In vitro selected Con1 subgenomic replicons resistant to 2'-C-methyl-cytidine or to R1479 show lack of cross resistance

Status presens of antiviral drugs and strategies: Part II: RNA VIRUSES (EXCEPT RETROVIRUSES)

More than 40 compounds have been formally licensed for clinical use as antiviral drugs, and half of these are used for the treatment of HIV infections. The others have been approved for the therapy of herpesvirus (HSV, VZV, CMV), hepadnavirus (HBV), hepacivirus (HCV) and myxovirus (influenza, RSV) infections. New compounds are in clinical development or under preclinical evaluation, and, again, half of these are targeting HIV infections. Yet, quite a number of important viral pathogens (i.e. HPV, HCV, hemorrhagic fever viruses) remain in need of effective and/or improved antiviral therapies.

Ribavirin antagonizes the in vitro anti-hepatitis C virus activity of 2'-C-methylcytidine, the active component of valopicitabine

Ribavirin antagonizes the in vitro anti-hepatitis C virus (HCV) activity of the pyrimidine nucleoside analogue 2'-C-methylcytidine, the active component of the experimental anti-HCV drug valopicitabine. In contrast, the combination of ribavirin with either the purine nucleoside analogue 2'-C-methyladenosine or the HCV protease inhibitor VX-950 resulted in an additive antiviral activity. These findings may have implications when planning clinical studies with valopicitabine.

Selection and characterization of replicon variants dually resistant to thumb- and palm-binding nonnucleoside polymerase inhibitors of the hepatitis C virus

Multiple nonnucleoside inhibitor binding sites have been identified within the hepatitis C virus (HCV) polymerase, including in the palm and thumb domains. After a single treatment with a thumb site inhibitor (thiophene-2-carboxylic acid NNI-1), resistant HCV replicon variants emerged that contained mutations at residues Leu419, Met423, and Ile482 in the polymerase thumb domain. Binding studies using wild-type (WT) and mutant enzymes and structure-based modeling showed that the mechanism of resistance is through the reduced binding of the inhibitor to the mutant enzymes. Combined treatment with a thumb- and a palm-binding polymerase inhibitor had a dramatic impact on the number of replicon colonies able to replicate in the presence of both inhibitors. A more exact characterization through molecular cloning showed that 97.7% of replicons contained amino acid substitutions that conferred resistance to either of the inhibitors. Of those, 65% contained simultaneously multiple amino acid substitutions that conferred resistance to both inhibitors. Double-mutant replicons Met414Leu and Met423Thr were predominantly selected, which showed reduced replication capacity compared to the WT replicon. These findings demonstrate the selection of replicon variants dually resistant to two NS5B polymerase inhibitors binding to different sites of the enzyme. Additionally, these findings provide initial insights into the in vitro mutational threshold of the HCV NS5B polymerase and the potential impact of viral fitness on the selection of multiple-resistant mutants.