Discovery and Biophysical Characterization of 2‑Amino-oxadiazoles as Novel Antagonists of PqsR, an Important Regulator of Pseudomonas aeruginosa Virulence

The human pathogen Pseudomonas aeruginosa employs alkyl quinolones for cell-to-cell communication. The <i>Pseudomonas</i> quinolone signal (PQS) regulates various virulence factors via interaction with the transcriptional regulator PqsR. Therefore, we consider the development of PqsR antagonists a novel strategy to limit the pathogenicity of <i>P. aeruginosa</i>. A fragment identification approach using surface plasmon resonance screening led to the discovery of chemically diverse PqsR ligands. The optimization of the most promising hit (<b>5</b>) resulted in the oxadiazole-2-amine <b>37</b> showing pure antagonistic activity in Escherichia coli (EC₅₀ = 7.5 μM) and <i>P. aeruginosa</i> (EC₅₀ = 38.5 μM) reporter gene assays. <b>37</b> was able to diminish the production of the PQS precursor HHQ in a PqsH-deficient <i>P. aeruginosa</i> mutant. The level of the major virulence factor pyocyanin was significantly reduced in wild-type <i>P. aeruginosa</i>. In addition, site-directed mutagenesis in combination with isothermal titration calorimetry and NMR INPHARMA experiments revealed that the identified ligands bind to the same site of PqsR by adopting different binding modes. These findings will be utilized in a future fragment-growing approach aiming at novel therapeutic options for the treatment of <i>P. aeruginosa</i> infections