Insights into the direct anti-influenza virus mode of action of Rhodiola rosea

Background: The anti-influenza A virus activities and contents of previously isolated most active flavonoids (rhodiosin and tricin) from a standardized hydro-ethanolic R. rosea root and rhizome extract (SHR-5 (R)), did not fully explain the efficacy of SHR-5 (R). Moreover, the mode of antiviral action of SHR-5 (R) is unknown.Purpose: To determine the anti-influenza viral principle of SHR-5 (R) we evaluated i) the combined anti-influenza virus effect of rhodiosin and tricin, ii) the impact of its tannin-enriched fraction (TE), iii) its anti-viral spectrum and mode of action, and iv) its propensity for resistance development in vitro.Methods: The combined anti-influenza virus effect of rhodiosin and tricin and the impact of TE were inves-tigated with cytopathic effect (CPE)-inhibition assays in MDCK cells. A tannin-depleted fraction (TD) and TE were prepared by polyamide column chromatography and dereplicated by LC-MS. Plaque-reduction assays provided insights into the anti-influenza virus profile, the mode of action, and the propensity for resistance development of SHR-5 (R).Results: Our results i) did not reveal synergistic anti-influenza A virus effects of rhodiosin and tricin, but ii) proved a strong impact of TE mainly composed of prodelphinidin gallate oligomers. iii) TE inhibited the plaque-production of influenza virus A(H1N1)pdm09, A(H3N2), and B (Victoria and Yamagata) isolates (including isolates resistant to neuraminidase and/or M2 ion channel inhibitors) with 50% inhibitory concentration values between 0.12 -0.53 mu g/ml similar to SHR-5 (R). Mechanistic studies proved a virucidal activity, inhibition of viral adsorption, viral neuraminidase activity, and virus spread by SHR-5 (R) and TE. iv) No resistance development was observed in vitro.Conclusion: For the first time a comprehensive analysis of the anti-influenza virus profile of a hydro-ethanolic R. rosea extract (SHR-5 (R)) was assessed in vitro. The results demonstrating broad-spectrum multiple direct anti-influenza virus activities, and a lack of resistance development to SHR-5 (R) together with its known augmentation of host defense, support its potential role as an adaptogen against influenza virus infection.