Structure-activity relationships and optimization of acyclic acylphloroglucinol analogues as novel antimicrobial agents.

Methicillin-resistant Staphylococcus aureus (MRSA) poses a serious threat to global public health, because it exhibits resistance to existing antibiotics and therefore high rates of morbidity and mortality. In this study, twenty-one natural product-based acylphloroglucinol congeners were synthesized, which possessed different side chains. Antibacterial screening against MRSA strains revealed that acyl moiety tailoring is a prerequisite for the antibacterial activity. Moreover, the lipophilicity, rather than the magnitude of the hydrophobic acyl tail dominates variability in activity potency. Compound 11j was identified as a promising lead for the generation of new anti-MRSA drug development. It was discovered by optimization of the side chain length in light of the potency, the breadth of the antibacterial spectrum, the rate of bactericidal action, as well as the membrane selectivity. Compound 11j exerted profound in vitro antibacterial activity against the MRSA strain (JCSC 2172), and its MIC was 3-4 orders of magnitude lower than that of vancomycin. A preliminary mode of action study of compound 11j at the biophysical and morphology levels disclosed that the mechanism underlying its anti-MRSA activity included membrane depolarization and, to a lesser extent, membrane disruption and cell lysis.