Discovery of quaternary ammonium salts fusing β-carboline skeleton as new anti-MRSA agents.

The excessive and indiscriminate use of antibiotics has led to a broad increase in bacterial resistance, highlighting the critical need for the rapid development of new antibacterial agents. Herein, a collection of quaternary ammonium salts based on β-carboline skeleton was designed and prepared. Analysis of structure-activity relationships (SARs) revealed that the length of the hydrophobic chain is a critical determinant for antibacterial efficacy against methicillin-resistant Staphylococcus aureus (MRSA). Among them, compound 6d emerged as the most promising lead candidate, exhibiting strong activity against MRSA with MIC ranging from 0.25 to 0.5 μg/mL across 11 clinical isolates, while maintaining low hemolytic activity and minimal cytotoxicity toward mammalian cells. Furthermore, compound 6d displayed rapid bactericidal action at a high concentration (1 μg/mL), low propensity for inducing resistance, and good plasma stability. Notably, compound 6d demonstrated superior in vivo anti-MRSA activity and low toxicity. Further multi-target mechanistic studies indicated that compound 6d suppressed biofilm formation, compromised the cell wall, and disrupted the cytoplasmic membrane, which were accompanied by membrane depolarization, enhanced permeability, and loss of membrane integrity. Additionally, compound 6d facilitated the accumulation of reactive oxygen species (ROS), reduced metabolic activity as well as bound to DNA. Overall, these findings provide key insights for the development of quaternary ammonium salt antibacterial fusing active natural skeleton.