Enhancing Antibacterial Efficacy: Combining Novel Bacterial Topoisomerase Inhibitors with Efflux Pump Inhibitors and Other Agents Against Gram-Negative Bacteria.

Background: The novel bacterial topoisomerase inhibitors (NBTIs) developed in our laboratory show potent on-target enzyme inhibition but suffer from low activity against Gram-negative bacteria.

Methods: With the aim of improving the antibacterial activity of our compounds against Gram-negative bacteria, we tested them in combination with different efflux pump inhibitors (EPIs), a strategy that showed promise in several other classes of antimicrobials. We also investigated the combined effect of NBTIs with ATP-competitive inhibitors of bacterial type II topoisomerases (ACIs), as well as the antibiofilm properties of our compounds and the combination with EPIs against early and mature biofilm.

Results: Our results demonstrate that combinations of NBTIs with EPI Phenylalanine-arginyl-β-naphthylamide significantly reduce the corresponding NBTIs' minimal inhibitory concentration values and show potentiation of biofilm inhibition as compared to NBTIs alone. Although combinations of NBITs and ACIs did not show synergistic effects, the FIC index value calculations revealed additive effects for all the combinations of a selected NBTI in combination with three ACIs in all the assayed Gram-negative bacteria from the ESKAPE group.

Conclusions: These results show for the first time that combinations of NBTIs with either EPIs or a different class of the topoisomerase inhibitors may be a beneficial strategy to combat difficult-to-treat bacterial infections.