Alternative RBV formulation

Optimal RBV dosages are essential in achieving a SVR. Maintenance of RBV in the therapeutic regimen has been proven to have an important additive effect in the overall success rate, leading to both increased RVR and reduced rates of relapses (as demonstrated by the PROVE-2 trial). As described in chapter 1, the main impediment in the administration of high-dose RBV is the dose-dependent development of hemolytic anemia. Although the addition of epoetin alfa has been useful in maintaining the highest possible RBV doses, new RBV-replacement compounds, with an improved side effects profile, are investigated.
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Taribavirin – formerly known as viramidine – (Valeant Pharmaceuticals International/Kadmon Pharmaceuticals LLC), is a prodrug of RBV, converted in the active form by adenosine deaminase. This nucleoside analog was studied for the treatment of CHC, due to the lower frequency of anemia, a benefit registered especially within the first 12 weeks of treatment, the period in which maintenance of the dose of RBV has been shown to be the most critical. The major conversion site of taribavirin is in the liver, enabling drug concentration in this location. Due to its lower uptake in red blood cells, taribavirin causes significantly less hemolytic anemia compared to RBV. While this effect was confirmed in several clinical studies, the rates of SVR were lower with taribavirin. In two phase III studies, taribavirin failed to prove noninferiority compared to RBV (SVR rates were 38% and 40% with taribavirin vs. 52% and 55% with RBV in the VISER 1 and VISER 2 trials, respectively), even if taribavirin caused lower rates of severe anemia (5% vs 24%). Suboptimal dosing of taribavirin (Marcellin 2010) seems to be the explanation, as recent studies with weight-based dosing of taribavirin confirmed reduced rates of anemia (7%-15% vs. 24% with RBV), while acquiring comparable SVR rates and lower relapse rates than RBV. Whether taribavirin will have a role in the future combination therapies including DAAs (most of which are also associated with a certain degree of anemia) remains to be seen.
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Direct-Acting Antivirals (DAAs)
Direct-acting antivirals (DAAs), also known as “specifically targeted antiviral therapy for hepatitis C” (STAT-C), are the most important new therapeutical options for CHC. In May 2011, two HCV protease inhibitors Telaprevir (Incivek™) and Boceprevir (Victrelis™) have been approved by the FDA. For the first time, we have now drugs with specific anti-HCV activity. Several other DAAs are at various stages of clinical development, the most advanced being alternative protease inhibitors and nucleoside and non-nucleoside polymerase inhibitors. Other tentative approaches include inhibitors of host cyclophilins, alphaglucosidase inhibitors, oligonucleotides and immune modulators (Soriano 2009).