Synthesis and antimicrobial activity of sulfonyl-imidazole linked fused isoxazolo[3,4-b][1,2,3]triazolo[4,5-d]pyridines:PEG-400 mediated one-pot reaction under ultrasonic irradiation

Introduction The rapid emergence of methicillin-resistant and vancomycin-resistant Staphylococcus aureus (MRSA and VRSA) represents a major global health challenge, necessitating the development of new antibacterial scaffolds with improved efficacy and safety. Methods In this study, a novel series of sulfonyl-imidazole-linked fused isoxazolo[3,4-b][1,2,3]triazolo[4,5-d]pyridine derivatives (6a–6o) was synthesized using a PEG-400-mediated, ultrasound-assisted one-pot Cu(I)-catalysed strategy under environmentally benign conditions. The synthesized compounds were evaluated for antibacterial activity against MSSA, MRSA, and VRSA strains, along with antibiofilm activity, hemolytic potential using mouse erythrocytes, and cytotoxicity in RAW 264.7, THP-1, and BoMac cells. Immunomodulatory effects were assessed through cytokine induction studies. Results Several derivatives exhibited potent antibacterial activity, with compound 6k emerging as the most active candidate, displaying MIC values of 1.56–3.12 μg/mL and outperforming the reference drug dicloxacillin. Selected compounds also showed significant antibiofilm activity against resistant S. aureus strains. Hemolysis and cytotoxicity assays demonstrated minimal toxicity, indicating good biocompatibility. Immunomodulatory analysis revealed moderate cytokine induction, suggesting a controlled immune response. Discussion Overall, compound 6k was identified as a promising lead with potent antibacterial, antibiofilm, and immunomodulatory properties, combined with a favourable safety profile, warranting further preclinical development against drug-resistant S. aureus infections.