SUMMARY: Saikosaponins represent a group of oleanane derivatives, usually as glucosides, that are found in a number of plant families. Saikosaponins isolated from medicinal plants such as Bupleurum spp., Heteromorpha spp. and Scrophularia scorodonia have been reported to possess various biological activities, specifically antihepatitis, antinephritis, antihepatoma, anti‐inflammation, immunomodulation and antibacterial effects. The aim of the present study was to examine the anticoronaviral activity of saikosaponins (A, B₂, C and D) and their mode of action. Using the 2,3‐bis[2‐methoxy‐4‐nitro‐5‐sulfophenyl]‐5‐[(phenylamino) carbonyl‐2H‐tetrazolium hydroxide] (XTT) assay, results showed that all saikosaponins tested demonstrated antiviral activity at concentrations of 0.25–25 µmol/L, with the strongest activity being noted for saikosaponin B₂ (IC₅₀ = 1.7 ± 0.1 µmol/L). Interestingly, both saikosaponins A (50% cellular cytotoxicity (CC₅₀) concentration = 228.1 ± 3.8 µmol/L; selectivity index (SI) = 26.6) and B₂ (CC₅₀ = 383.3 ± 0.2 µmol/L; SI = 221.9) exhibited no cytotoxic effects on target cells at concentrations that achieved antiviral activity. In the time‐of‐addition studies, saikosaponin B₂, at 6 µmol/L, significantly inhibited human coronavirus 229E infection following its addition at various time pre‐infection (−4 to −1 h), coinfection (0 h) and post‐infection (1–4 h). Furthermore, saikosaponin B₂ also showed an inhibitory effect on viral attachment and penetration. The present results indicate that saikosaponin B₂ has potent anticoronaviral activity and that its mode of action possibly involves interference in the early stage of viral replication, such as absorption and penetration of the virus.

A number of Plantago spp. especially P. major has long been used in the treatment of diseases such as infection, inflammation and cancer. In this study, we evaluated the immunomodulatory activities of five chemical classes of pure compounds obtained from the Plantago genus on human peripheral blood mononuclear cells (PBMC). Studies were conducted on lymphocyte transformation by BrdU immunoassay and secretion of interferon-gamma (IFN-γ) using an ELISA assay. Results showed that the water-soluble compounds, namely aucubin, chlorogenic acid, ferulic acid, p-coumaric acid and vanillic acid, enhanced the activity of human lymphocyte proliferation and secretion of IFN-γ. Among the water-insoluble compounds, with the exception of luteolin, both baicalein and baicalin showed an enhancement of the human PBMC. Although oleanolic acid and ursolic acid of the triterpenoids did not significantly affect the proliferation of PBMC, they exhibited a strong stimulation of IFN-γ secretion. Linalool, a monoterpenoid, showed a similar immunomodulatory activity as the triterpenoids. The present study concludes that the tested compounds, which possess immunostimulating activities, may contribute to the traditional claims of Plantago-based natural products used in treating cancers and infectious diseases.