In vitro neuraminidase inhibitory effect and activity against influenza virus A H1N1 of herbal drugs used for common cold

Influenza viruses are causative agents of common respiratory infections, often underestimated and badly managed. Monomolecular anti-inflammatory and anti-pyretic drugs are extensively used in infection related symptoms, but they do not trigger virus replication. Also herbal drugs are traditionally used [1], although specific anti influenza activities are rarely recorded and little investigated. By means of validated in vitro tests, in this study we investigated the effect of ten herbal extracts and poplar propolis commonly used for respiratory infections. We tested their effectiveness against an influenza A H1N1 strain in pre- and post cell infection and neuraminidase activity inhibition. Camellia sinensis Kuntze leaves dried extract 60% catechins (CSE), Pelargonium sidoides DC. roots standardized d.e. (PS) and Cistus creticus L. subsp. eriocephalus (Viv.) Greuter & Burdet (formerly Cistus incanus L.) aerial parts commercial d.e. 20% polyphenols (CIS) showed a strong anti-neuraminidase effect (IC50<25 μg/ml). Only these extracts were efficacious in in vitro inhibiting influenza virus entry and (in a weaker manner) virus replication in MDCK cells; toxic/effective concentration ratio was >4 only for CIS. Chemical analyses showed that galloil derivatives and flavan-3-ols oligomers mainly occur in CIS and, similarly to PS and CSE, these constituents are likely to contribute more to the antiviral efficacy [2], [3]. Our findings suggest that C. creticus subsp. eriocephalus is worth to be better investigated for its anti-influenza efficacy and ongoing researches are aiming to investigate molecular mechanisms of CIS constituents on viral targets such as the influenza virus glycoproteins, emoagglutinin and neuraminidase.