Design, Semi-Synthesis, and Antibacterial Activities of Triterpenoid-Hydrazone Conjugates

Objective: To develop novel ocotillol-derived triterpenoid antibacterial agents by introducing hydrazone, acylhydrazide, and amino groups at the C-3 position, aiming to enhance their antibacterial potency, water solubility, and broaden their spectrum of activity, particularly against drug-resistant bacteria. Methods: Compounds (VIIIa–VIIIm) were synthesized via a multistep procedure using PPD as the starting material. The synthetic route included hydroxyl group protection, epoxidation, oxidation, and condensation with acylhydrazides. Antibacterial activity was evaluated using the broth microdilution method in LB medium against both drug-sensitive and drug-resistant bacterial strains. Cytotoxicity was assessed using the MTT assay on HEK-293, HK-2, and MCF-7 cell lines. Additional experiments included resistance development assays, bactericidal time-kill kinetics, biofilm disruption evaluation, and in silico prediction of pharmacokinetic properties. Results and Discussion: In this study, we synthesized a diverse array of hydrazone derivatives derived from ocotillol to evaluate their efficacy as antibacterial agents. The results showed that compound (VIIIm) exhibited exceptional potency, with minimum inhibitory concentrations (MICs) as low as 2–4 μg/mL against S. aureus and B. subtilis, and 8 μg/mL against E. coli. When tested against MRSA, compound (VIIIm) also demonstrated strong antibacterial activity, achieving an MIC of 8 μg/mL. Moreover, these hydrazone derivatives significantly enhanced the efficacy of kanamycin and chloramphenicol against MRSA and E. coli, as evidenced by fractional inhibitory concentration index (FICI) values below 0.5. Compound (VIIIm) was particularly notable for its ability to disrupt established bacterial biofilms and to delay the development of bacterial resistance compared to conventional antibiotics such as norfloxacin and colistin. Time-kill kinetics analysis provided compelling evidence that compound (VIIIm), at a concentration of 3 × MIC, exerted bactericidal activity against MRSA and E. coli. Furthermore, drug-likeness and ADME (absorption, distribution, metabolism, and excretion) predictions for the synthesized derivatives indicated overall compliance with Lipinski’s rule of five—with the exception of molecular weight—and suggested moderate oral bioavailability and intestinal absorption in humans. Mechanistic studies revealed that compound (VIIIm) exerted its antibacterial effect by disrupting the structural integrity of bacterial cell membranes, leading to leakage of intracellular components and subsequent cell death. Conclusions: In vivo studies supported the in vitro findings, demonstrating that compound (VIIIm) exhibited a moderate level of antibacterial efficacy in S. aureus infection models when administered at a dose of 80 mg/kg.