Modeling the antiviral activity of ribavirin against hepatitis C virus in cell culture

W hile the recent inclusion of direct-acting antiviral (DAA) therapies has recently improved the standard of care (SOC) for patients with hepatitis C virus (HCV) genotype 1 infection; the remaining limitations of efficacy, side effects, and high costs remain challenges for improving therapy. A foreseeable goal is an exclusively orally administered treatment regimen, free of interferon (IFN) and IFN-associated side effects. While the current SOC for patients with genotype 1 infection is composed of pegylated IFN alpha with ribavirin (RBV) and either telaprevir or boceprevir, treatment is anticipated to improve by the inclusion of a secondgeneration protease inhibitor and/or DAAs targeting the viral polymerase or NS5A protein, and eventually removal of IFN. A remaining arm of anticipated future treatment is the guanosine nucleotide analog, RBV. Recent results with next-generation DAAs including sofosbuvir have highlighted RBV’s role in the upcoming anti-HCV regimens. Even though RBV has been employed in treating hepatitis C for more than 20 years, the primary mechanism of its action is still unclear. This lack of clarity is hindered by the current state-of-the-art Huh7 cell-based models of HCV infection poorly reflecting the in vivo activity of RBV at clinical concentrations. There is evidence supporting multiple mechanisms of RBV’s anti-HCV activity (Fig. 1). While there is an extrahepatic immunomodulatory role of RBV that alters helper T-cell responses to favor Th1 responses while suppressing Th2 responses, the majority of the evidence indicates that the more important mechanisms of RBV-mediated HCV clearance occur most likely within the hepatocyte. The proposed intrahepatic mechanisms of RBV action involve conversion of RBV to its monophosphate form (RMP) by host enzyme adenosine kinase (ADK), followed by subsequent conversion into triphosphate form (RTP). Putative mechanisms of RBV anti-HCV activity include direct inhibition of viral polymerase activity, introduction of terminating numbers of mutations resulting in error catastrophe, and inhibition of inosine-monophosphate-dehydrogenase (IMPDH) that would result in depletion of guanosine triphosphate (GTP) pools. Furthermore, recent evidence suggests that an important mechanism of RBV is in potentiating inflammatory defenses through activation of IFN-stimulated genes (ISGs) beyond the stimulation of IFN alone. The role of RBV conversion to RTP as a preceding step for ISG activation is not well defined, but since the addition of guanosine reverses ISG stimulation, it is likely important. The contribution and role of all of these possible antiviral mechanisms has been difficult to ascertain since the hepatoma cell line Huh-7, which until recently has been exclusively capable of sustaining HCV replication, is resistant to RBV treatment at clinically relevant levels. In this issue of HEPATOLOGY, Mori et al. contribute a scientific advance to address this puzzle by investigating RBV’s anti-HCV activity utilizing a previously identified hepatoma cell line, distinct from Huh7, which is capable of sustaining HCV replication. This cell line, Li23, displayed a different gene expression profile from Huh7 and, in contrast to Huh7-based cell lines, the anti-HCV activity of RBV was effective at more clinically relevant concentrations. The authors took advantage of this RBV-sensitive phenotype by conducting a comparative microarray analysis of transcript levels between an Huh-7-derived line and an Li23-derived line, both altered to report HCV replication. The RBVsensitive line had more than 4-fold more ADK transcript expression than the RBV-resistant line, and 16Abbreviations: ADK, adenosine kinase; DAA, direct acting antiviral; GTP, guanosine triphosphate; HCV, hepatitis C virus; IFN, interferon; IMP, inosine5-monophosphate; IMPDH, inosine-monophosphate-dehydrogenase; IRES, internal ribosome entry site; ISGs, IFN stimulated genes; PHH, primary human hepatocytes; RBV, ribavirin; RMP, RBV monophosphate; RTP, ribavirin triphosphate; SOC, standard of care; SVR, sustained virologic response; UTR, untranslated region. Supported by Inserm, ANRS (13081), University of Strasbourg, the European Union (INTERREG-IV-Rhin Sup erieur-FEDER-Hepato-Regio-Net 2009 and 2012, FP7 HEPAMAB GAN 305600), ERC AdG HEPCENT 233130, Direction G en erale de l’Offre de Soins, and the Laboratoire d’excellence HEPSYS (ANR-10-LAB-28). D.J.F. supported by an EASL fellowship. Address reprint requests to: Thomas F. Baumert, M.D., Inserm Unit e 1110, Hôpitaux Universitaires de Strasbourg, Universit e de Strasbourg, 3 Rue Koeberle, F-67000 Strasbourg, France. E-mail: Thomas.Baumert@unistra.fr; fax: 133 3 68 85 37 24. Copyright VC 2013 by the American Association for the Study of Liver Diseases. View this article online at wileyonlinelibrary.com. DOI 10.1002/hep.26493 Potential conflict of interest: Nothing to report.