New 1,2,4-oxadiazole/pyrrolidine hybrids as topoisomerase IV and DNA gyrase inhibitors with promising antibacterial activity.

A series of hybridized pyrrolidine compounds with a 1,2,4-oxadiazole moiety were synthesized to develop effective molecules against the enzymes DNA gyrase and topoisomerase IV (Topo IV). Compounds 8-20 were developed based on a previously disclosed series of compounds from our lab, but with small structural modifications in the hopes of increasing the compounds' biological activity. In comparison to novobiocin, with IC50  = 170 nM, the findings of the DNA gyrase inhibitory assay revealed that compounds 16 and 17 were the most potent of all synthesized derivatives, with IC50 values of 180 and 210 nM, respectively. Compound 17 had the strongest inhibitory effect against Escherichia coli Topo IV of all the synthesized compounds, with an IC50 value of 13 µM, which was comparable to novobiocin (IC50  = 11 µM). Therefore, hybrids 16 and 17 appeared to be potential dual-target inhibitors. In the minimal inhibitory concentration (MIC) assays, compound 17 outperformed ciprofloxacin against E. coli, with an MIC of 55 ng/ml, compared to 60 ng/ml for ciprofloxacin. Finally, the docking study, along with the in vitro experiments, supports our promising approach to effectively develop potent leads for further optimization as dual DNA gyrase and Topo IV inhibitors.