A series of 11 pyrrolo[1,2-a]quinoxaline derivatives, 1a 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 g/ml, and by the checkerboard method, compound 1e (MIC decrease, 16-fold; fractional inhibitory concentration index [⌺FIC], 0.18) appeared to be more active than compounds 1b to 1d, reserpine, and omeprazole (MIC decrease, eightfold; ⌺FIC, 0.31), followed by compounds 1a and 1f (MIC decrease, fourfold; ⌺FIC, 0.37) and 1g to 1k (MIC decrease, twofold; ⌺FIC, 0.50 to 0.56). By time-kill curves combining norfloxacin (1/4 MIC) and the most efficient EPIs (128 g/ml), compound 1e persistently restored the bactericidal activity of norfloxacin (inoculum reduction, 3 log 10 CFU/ml at 8 and 24 h), compound 1f 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 10 CFU/ml at 8 h but further regrowth), while compound 1a and reserpine slightly enhanced norfloxacin activity. The bacterial uptake of norfloxacin monitored by high-performance liquid chromatography confirmed that compounds 1a to 1f increased antibiotic accumulation, as did reserpine and omeprazole. Since these EPIs did not disturb the ⌬ and ⌬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 1e to 1f. However, further pharmacomodulation is required to obtain therapeutically applicable derivatives.
