Synergy Potential of Ursolic Acid-Based Hybrid Molecules

Background: Ursolic acid (UA, 3β-hydroxy-urs-12-en-28-oic acid), a pentacyclic triterpenoid from various medicinal plants, has been blessed with proven biological effects, including antiinflammatory, anticancer, antidiabetic, antioxidant and antibacterial, but its bioavailability and solubility limit its clinical application. Objective: Synthesis of UA-based hybrid molecules to explore their antibacterial and synergy potential in combination with azithromycin (AZT) for the treatment of multidrug-resistant (MDR) bacterial infections. Methods: Hybrid molecules of UA with menthol, eugenol, and nalidixic acid (NAL) along with some other ester derivatives were synthesized, and evaluated for their antibacterial and synergy potential in combination with AZT against the clinical isolate of Escherichia coli in terms of their minimum inhibitory concentration (MIC), fold reduction in MIC, fractional inhibitory concentration index (FICI) and type of interaction. In silico screening of pharmacokinetic parameters, docking affinity against efflux pump proteins AcrA, AcrB, and TolC was performed on the most potent derivative 7 (3-O-nalidixoyl UA). Results: Derivative 7 showed MIC of 62.5 μg/mL and a strong synergistic effect with AZT reducing the MIC of AZT from 100 to 0.19 μg/mL (512-fold reduction) against E. coli at a concentration of 12.5 μg/mL. Other derivatives neither showed antibacterial activity of their own (MIC > 1000 μg/mL) nor any significant synergistic interaction in combination with AZT. The in silico studies on 7 revealed improved druggability parameters over the parent UA and NAL. Conclusion: The findings highlight derivative 7 as strong synergistic agent in combination with AZT which may be further investigated to render its efficient use for the treatment of MDR bacterial infections.