Anti-influenza activity of Alchemilla mollis extract: possible virucidal activity against influenza virus particles

Influenza virus infection is a major public health problem that leads to significant morbidity and mortality. The emergence of resistance to the currently available anti-influenza agents has necessitated the development of new drugs with novel targets. Studying known ethno-medicinal plants is a promising approach for the discovery of new antiviral compounds. Alchemilla mollis is used in traditional medicine in Europe for different indications, including minimizing the symptoms of a sore throat. In this study, we found that A. mollis extract has anti-influenza activity, and investigated the mechanism underlying its inhibition of influenza virus replication. Plaque assays demonstrated that treatment of cells with A. mollis extract prior to infection did not inhibit influenza virus infection. However, plaque formation was markedly reduced when infected cells were overlaid with an agarose gel containing A. mollis extract. In addition, exposure of the virus to A. mollis extract prior to infection and treatment of cells during virus infection significantly suppressed plaque formation. Influenza virus-induced hemagglutination of chicken red blood cells was inhibited by A. mollis extract treatment. The inhibitory effect was observed against influenza A virus subtypes H1N1, H3N2, and H5N2. These findings suggest that A. mollis extract has virucidal or neutralizing activity against influenza virus particles. Furthermore, inhibitory effect of zanamivir synergistically increased after combination with A. mollis extract. Our results suggest that A. mollis extract has the potential to be developed as an antiinfluenza agent.