Discovery of potent salicylanilide derivatives as effective antimicrobial agents against multidrug-resistant gram-positive pathogens.

The increasing prevalence of multidrug-resistant bacteria underscores the need for antibacterial agents with mechanisms distinct from those of conventional antibiotics. Inspired by the amphiphilic organization and membrane-active properties of host defense peptides (HDPs), we synthesized 22 salicylanilide derivatives and related analogues by systematically varying the ortho-phenolic hydroxyl group and substituents on the aromatic rings to modulate structural features associated with molecular polarity and hydrophobicity. Structure-activity relationship (SAR) analysis identified D2, bearing 3,5-dibromo and 2,5-bis(trifluoromethyl) substitutions, as the lead compound. D2 exhibited potent activity against Gram-positive pathogens, with MIC values of 0.2 - 12.5 μg/mL and MICs of 0.8 - 1.6 μg/mL against multiple MRSA strains. Mechanistic studies showed that D2 induced membrane depolarization and reduced intracellular ATP levels without detectable gross membrane permeabilization under the tested conditions, consistent with a membrane-associated antibacterial effect. No detectable increase in the MIC of D2 was observed over 21 serial passages, and D2 also showed substantial activity against established biofilms. In murine models of MRSA pneumonia and keratitis, D2 reduced bacterial burdens at the sites of infection and attenuated infection-associated pathological damage. Collectively, these findings identify D2 as a promising salicylanilide-based antibacterial lead and provide a structure-activity framework for further optimization of this chemotype against drug-resistant Gram-positive pathogens.