The increasing incidence of bacterial resistance against antibacterial agents, which is a consequence of the wide use of antibiotics, is becoming a global health problem. For the control of bacterial infections discovery of antibacterial agents with novel mechanism of action and the development of therapeutic approaches to overcome resistance mechanisms is of great importance. Benzothiazole-based DNA gyrase inhibitors, which target the binding pocket for ATP located at the GyrB, have shown high potential in the search for new antibacterial agents. One of the possible approaches to surpass the resistance mechanisms with the origin in the cell wall is the use of siderophore-antibacterial agent conjugates. The principle comes from nature, where such conjugates are produced by some bacteria and are in competition for iron released into the environment, where they are taken up by other bacteria via specific ATP depended iron uptake system.
In order to search for new antibacterial compounds with novel mechanisms of action and approaches to overcome bacterial resistance, starting from already known benzothiazole DNA gyrase inhibitors, we designed and synthesized benzothiazole inhibitors with a modified scaffold and formed conjugates with siderophore mimetics.
Five final compounds were synthesized and tested for their enzymatic inhibitory and antibacterial activity. Compounds 22 and 32 displayed the best enzymatic activity of the tested compounds. The most potent antibacterial activity against Gram-positive bacteria displayed compound 22, however, compounds were devoid of activity against Gram-negative bacteria. We made the assumption that there was no need for cleavage of the siderophore mimetic, because in the case of compounds 7 and 13 the siderophore fragment points towards the solvent and in the case of compound 32 it is directed into the hydrophobic pocket of the binding site, where it forms interactions that favorably affect the binding affinity. According to the results of the biochemical assays, it is evident that the synthesized benzothiazoles inhibit DNA gyrase in the nanomolar range. In the antibacterial activity evaluation, no positive effect of the use of conjugates with siderophore mimetics was observed. Synthesized benzothiazole inhibitors with a hydroxy group on the scaffold are an excellent starting point for further design and optimization of DNA gyrase inhibitors.