Exploration of the novel fluoroquinolones with high inhibitory effect against quinolone-resistant DNA gyrase of Salmonella Typhimurium.

Quinolone-resistant nontyphoidal Salmonella , one of the prominent pathogens causing acute gastroenteritis, has become a public health concern globally. The World Health Organization has ranked fluoroquinolone-resistant Salmonella as a high-priority pathogen for researching and developing new antibiotics. WQ-3034 and WQ-3154 are relatively new synthetic fluoroquinolones with distinctive structures. WQ-3034 has 6-amino-3,5-difluoropyridine-2-yl at R 1 , 3-hydroxyazetidinyl at R 7 , and the addition of chlorine atom at R 8 . WQ-3154 has a similar basic pharmacophore to WQ-3034 except for the modification at R 8 with a methyl group. In this study, the inhibitory effect and DNA cleavage effect against wild-type (WT) and mutant Salmonella Typhimurium DNA gyrases of WQ-3034 and WQ-3154 were examined along with WQ-3810 and ciprofloxacin by measuring the drug concentration that inhibits half of the enzyme activity (IC 50 ) and the drug concentration that induces 25% of maximum DNA cleavage (CC 25 ). The minimum inhibitory concentration (MIC) of the compounds was assessed against Salmonella Typhimurium and Salmonella Enteritidis. Among four compounds, WQ-3034 demonstrated the highest inhibitory effect against both WT and mutant Salmonella Typhimurium DNA gyrases with amino acid substitution at codon 83 and/or 87, while ciprofloxacin showed the lowest inhibitory effect. Remarkably, WQ-3034 and WQ-3154 exhibited a significantly higher inhibitory effect than ciprofloxacin against Salmonella Typhimurium DNA gyrase with double amino acid substitution, Ser83Phe-Asp87Asn. Similarly, CC 25 of WQ-3034 against mutant Salmonella Typhimurium DNA gyrase was lower than ciprofloxacin. Notably, MICs of WQ-3034 and WQ-3154 were higher than ciprofloxacin. In conclusion, this study revealed that WQ-3034 and WQ-3154 could potentially be effective therapeutic agents against quinolone-resistant nontyphoidal Salmonella . IMPORTANCE Quinolone-resistant nontyphoidal Salmonella is a pressing public health concern, demanding the exploration of novel treatments. In this study, we focused on two innovative synthetic fluoroquinolones, WQ-3034 and WQ-3154. Our findings revealed that these new compounds demonstrate potent inhibitory effects, even against mutant strains that cause resistance to existing quinolones. Hence, WQ-3034 and WQ-3154 could potentially be effective therapeutic agents against quinolone-resistant Salmonella Typhimurium. Furthermore, the data obtained in this study will be baseline information for antimicrobial drug development.