner, PcephPT leads to inhibition of NDM-1, a β-lactamase that catalyzes hydrolysis using enzyme-bound zinc ions. Bacterial pathogens expressing metallo-β-lactamases such as NDM-1 are difficult to overcome with antibacterial treatment, and the success of PcephPT in inhibiting NDM-1 is a promising indication that chelator-releasing prodrugs (“prochelators”) are a useful strategy for metallo-β-lactamase inhibitor development. Enzymatic and spectroscopic assays reveal important information about the mechanism of PcephPT-mediated NDM-1 inhibition, which involves formation of a ternary complex with the zinc-containing active site and PT, one of the products of PcephPT hydrolysis. We next present progress on synthesizing and evaluating new prochelators that are designed to have increased specificity for drug-resistant strains. These compounds have core structures based on β-lactamase inhibitors, which are not inherently antibacterial, but may still release chelators for both antibacterial activity and metallo-β-lactamase inhibition. Finally, we discover a new class of metallo-β-lactamase inhibitors based on benzimidazole and benzoxazole zinc-chelating agents that form a ternary complex in the active site of NDM-1 and have the potential to be further optimized for inhibitory properties and targeting of drug-resistant bacteria.