Novel Bacterial Topoisomerase Inhibitors: A New Front in an Old War

Antibacterial agents have transformed modern medicine. However, the rise of antimicrobial resistance threatens these advances, creating an urgent need for new antibacterial classes with novel mechanisms of action to be developed. The bacterial type II topoisomerases, gyrase and topoisomerase IV, are validated targets for antibacterial drug discovery. Fluoroquinolones have historically exploited these targets with great clinical success despite their growing resistance. This review examines gyrase and topoisomerase IV as antibacterial targets and the emergence and evolution of fluoroquinolone resistance. It will focus on the development of the Novel Bacterial Topoisomerase Inhibitors (NBTIs), a mechanistically distinct class of agents designed to overcome the limitations of fluoroquinolones. We cover the development of NBTI scaffolds, highlighting key structural features (DNA-binding motif, central linker, secondary amine, and enzyme-binding motif) that govern potency, selectivity, and safety. Particular emphasis is placed on optimization strategies used to balance antibacterial efficacy with cardiovascular safety by mitigating hERG potassium channel inhibition, a common roadblock in the clinical pursuit of NBTI development. Advances from both industrial and academic discovery programs illustrate how subtle modulation of basicity, polarity, and conformational rigidity has enabled improved pharmacological profiles. The clinical translation of this class is exemplified by gepotidacin, the first-in-class triazaacenaphthylene approved in March 2025, which demonstrates balanced dual-targeting of gyrase and topoisomerase IV and maintains activity against fluoroquinolone-resistant pathogens. Ongoing clinical development of next-generation NBTIs, including BWC0977, underscores the continued promise of this class. Collectively, these efforts highlight NBTIs as a critical and expanding weapon in the fight against antimicrobial resistance.