Synthesis and Evaluation of 2-Aminothiophene Derivatives as Staphylococcus aureus Efflux Pump Inhibitors

2-aminothiophene derivatives (2AT) in which the thiophene ring is fused with a cycloalkyl or a N-acylated piperidine ring by positions 5 and 6 and carrying a 3-carbethoxy group were synthesized and their bacterial growth and enzyme inhibitory effects against efflux proteins of Staphylococcus aureus leading to resistance to fluoroquinolones and erythromycin (ERY) were investigated. Compounds that most effectively decreases the minimum inhibitory concentrations (MICs) of ciprofloxacin (CIP) were assayed for their dose and time effects on the accumulation and efflux of ethidium bromide (EtBr) in the SA-1 strain. None of the compounds displayed antibacterial activity however, three derivatives carrying 2-amino, 2-aminoacetyl and 2-aminotrifluoroacetyl group enhanced the activity of CIP and ERY by 8- and 16-fold, respectively, and were able to restore the sensitivity of resistant strains, acting as typical efflux pump inhibitors (EPIs). The 2-aminoacetyl and 2-aminotrifluoroacetyl derivatives and two other piperidinyl 2-aminotrifluoroacetyl derivatives increased EtBr accumulation in a dose- and time-dependent manner, and one of them was also able to inhibit the EtBr efflux. Taken together, these results represent an important advance in the development of new EPIs, and demonstrate that 2AT represent a good scaffold for developing new antibiotic adjuvants.