Screening of FDA-Approved Small Molecules to Discover Inhibitors of the <i>Pseudomonas aeruginosa</i> Quorum-Sensing Enzyme, PqsE

<i>Pseudomonas aeruginosa</i> is a notorious pathogen that is a leading cause of hospital-acquired infections, for which there are few treatment options. The quorum sensing (QS) pathway governs many pathogenic behaviors that allow for <i>P</i>. <i>aeruginosa</i> to stage infections. Within the QS pathway, there is a key protein-protein interaction between an enzyme, PqsE, and one of the master QS regulators, RhlR. Although its catalytic function is dispensable for its interaction with RhlR, previous mutagenic work characterizing the active site of PqsE identified active site mutations that induce a conformational change in PqsE, preventing it from forming a complex with RhlR. These active site mutations, when introduced stably into the genome of <i>P</i>. <i>aeruginosa</i>, also lead to a significant decrease in production of a key toxin, pyocyanin, and prevent colonization in the lungs of a murine host. Here, we performed a fluorescence polarization screen of an FDA-approved drug library to identify molecules that bind in the active site of PqsE. Three molecules were identified, two of which showed inhibitory activity consistent with a competitive mode of inhibition. One hit molecule, Apomorphine, had a distinctly different inhibitory profile and is potentially binding outside of the active site to allosterically inhibit enzyme activity of PqsE. All three hit molecules were tested in a cellular enzyme assay, and one of the competitive inhibitors, Vorinostat, was found to inhibit intracellular PqsE. Vorinostat is now being explored as a candidate for synthetic derivatization to inhibit the PqsE-RhlR protein-protein interaction via binding in the PqsE active site.