N,N'‑Disubstituted Tetrandrine Derivatives: Enhanced Antibacterial Activity via Double Quaternization

A series of N,N'-disubstituted tetrandrine derivatives, comprising both newly synthesized and previously reported compounds, were investigated as a strategy to enhance antibacterial activity. Four derivatives (DIT, DAT, DNT, and DBT) had been previously reported by our group, whereas three new analogues (DQT, DAcT, and DeBT) were synthesized and fully characterized. Antibacterial effects were evaluated by the broth microdilution method against Escherichia coli, Staphylococcus aureus, Enterococcus faecalis, and Pseudomonas aeruginosa. In contrast to natural tetrandrine, quaternized derivatives exhibited markedly enhanced antibacterial potency, particularly against S. aureus, highlighting DAcT (minimum inhibitory concentration (MIC) = 0.04 μg/mL), DQT (MIC = 0.11 μg/mL), and DNT (MIC = 0.14 μg/mL), with activities comparing favorably to those of reference antibiotics such as ampicillin, chloramphenicol, and ciprofloxacin. Antioxidant activity was also assessed using ABTS^•+ and DPPH^• assays, with the highest response observed in the ABTS^•+ assay for DeBT (63.72% inhibition, IC₅₀ = 21.53 μg/mL), followed by DNT (40.97% inhibition). Cytotoxicity studies of the newly synthesized derivatives against ARPE-19, MCF-7, HeLa, and A549 cell lines revealed low toxicity (IC₅₀ > 50 μg/mL), consistent with previously reported analogues, suggesting a favorable selectivity profile. DNA-binding studies by fluorescence spectroscopy indicated efficient interaction with double-stranded DNA. In addition, molecular modeling and docking studies against relevant bacterial targets provided mechanistic insight and showed good agreement with the experimental antibacterial results. Overall, the combined in vitro and in silico findings identify N,N'-disubstituted tetrandrine derivatives as promising scaffolds for the development of new antibacterial agents.