Hepatitis C affects about 3% of the world population, yet its current treatment options are limited to interferon-ribavirin drug regimens which achieve a 50-70% cure rate depending on the hepatitis C virus (HCV) genotype. Besides extensive screening for HCV-specific compounds, some wellestablished medicinal drugs have recently demonstrated anti-HCV effect in HCV replicon cells. One of these drugs is arbidol (ARB), a Russian-made broad spectrum antiviral agent, which we have previously shown to inhibit acute and chronic HCV infection. Here we show that ARB inhibits the cell entry of HCV pseudoparticles of genotypes 1a, 1b and 2a in a dose-dependent fashion. ARB also displayed a dose-dependent inhibition of HCV membrane fusion, as assayed by using HCV pseudoparticles (HCVpp) and fluorescent liposomes. ARB inhibition of HCVpp fusion was found more effective on genotype 1a than on genotypes 1b and 2a. In vitro biochemical studies revealed ARB association with membrane-like environments such as detergents, and with lipid membranes. This association was particularly prominent at acidic pH which is optimal for HCV-mediated fusion. Our results suggest that the affinity of ARB for lipid membranes could account for its anti-HCV actions, together with a differential level of interaction with key motifs in HCV glycoproteins of different genotypes.The hepatitis C virus (HCV) infects an estimated 3% or 170 million of the world's population, and hepatitis C is now the most frequent indication for liver transplantation. Current treatment options are limited to pegylated recombinant interferon alpha (IFN-α) in combination with ribavirin. However viremia eradication is variably achieved depending on the genotype, with only 50% of virus eradication in genotype 1-infected patients. This is clearly a problem in North America, Europe and Japan, where genotype 1 is the most prevalent genotype. HCV † This work was supported by the ANRS (Agence Nationale de Recherches sur le SIDA et les hepatites virales) to E.I.P., by the French CNRS and INSERM, by LSHB-CT-2004-005246 (COMPUVAC) to F.L.C., by the Ligue Nationale Contre le Cancer and the Rhône-Alpes Region. Y.S.B. was partially supported by the Fulbright Visiting Scholar Program. S.J.P. is supported by NIH grants RO1 DK62187 and U19 A1066328. * Corresponding author, IBCP, UMR 5086 CNRS-UCBL, 7 passage du Vercors, 69367 Lyon Cedex 07, France. Phone: ; E-mail: e.pecheur@ibcp.fr. ◊ These authors contributed equally to this work. NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2008 September 9. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript therefore appears resistant to IFN antiviral therapy, and this is likely to be due to some factors of viral origin (1).Historically, the development of new anti-HCV drugs has been hampered due to the lack of cell culture and small animal models that are required for pre-clinical evaluations of antiviral compounds. The generation of an HCV replicon system (2) has afforded massive an...