Silibinin mode(s) of action against hepatitis C virus: A controversy yet to be resolved

Wagoner et al.1, 2 suggested that Legalon-SIL (SIL), a commercially available intravenous preparation of silibinin, has an effect on hepatitis C virus (HCV) entry and cell-to-cell spread in vitro with only marginal suppression of HCV nonstructural protein 5B RNA–dependent RNA polymerase (RdRp) activity, a finding that is in contrast with the findings of Ahmed-Belkacem et al.3 Three clinical studies have reported the viral response during SIL therapy.4-6 The protocols were similar and consisted of daily injection of SIL for 7 days followed by pegylated interferon plus ribavirin; however, in Biermer and Berg,5 ribavirin was administered before and during silibinin treatment. Viral decline after the initiation of SIL was monophasic until day 7 in the two case reports and in the majority of subjects in the study by Ferenci et al.4 (Fig. 1, red squares). Interestingly, a monophasic pattern of viral decline (Fig. 1, blue curves) was also observed in about half of patients (N = 31) given 14 days of monotherapy with RG7128, a nucleoside HCV-RdRp inhibitor (unpublished data), and in 3 subjects (N = 5) in Le Pogam et al. (figure 1A in that article).7 This monophasic decline is strikingly different from the biphasic viral decline typically observed in patients treated with protease inhibitors or (pegylated)interferon-α–based therapies8 (Fig. 1, triangles). The fact that both SIL and RG7128 led to a monophasic HCV decline in some patients is interesting and tends to support the findings of Ahmed-Belkacem et al.3 Representative serum HCV RNA decline from baseline during the first week of treatment with silibinin monotherapy6 (red squares), RG7128 1500-mg BID (blue circles; manuscript in preparation), daily 10 MIU IFN9 (black triangles) and telaprevir+PegIFN8 (gray triangles). Solid lines were used to emphasize plausible phases of viral decline. According to the standard HCV infection model,9 a monophasic viral decline pattern results when viral infection is blocked, which tends to support the results of Wagoner et al.1, 2 On the other hand, one can also predict a monophasic decline of virus if one assumes in the standard viral kinetic model a gradual reduction in viral production (unpublished observation), rather than an immediate high antiviral effectiveness in reducing viral production, as is the case with interferon-α or protease inhibitors. This gradual reduction in viral production could be related to drug pharmacokinetic and pharmacodynamic properties, which could explain the similarity in the pattern of viral decline observed under treatment with SIL and RG7128 and possibly will shed light on why some patients treated with either of these two agents had a monophasic viral decline pattern. In summary, to further investigate this controversy we suggest that pharmacokinetic and pharmacodynamic studies of SIL are needed to better understand the nature of the observed monophasic viral decline in treated patients. If SIL-resistant strains can be identified, the nature of the resistance mutations would provide information about the mechanism of action. If resistance mutations are found in the HCV polymerase, it would favor an HCV-RdRp inhibitor mechanism, whereas if resistance mutations exist in HCV E1/E2, it would support an entry inhibitor mechanism. Further in vitro experiments10 that include detailed kinetics of both intracellular and extracellular HCV RNA during treatment with SIL are likely to provide more insights into its mechanism(s) of action against HCV. Harel Dahari Ph.D.* , Jeremie Guedj Ph.D.*, Alan S. Perelson Ph.D.*, * Theoretical Biology and Biophysics, Los Alamos National Laboratory, Los Alamos, NM, Department of Medicine, University of Illinois at Chicago, Chicago, IL.