STRUCTURAL INVESTIGATIONS OF THE 2-PHENYL-4- HYDROXYQUINOLINE CLASS OF S. AUREUS NORA EPIS

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 blockage of the activity of such pumps through the use of efflux pump inhibitors (EPIs) might thus be a powerful 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. MRSA accounted for 44% of healthcare-associated infections and 22% 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%).(2) In S. aureus, the NorA protein is able to extrude from the bacterial cell several antibacterials, including hydrophilic fluoroquinolones such as ciprofloxacin, and dyes. Taking into account the strong inhibitory activity on NorA efflux pump displayed from our previously reported 2-pheny-4- hydroxylquinoline derivatives,(3,4) in this work we have explored the effects of the introduction of a single methoxy group, a substituent frequently recurrent in both natural and synthetic NorA EPIs, at different positions of the quinoline core, maintaining at C-4 position the O-alkylamino chains that previously showed the best EPI activities (Fig. 1). The main aim was to obtain new and potent small molecules capable of restoring CPX activity on S. aureus resistant strains through the inhibition of NorA efflux pump and to get data for the refinement of our pharmacophore model,(4) that highlights the structural requirements necessary for the inhibition activity of NorA EPIs. Activity data of 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.