The Direct and Potentiating Antimicrobial Activities of Zafirlukast

Background: Antimicrobial resistance is at alarming levels. Methicillin-resistant Staphylococcus aureus is a clinical concern and has been classified as one of the so-called ESKAPE pathogens. The increase in resistance coupled with the lack of new agents in development requires innovative ways of developing new antimicrobials. Methods: Here, we investigate an approved drug (Zafirlukast), which has been previously reported to have activity against Mycobacterium, for activities against other bacterial strains. Antibacterial activity was assessed using standard MIC testing along with checkerboard analysis to investigate potential synergies/antagonisms. In order to define the mechanism of action (MOA), a number of Zafirlukast-resistant mutant strains of MRSA (generated via serial passage) were sequenced. Biochemical analysis was performed on the target pathway to confirm the putative target of Zafirlukast in Staphylococcus aureus. Results: Zafirlukast showed modest direct-acting antimicrobial activity (0.5–32 μg/mL) against a panel of Gram-positive bacteria, including MRSA. It was also found that Zafirlukast synergised with a number of existing antibiotics. Remarkably, mutants forced to be resistant to Zafirlukast became resensitised to the β-lactams that they were originally resistant to. This is potentially a very useful property of a potentiator drug. Mutational analysis suggested the target of Zafirlukast in Staphylococci is LtaS (Lipoteichoic acid synthase), of which there are no currently approved inhibitors. Teichoic acid pathway involvement was further confirmed via biochemical analysis. Conclusions: Here, we establish that Zafirlukast has activities beyond Mycobacterium and Gingivalis, encompassing many Gram-positive bacteria including MRSA, and, for the first time, we report its putative target as LtaS. The repurposing of safe-to-use drugs is an attractive strategy to derisk drug discovery programmes. Zafirlukast has many attractive antimicrobial activities, which may encourage further investigations in this field. However, it should be noted that it may represent a safe starting scaffold, and further chemistry is likely to be needed for it to be used in the antimicrobial setting.