Hepatitis B is an infectious disease of the liver caused by the Hepatitis B virus (HBV). The illness can be acute causing liver inflammation, vomiting, jaundice and in some rare instances of severe fulminant disease, death. The majority of infections result in chronic illness that can be either asymptomatic or resulting in chronic liver inflammation leading to cirrhosis of the liver and an increased incidence in the development of hepatocellular carcinoma (HCC).
HBV infection is a global problem with approximately >350 million people world wide chronically infected and 600,000 die each year from HBV-related liver disease or HCC. The disease has caused epidemics in Asia and Africa and is endemic in China. Transmission of HBV is via infectious blood or body fluids. There are currently vaccines for the prevention of HBV infection. However, the vaccine is prophylatic and cannot be used to treat already infected patients.
There are several approved chemotherapeutic treatments for chronic hepatitis B (lamivudine, adefovir, entacavir and telbivudine) that prevent replication of HBV by blocking the action of the HBV polymerase. None of the treatments result in complete clearance of the virus and these treatment result in drug-resistant HBV variants developing after prolonged treatment. Hepatitis B can also be treated with pegylated Interferon-alpha2a but this treatment is associated with severe side effects and is not effective in all patients.
HBV is a member of the Hepadanvirus family and is divided into 4 major stereotypes (adr, adw, ayr, ayw) based on antigenic epitopes. The virus is also classed into eight genotypes (A-H based on genomic sequence. Genotype A is common in the Americas, Africa, India and Western Europe. Genotype B and C are found in Asia and the US. Genotype D is most common in Southern Europe and India. Genotype E is found in Western and Southern Africa. Genotype F and H are commonly found in Central and Southern America. Genotype G is commonly found in Europe and the U.S.
HBV genome consists of a circular strand of DNA that is partially double stranded. The genome is ˜3.3 Kb in length. It encodes 4 known genes (C, X, P and S). HBV is one of the few DNA viruses that utilize reverse transcriptase in the replication process. HBV replication involves multiples stages including entry, uncoating and transport of the virus genome to the nucleus. Initially, replication of the HBV genome involves the generation of an RNA intermediate that is then reverse transcribed to produce the DNA viral genome.
Since current therapies are limited due to their ineffectiveness, serious side effects or due to the generation of drug resistant variants, there is a clinical need for the development of new therapies to treat HBV infection.
Alteration of viral gene expression, specifically HBV gene expression, through RNA interference (hereinafter “RNAi”) is one approach for meeting this need. RNAi is induced by short single-stranded RNA (“ssRNA”) or double-stranded RNA (“dsRNA”) molecules. The short dsRNA molecules, called “short interfering nucleic acids (“siNA”)” or “short interfering RNA” or “siRNA” or “RNAi inhibitors” silence the expression of messenger RNAs (“mRNAs”) that share sequence homology to the siNA. This can occur via cleavage of the mRNA mediated by an endonuclease complex containing a siNA, commonly referred to as an RNA-induced silencing complex (RISC). Cleavage of the target RNA typically takes place in the middle of the region complementary to the guide sequence of the siNA duplex (Elbashir et al., 2001, Genes Dev., 15:188). In addition, RNA interference can also involve small RNA (e.g., micro-RNA or miRNA) mediated gene silencing, presumably through cellular mechanisms that either inhibit translation or that regulate chromatin structure and thereby prevent transcription of target gene sequences (see for example Allshire, 2002, Science, 297:1818-1819; Volpe et al., 2002, Science, 297:1833-1837; Jenuwein, 2002, Science, 297:2215-2218; and Hall et al., 2002, Science, 297:2232-2237).
Several studies have attempted to use RNAi for the treatment of HBV and this approach has been comprehensively reviewed in RNAI for treating Hepatitis B Viral Infection, Chen, Y. et al., Pharmaceutical Research, 2008 Vol. 25, No. 1, pgs 72-86. Yet, as noted in the above reference transfection of a single siRNA often fails to provide adequate gene silencing. Id. Thus, despite significant advances in the field of RNAi, there remains a need for agents that can effectively inhibit HBV gene expression and that can treat disease associated with HBV expression such as liver disease and cancer.