Combination therapy has been a central component of treatment for certain viral diseases for more than 15 years. The benefits of combination therapy can arise from activation of multiple host pathways, suppression of mutational variants that can lead to viral escape, or perhaps both.
In HIV, the benefit of combination therapy is due to suppression of viral resistance, which is the result of using multiple agents acting at distinct sites within the virus life cycle. In hepatitis C (which I’ll abbreviate as HCV) the addition of ribavirin to interferon turned what was primarily an on-treatment lowering of viral titers into the first significant rate of viral clearance that persisted even after therapy was stopped. This sustained virological response, known as SVR, has become the primary measurement of clinical benefit in HCV. The mechanism underlying the dramatic effect of combining ribavirin with interferon is not clear. The benefit could be due to a pharmacologic interaction between pathways activated by interferon and pathways activated by ribavirin, or it may be the result of a modest ribavirin antiviral effect added to an “antiviral state” induced by interferon.
Just this year, companies in the HCV field began exploring the use of direct antiviral combinations. It is hoped that by appropriately choosing complementary targets, benefits of combination similar to what was seen in HIV may soon be seen in HCV therapy. Whether or not the combination of direct antivirals will permit the elimination of interferon and/or ribavirin remains unknown at this time, and is perhaps the most highly anticipated answer in the HCV field today.
Combinations of antivirals today
Three companies have moved into the clinical stage of exploring direct antiviral combinations for HCV.
Roche is most advanced in combination studies of direct antivirals with its INFORM-1 study. In this study, HCV patients were treated for 14 days with various dose levels of two drug candidates that inhibit different parts of the virus life cycle. These drug candidates are RG7128, a nucleoside polymerase inhibitor licensed from Pharmasset, and RG7227, a protease inhibitor licensed from Intermune. Data from the first several dosing cohorts was disclosed this past April at the annual meeting of the European Association for the Study of the Liver. Additional data, including responses at higher doses and in patients who previously failed interferon/ribavirin, will is being reported at the annual meeting of the American Association for the Study of Liver Diseases conference in Boston.
The INFORM-1 study clearly shows that two antiviral agents can act in concert to produce a greater antiviral effect over 14 days than either agent produced alone. At the same time, critical questions remain for longer studies — Can direct antivirals alone retain viral titers at undetectable levels over longer periods of treatment? Even more important, will a state of virus negativity elicited by a direct antiviral combination afford the same rate of SVR once therapy is stopped as when virus negativity is induced by the interferon/ribavirin combination? Is there anything special
