Synthesis of omeprazole analogues and evaluation of these as potential inhibitors of the multidrug efflux pump NorA of Staphylococcus aureus

A series of 11 pyrrolo[1,2-a]quinoxaline derivatives, la to 1k, sharing structural analogies with omeprazole, a eukaryotic efflux pump inhibitor (EPI) used as an antiulcer agent, was synthesized. Their inhibitory effect was evaluated using Staphylococcus aureus strain SA-1199B overexpressing NorA. By determinations of the MIC of norfloxacin in the presence of these EPIs devoid of intrinsic antibacterial activity and used at 128 mu g/ml, and by the checkerboard method, compound le (MIC decrease, 16-fold; fractional inhibitory concentration index [Sigma FIC], 0.18) appeared to be more active than compounds 1b to Id, reserpine, and omeprazole (MIC decrease, eightfold; Sigma FIC, 031), followed by compounds la and If (MIC decrease, fourfold; Sigma FIC, 0.37) and 1g to 1k (MIC decrease, twofold; Sigma FIC, 0.50 to 0.56). By time-kill curves combining norfloxacin (1/4 MIC) and the most efficient EPIs (128 mu g/ml), compound le persistently restored the bactericidal activity of norfloxacin (inoculum reduction, 3 log(10) CFU/ml at 8 and 24 h), compound If led to a delayed but progressive decrease in the number of viable cells, and compounds 1b to 1d and omeprazole acted synergistically (inoculum reduction, 3 log,0 CFU/ml at 8 h but further regrowth), while compound la and reserpine slightly enhanced norfloxacin activity. The bacterial uptake of norfloxacin monitored by high-performance liquid chromatography confirmed that compounds la to If increased antibiotic accumulation, as did reserpine and omeprazole. Since these EPIs did not disturb the Delta psi and Delta pH, they might directly interact with the pump. A structure-activity relationships study identified the benzimidazole nucleus of omeprazole as the main structural element involved in efflux pump inhibition and highlighted the critical role of the chlorine substituents in the stability and efficiency of compounds le to If. However, further pharmacomodulation is required to obtain therapeutically applicable derivatives.