1-(1H-Indol-3-yl)ethanamine Derivatives as Potent Staphylococcus aureus NorA Efflux Pump Inhibitors

The synthesis of 37 1-(1H-indol-3-yl)ethanamine derivatives, including 12 new compounds, was achieved through a series of simple and efficient chemical modifications. These indole derivatives displayed modest or no intrinsic anti-staphylococcal activity. By contrast, several of the compounds restored, in a concentration-dependent manner, the antibacterial activity of ciprofloxacin against Staphylococcus aureus strains that were resistant to fluoroquinolones due to overexpression of the NorA efflux pump. Structure-activity relationships studies revealed that the indolic aldonitrones halogenated at position 5 of the indole core were the most efficient inhibitors of the S. aureus NorA efflux pump. Among the compounds, (Z)-N-benzylidene2-(tert-butoxycarbonylamino)-1-(5-iodo-1H-indol-3-yl) ethanamine oxide led to a fourfold decrease of the ciprofloxacin minimum inhibitory concentration against the SA-1199B strain when used at a concentration of 0.5 mg L-1. To the best of our knowledge, this activity is the highest reported to date for an indolic NorA inhibitor. In addition, a new antibacterial compound, tert-butyl (2-(3-hydroxyureido)-2-(1H-indol-3-yl)ethyl)carbamate, which is not toxic for human cells, was also found.