Antiviral Research 76 (2007) 38–47 A polyphenol rich plant extract, CYSTUS052, exerts anti influenza virus activity in cell culture without toxic side effects or the tendency to induce viral resistance Christina Ehrhardt a , Eike R. Hrincius a , Virginia Korte a , Igor Mazur a , Karoline Droebner b , Anne Poetter c , Stephan Dreschers d , Mirko Schmolke a , Oliver Planz b , Stephan Ludwig a,∗ a Institute of Molecular Virology (IMV), ZMBE, Westfaelische-Wilhelms-Universitaet, Von Esmarch-Str. 56, D-48159 Muenster, Germany b Institute of Immunology, Friedrich-Loeffler-Institute (FLI), Paul-Ehrlich-Str. 28, D-72076 Tuebingen, Germany c Dr. Pandalis NatUrprodukte GmbH, Fuechtenweg 3, D-49219 Glandorf, Germany d Department of Molecular Biology, University of Duisburg-Essen, Hufelandstr. 55, D-45122 Essen, Germany Received 20 November 2006; accepted 7 May 2007 Abstract Infections with influenza A viruses still pose a major threat to humans and several animal species. The occurrence of highly pathogenic avian influenza viruses of the H5N1 subtype capable to infect and kill humans highlights the urgent need for new and efficient countermeasures against this viral disease. Here we demonstrate that a polyphenol rich extract (CYSTUS052) from the Mediterranean plant Cistus incanus exerts a potent anti-influenza virus activity in A549 or MDCK cell cultures infected with prototype avian and human influenza strains of different subtypes. CYSTUS052 treatment resulted in a reduction of progeny virus titers of up to two logs. At the effective dose of 50 g/ml the extract did not exhibit apparent harming effects on cell viability, metabolism or proliferation, which is consistent with the fact that these plant extracts are already used in traditional medicine in southern Europe for centuries without any reported complications. Viruses did not develop resistance to CYSTUS052 when compared to amantadine that resulted in the generation of resistant variants after only a few passages. On a molecular basis the protective effect of CYSTUS052 appears to be mainly due to binding of the polymeric polyphenol components of the extract to the virus surface, thereby inhibiting binding of the hemagglutinin to cellular receptors. Thus, a local application of CYSTUS052 at the viral entry routes may be a promising approach that may help to protect from influenza virus infections. © 2007 Elsevier B.V. All rights reserved. Keywords: Influenza A virus; CYSTUS052; Polyphenols; Plant extract; Virus entry 1. Introduction Influenza A viruses are negative strand RNA viruses with a segmented genome that belong to the family of orthomyx- oviridae (Lamb and Krug, 2001). While the natural reservoir of these viruses is within wild living water fowl, influenza A viruses also infect humans and several animal species (Webster et al., 1992). In fact, influenza A viruses still pose a major bur- den to human health and cannot be eradicated due to their large natural reservoir. Thus, introduction of avian virus genes into the human population can happen at any time and may give rise to a new pandemic. There is even the likeliness of a direct infection ∗ Corresponding author. Tel.: +49 251 83 57791; fax: +49 251 83 57793. E-mail address: ludwigs@uni-muenster.de (S. Ludwig). of humans by avian viruses, as evidenced by the emergence of highly pathogenic avian influenza viruses of the H5N1 subtype that were capable to infect and kill humans (Shortridge et al., 2000; Webby and Webster, 2001). Although vaccination is the best option to protect from influenza virus infections, this approach is difficult with regard to pathogenic avian strains or human reassortants with avian glycoprotein genes. To date only two classes of anti-influenza drugs have been approved; inhibitors of the M2 ion channel such as amantadine and rimantadine or neuraminidase inhibitors such as oseltamivir or zanamivir (De Clercq, 2004). Treat- ment with amantadine and derivatives rapidly results in the emergence of resistant variants and is not recommended for a general and uncontrolled use (Fleming, 2001; Hayden and Hay, 1992). Among H5N1 isolates from Thailand and Viet- nam 95% of the strains have been shown to harbor genetic 0166-3542/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.antiviral.2007.05.002