HBV infection is global health problem with over 350 million HBV carriers worldwide and over one million deaths occurring annually due to HBV-induced liver diseases.Current therapy which includes IFN-? and nucleoside analogs such as lamivudine are partially effective. Disadvantages of these drugs include low efficacy, severe side effects, and occurrence of resistance due to HBV mutations. There is need for an ideal treatment that would be very short duration, have high cure rates, fewer side effects and broad coverage.
RNA interference (RNAi) is a process by which small interfering RNA (siRNA) with specific sequences induce silencing of homologous genes by binding to their complementary mRNA and inducing the elimination of the mRNA. RNAi has shown antiviral effect against HBV, HCV, HPV, and HIV in many studies, and unlike HCV and HIV, the small size genome and presence overlapping reading frames (ORFs) in HBV makes it more susceptible to RNAi.
In terms of attributes, RNAI-based therapy holds a lot of advantages over current therapies. For example, siRNA mediated inhibition of gene expression does not require any viral DNA replication, and siRNA combinations have the ability to target conserved regions of the viral genome. Also, there is an obvious reduction of HBV DNA from infected hepatocyte. Furthermore, because RNAi-based drugs can be expressed from introduced genes, they offer the possibility for a sustained therapeutic response. They are also specific and potent in their actions.
The methods of synthesis of siRNA are mainly three discussed briefly below.
- Chemical synthesis: this involves the production of sense and antisene strands, annealing of the strands, adding stable chemical entities and 2 nt overhangs at 3? end. A particular research recommended the following guidelines for designing siRNAs:
- beginning with the AUG of the target gene transcript, search downstream for AA dinucleotide sequences, each AA and the 3? adjacent 19 nt are potential siRNAs;
- blast the potential sequences against the species-specific genome database to eliminate cross-silencing phenomenon with non-target genes.
- Web-based software called siDirect algorithm which would incorporated the above guidelines for designing siRNAs.
- Endogenous vector Expression of Anti-HBV shRNA which involves the use of Plasmid to express shRNA that are converted into siRNA in cells.
Though RNAi-based therapies are obviously effective, there are many factors and challenges militating against their application. One of these is the delivery mechanisms to the cells/organs and the issue of bioavailability and stability of siRNAs.
However, the issue of delivery methods have been resolved through the uses of cationic liposome which contains lipid with positively charged group (e.g cholesteryl spermine ) that form complexes with siRNA and cationic polymer which like liposome, also form complexes with the negatively charged phosphate groups of the siRNA e.g. NAG-MLP. Also, in vivo stability of siRNA has been improved via chemical modification of backbone with 2?F, 2?O-Methyl, and 2’H to substitute 2’-OH residues required for nuclease activity
One of the drugs which have been developed to treat HBV infection through the use of RNAi is ARC-520, a drug designed to target the X protein of the virus. This has been found out to lead to reduction in viral replication, viral protein synthesis, and ultimately to reduction in viral antigen and immune suppression.
In laboratory animals experiments, a single low dose of ARC-520 was found to result in above 95% reduction of HBV RNA, proteins (eg. HBeAg and HBsAg), and viral DNA with long duration of effect in mouse and chimpanzee models of HBV infection. In chimpanzee, levels of HBV DNA, eAg ,sAg only return to baseline after 43, 43, and 71 days respectively of dosing.
As at 2013, ARC-520 has gone through phase-1-clinical trials. Results of a phase 1 from first-in-human safety and tolerability studies conducted among 36 patients with chronic HBV showed that the drug was reasonably safe and well tolerated. No dropout was recorded for any reason and no serious adverse effect was experienced.
In conclusion, the use of RNAi pathway as a new approach in antiviral drug discovery is promising as the genetic distance between mammalian and HBV genome represents an advantage in minimizing off-target hits and reducing possible side effects. Also, the ability of RNAI to effectively and durably halt viral protein production can lead to complete HBsAg loss and conversion to seronegative status which eliminates long treatments and patient compliance issues. However, improvements in safety and efficacy of delivery methods remain an important objective.