Novel Bacterial Topoisomerase Inhibitors with Potent Broad-Spectrum Activity against Drug-Resistant Bacteria

The novel bacterial topoisomerase inhibitor class is an investigational type of antibacterial inhibitor of DNA gyrase and topoisomerase IV that does not have cross-resistance with the quinolones. Here, we report the evaluation of the <i>in vitro</i> properties of a new series of this type of small molecule. Exemplar compounds selectively and potently inhibited the catalytic activities of <i>Escherichia coli</i> DNA gyrase and topoisomerase IV but did not block the DNA breakage-reunion step. Compounds showed broad-spectrum inhibitory activity against a wide range of Gram-positive and Gram-negative pathogens, including biodefence microorganisms and <i>Mycobacterium tuberculosis</i> No cross-resistance with fluoroquinolone-resistant <i>Staphylococcus aureus</i> and <i>E. coli</i> isolates was observed. Measured MIC₉₀ values were 4 and 8 μg/ml against a panel of contemporary multidrug-resistant isolates of <i>Acinetobacter baumannii</i> and <i>E. coli</i>, respectively. In addition, representative compounds exhibited greater antibacterial potency than the quinolones against obligate anaerobic species. Spontaneous mutation rates were low, with frequencies of resistance typically <10^-8 against <i>E. coli</i> and <i>A. baumannii</i> at concentrations equivalent to 4-fold the MIC. Compound-resistant <i>E. coli</i> mutants that were isolated following serial passage were characterized by whole-genome sequencing and carried a single Arg38Leu amino acid substitution in the GyrA subunit of DNA gyrase. Preliminary <i>in vitro</i> safety data indicate that the series shows a promising therapeutic index and potential for low human ether-a-go-go-related gene (hERG) inhibition (50% inhibitory concentration [IC₅₀], >100 μM). In summary, the compounds' distinct mechanism of action relative to the fluoroquinolones, whole-cell potency, low potential for resistance development, and favorable <i>in vitro</i> safety profile warrant their continued investigation as potential broad-spectrum antibacterial agents.