Design, Synthesis, and Mechanistic Pharmacological Evaluation of Novel Quinoline–1,3,4-Oxadiazole Hybrid Molecules as Therapeutic Agents Against Multidrug-Resistant Infections

Background The rapid emergence of multidrug-resistant (MDR) bacterial strains poses a critical global health challenge, significantly reducing the efficacy of existing antimicrobial agents such as fluoroquinolones and βlactams. Quinoline derivatives are well-established antimicrobial pharmacophores, while 1,3,4-oxadiazole scaffolds are known to enhance biological activity and pharmacokinetic properties. Hybridization of these two moieties offers a promising strategy to develop potent agents against resistant pathogens. Objective To design, synthesize, and evaluate novel quinoline–1,3,4-oxadiazole hybrid molecules for their antimicrobial activity against MDR bacterial strains and to investigate their possible mechanisms of action. Methods A series of ten quinoline–oxadiazole hybrid compounds (QO-1 to QO-10) were synthesized via cyclization of quinoline-based hydrazide intermediates under reflux conditions. The structures were confirmed using IR, ¹H NMR, ¹³C NMR, and mass spectrometry. Antimicrobial activity was assessed against MDR strains of Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa using broth microdilution method to determine minimum inhibitory concentration (MIC). Ciprofloxacin was used as a standard reference drug. Mechanistic studies included membrane permeability assay, DNA binding analysis (UV-visible spectroscopy), and DNA gyrase inhibition assay. Cytotoxicity was evaluated using MTT assay on HEK-293 cell lines. Results All synthesized compounds exhibited moderate to significant antimicrobial activity. Among them, compounds QO-4, QO-7, and QO-9 demonstrated the most potent effects. MIC values ranged from 1.56 to 25 µg/mL against tested strains. QO-7 showed highest activity with MIC: S. aureus: 1.56 µg/mL, E. coli: 3.12 µg/mL, P. aeruginosa: 6.25 µg/mL, showing comparable activity to ciprofloxacin (MIC: 0.78–1.56 µg/mL). Mechanistic evaluation revealed increased membrane permeability (up to 65% leakage of intracellular material), significant DNA binding affinity (hypochromic shift ~18%), and DNA gyrase inhibition (IC₅₀ = 2.8 µM for QO7). Cytotoxicity studies indicated low toxicity, with cell viability >85% at 50 µg/mL, suggesting a favorable safety profile.Conclusion The synthesized quinoline–1,3,4-oxadiazole hybrids exhibited promising antimicrobial activity against MDR pathogens, with QO-7 emerging as the most potent candidate. The mechanism of action is likely mediated through bacterial membrane disruption and DNA gyrase inhibition. These findings highlight the potential of these hybrid molecules as lead compounds for the development of new antimicrobial agents.