A Novel Dibenzoxazepine Attenuates Intracellular Salmonella Typhimurium Oxidative Stress Resistance

Salmonella enterica serovar Typhimurium is the leading cause of invasive nontyphoidal salmonellosis. Additionally, the emergence of multidrug-resistant S. Typhimurium has further increased the difficulty of controlling its infection. Previously, we showed that an antipsychotic drug, loxapine, suppressed intracellular Salmonella in macrophages. To exploit loxapine's antibacterial activity, we simultaneously evaluated the anti-intracellular Salmonella activity and cytotoxicity of newly synthesized loxapine derivatives using an image-based high-content assay. We identified that SW14 exhibits potent suppressive effects on intramacrophagic S. Typhimurium with an 50% effective concentration (EC50) of 0.5 mM. SW14 also sensitized intracellular Salmonella to ciprofloxacin and cefixime and effectively controlled intracellular multidrug- and fluoroquinolone-resistant S. Typhimurium strains. However, SW14 did not affect bacterial growth in standard microbiological broth or minimal medium that mimics the phagosomal environment. Cellular autophagy blockade by 3-methyladenine (3-MA) or shATG7 elevated the susceptibility of intracellular Salmonella to SW14. Finally, reactive oxygen species (ROS) scavengers reduced the antibacterial efficacy of SW14, but the ROS levels in SW14-treated macrophages were not elevated. SW14 decreased the resistance of outer membrane-compromised S. Typhimurium to H2O2. Collectively, our data indicated that the structure of loxapine can be further optimized to develop new antibacterial agents by targeting bacterial resistance to host oxidative-stress defense.