An aryloxyphenol potentiates polymyxin against multidrug-resistant

Background: The increasing prevalence of antibiotic-resistant pathogens highlights the urgent need for new strategies to overcome antimicrobial resistance. Adjuvant compounds that potentiate antibiotic activity represent a promising alternative to the development of new antibiotics. Methods: Through high-throughput screening of a chemical library, an aryloxyphenol compound, PA74, was identified as a polymyxin adjuvant candidate. Checkerboard analyses, serial passage assays, cytotoxicity assays, and functional analyses using FabI-overexpressing and point-mutant strains were performed. In vivo efficacy was evaluated using a murine skin infection model. Results: PA74 potentiated polymyxin B against polymyxin-resistant Acinetobacter baumannii, Klebsiella pneumoniae, and Escherichia coli. Among six antibiotic classes tested, PA74 additionally enhanced the activity of tetracycline and chloramphenicol. Structurally, PA74 is a triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) analogue in which the 2,4-dichlorophenol moiety is replaced by 4-amino-3-chlorophenol. PA74 exhibited a markedly lower propensity for resistance development over 15 days compared with triclosan. PA74 also showed minimal cytotoxicity across mammalian cell lines. Increased FabI abundance attenuated the synergistic effect of the polymyxin B-PA74 combination, indicating that FabI contributes to PA74-mediated activity. Conclusions: PA74 co-treatment significantly improved therapeutic outcomes in a murine skin infection model, establishing PA74 as a promising adjuvant candidate for polymyxin B against drug-resistant Gram-negative pathogens.