NS5A: A new target for antiviral drugs in the treatment of hepatitis C virus infection

H epatitis C virus (HCV) is a major cause of liver disease and mortality affecting 170 million people worldwide. Chronic infection with HCV can lead to liver fibrosis, cirrhosis, and hepatocellular carcinoma and is the most common reason for liver transplantation in the United States. HCV is an enveloped single-strand RNA virus that belongs to the flaviviridae family. It has a 9.6-kb RNA genome encoding a single polyprotein of 3,010 amino acids. This polyprotein is composed of structural and nonstructural proteins. HCV entry into hepatocytes is mediated by the structural proteins, including glycoproteins E1 and E2, which interact with cellular receptors. Once HCV enters the host cell, replication of its genome is orchestrated by the nonstructural proteins, including proteases (NS2-3, NS3-4A), helicases (NS3), polymerases (NS5B), and NS5A, a protein with no known enzymatic activity. NS5A is essential to the replication machinery of the virus and critical in the assembly of infectious viral particles. However, its specific role remains unclear. A number of properties of this viral protein have been described. NS5A interacts with a variety of host proteins, including some which modulate host cellular signaling pathways. Studies have demonstrated that NS5A interacts with p53 and p21, affecting cell cycle control. More recently, NS5A was found to interact with proteins related to focal adhesions, gap-junction, and host signaling pathways. NS5A also interacts with other nonstructural viral proteins. During HCV viral replication, NS5A interacts with the RNAdependent RNA-polymerase NS5B, which is essential to maintain HCV replication in cell culture. Through its domain 1 NS5A has RNA-binding capacity, a property that can be critical for RNA replication. In addition to being involved in HCV replication, NS5A is involved in the assembly of the virus by forming stable complexes with NS2, the main protein involved in HCV assembly. NS5A attracted attention due to its interferon-alpha sensitivity determining region (ISDR), which can confer resistance to interferon treatment. Moreover, NS5A has an effect on interferon activity by up-regulating interleukin (IL)-8, which has been reported to attenuate the antiviral properties of interferon. These observations suggest that NS5A plays a critical role in many aspects of the life cycle of HCV and is therefore an attractive target for antiviral therapy (Fig. 1). Until recently, the standard treatment for HCV infection has been pegylated interferon-alpha and ribavirin. These drugs have significant and often serious side effects. Moreover, in patients with genotype 1 (the most prevalent HCV genotype), sustained viral clearance is achieved in less than 50% of cases. In recent years, several viral inhibitors targeting nonstructural proteins of HCV have been developed (direct-acting antiviral agents [DAA]). Most of the research has been focused on inhibitors of the NS3-NS4A and NS5B proteins, mainly because of a better understanding of the role of these proteins. A variety of NS3-NS4A and NS5B inhibitors are currently in various stages of development in preclinical and clinical studies. Telaprevir and boceprevir, which are both inhibitors of the NS3-NS4A serine protease, have recently been approved for use in combination with pegylated interferon and ribavirin in patients with genotype 1 HCV infection. Treatment with pegylated interferon, ribavirin, and either of these protease inhibitors results in improved rates of viral eradication (both in previously untreated and in those who have failed prior treatment). However, the recommended treatment regimens are complex and are associated with significant side effects from interferon and ribavirin, as well as adverse events from the protease inhibitors. Furthermore, these protease inhibitors need to be administered every 8 hours with food and there is also a very real problem of interaction with other drugs. In addition, the rapid replication of HCV makes the Abbreviations: DAA, direct-acting antiviral agents; HCV, hepatitis C virus; ISDR, interferon-alpha sensitivity determining region. Address reprint requests to: Coleman I. Smith, M.D., Department of Medicine, Division of Gastroenterology and Hepatology, University of Minnesota, 420 Delaware Street SE, MMC-36, Minneapolis, MN 55455. E-mail: smith136@umn.edu. CopyrightVC 2012 by the American Association for the Study of Liver Diseases. View this article online at wileyonlinelibrary.com. DOI 10.1002/hep.25696 Potential conflict of interest: Dr. Smith is on the speakers’ bureau of, consults for, and received grants from Gilead and Vertex. He received grants from Mochida, Abbott, and Jannsen. He received grants from and is on the speakers’ bureau of Merck. He is on the speakers’ bureau of and consults for Bayer. He also consults for Bristol-Myers Squibb.