Structural optimization of the 2-Phenyl-4-hydroxyquinoline Class of S. aureus NorA Efflux Pump Inhibitors

The worldwide increase of bacteria resistant to almost every common antibacterials is now considered an alarming health emergency (1). Among the strategies by which resistance can be acquired, overexpression of the efflux pumps leads to a sub-lethal antibacterial concentration at the target site that in turn may predispose to the development of high-level target-based resistance. The inhibition of such pumps is thus an important approach for improving the efficacy and/or extending the clinical utility of existing antibiotics, giving new life to old drugs with secure economic, social and health benefit. Of particular concern is the overwhelming rise of methicillin-resistant S. aureus (MRSA), which is highly virulent and contagious (2). MRSA accounted for 44% (n = 171200) of healthcare-associated infections and 22% (n = 5400) of attributable extra deaths in 2008 in EU. The increased expression of one or more MDR efflux pump genes was identified in 151 out of 309 S. aureus clinical strains (49%). Among those overexpressing a single gene, norA was most common (43%), followed by its strict homologue norB (23.2%) and mepA (9.9%)(3). In S. aureus, the NorA protein is able to extrude from the bacterial cell several antibacterials, including hydrophilic fluoroquinolones such as ciprofloxacin, as well as dyes. Taking into account the strong inhibitory activity on NorA efflux pump displayed from our previously reported 2-pheny-4-hydroxylquinoline derivative PQQ4O (4), in this work we have modified, with the help of a ligand-based pharmacophore model, the less explored C-4 position of the quinoline scaffold, with the aim to improve the inhibitory activity and to complete the information about the SAR of this new class of NorA efflux pump inhibitors. To this purpose, a combinatorial library (~50k compounds) was enumerated in silico using the CombiGlide program and virtually screened by the pharmacophore model, helping in the selection of the compounds to be synthesized. Activity data for the synthesized compounds, regarding the ethidium bromide (EtBr) efflux inhibition and the synergistic activity with ciprofloxacin against several S. aureus strains having different levels of NorA pump expression will be reported together with our most recent preliminary data showing that PQQ4O is a strong inhibitor of S. aureus biofilm formation. (1) Piddock, L.J. Lancet Infect. Dis. 2012, 12, 249-253; (2) Payne, D.J. Science, 2008, 321, 1644-1645; (3) Patel, D.; Kosmidis, C.; Seo, S.M.; Kaatz, G.W. Antimicrob. Agents Chemother. 2010, 54, 5070-5073; (4) Sabatini, S.; Gosetto, F.; Manfroni, G.; Tabarrini, O.; Kaatz, G.; Patel, D.; Cecchetti, V. J. Med. Chem. 2011, 54, 5722−5736.