Design, synthesis, and evaluation of N¹,N³-dialkyldioxonaphthoimidazoliums as antibacterial agents against methicillin-resistant Staphylococcus aureus

Increasing antibiotic resistance of bacterial pathogens poses a serious threat to human health worldwide. Methicillin-resistant Staphylococcus aureus (MRSA) is among the most deleterious bacterial pathogens owing to its multidrug resistance, necessitating the development of new antibacterial agents against it. We previously identified a novel dioxonaphthoimidazolium agent, c5, with moderate antibacterial activity against MRSA from an anticancer clinical candidate, YM155. In this study, we aimed to design and synthesize several novel cationic amphiphilic N¹,N³-dialkyldioxonaphthoimidazolium bromides with enhanced lipophilicity of the two side chains in the imidazolium scaffold and improved antibacterial activities compared to those of c5 against gram-positive bacteria in vitro and in vivo. Our new antibacterial lead, N¹,N³-n-octylbenzyldioxonaphthoimidazolium bromide (11), exhibited highly potent antibacterial activities against various gram-positive bacterial strains (MICs: 0.19-0.39 μg/mL), including MRSA, methicillin-sensitive S. aureus, and Bacillus subtilis. Moreover, antibacterial mechanism of 11 against MRSA based on the generation of reactive oxygen species (ROS) was evaluated. Although compound 11 exhibited cytotoxic effects in vitro and lacked a therapeutic index against the HEK293 and HDFa mammalian cell lines, it exhibited low toxicity in the Drosophila animal model. Remarkably, 11 exhibited better in vivo antibacterial efficacy than c5 and the clinically used antibiotic, vancomycin, in SA3-infected Drosophila model. Moreover, the development of bacterial resistance to 11 was not observed after 16 consecutive passages. Therefore, rational design of antibacterial cationic amphiphiles based on ROS-generating pharmacophores with optimized lipophilicity can facilitate the identification of potent antibacterial agents against drug-resistant infections.