Antisense therapy of hepatitis B virus infection

Antisense oligonucleotides: recent progress in the treatment of various diseases

Background

Antisense oligonucleotides are a promising novel class of therapeutic agents to treat different diseases in living things. They provide an efficient method for making target-selective agents because they change gene expression sequences. Therefore, the malfunctioning protein could be stopped, and the source of disease would be obliterated. The existing reviews of antisense oligonucleotides are focusing on discovery, development and concept. However, there is no review paper concerning the latest development of antisense oligonucleotides and their different therapeutic uses. Therefore, the present work has been targeting a comprehensive summary of newly synthesized antisense oligonucleotides and their biological activities.

Main body

Antisense oligonucleotides are different from traditional therapeutic agents that are planned to interact with mRNA and modulate protein expression through a unique mechanism of action. In the last three decades, several researchers revealed the newer antisense oligonucleotides found with a high therapeutic profile due to more selective action on the drug target and thus producing a lesser side effect and low toxicity. This review emphasizes the research work on antisense oligonucleotides and their therapeutic activities.

Short conclusion

With the support of the literature review, here we enlisted various antisense oligonucleotides that were prepared by appropriate technique and explored their pharmacological activities. To the best of our knowledge, it is the right time to consider the antisense oligonucleotides as a perfect choice of treatment for different diseases due to conceptual simplicity, more selective action, lesser side effects, low toxicity and permanent cure.

Graphical abstract

In vivo reduction of hepatitis B virus antigenemia and viremia by antisense oligonucleotides

BACKGROUND & AIMS

Current treatment of chronic hepatitis B virus infection (CHB) includes interferon and nucleos(t)ide analogues, which generally do not reduce HBV surface antigen (HBsAg) production, a constellation that is associated with poor prognosis of CHB. Here we evaluated the efficacy of an antisense approach using antisense oligonucleotide (ASO) technology already in clinical use for liver targeted therapy to specifically inhibit HBsAg production and viremia in a preclinical setting.

METHODS

A lead ASO was identified and characterized in vitro and subsequently tested for efficacy in vivo and in vitro using HBV transgenic and hydrodynamic transfection mouse and a cell culture HBV infection model, respectively.

RESULTS

ASO treatment decreased serum HBsAg levels ⩾2 logs in a dose and time-dependent manner; HBsAg decreased 2 logs in a week and returned to baseline 4 weeks after a single ASO injection. ASO treatment effectively reduced HBsAg in combination with entecavir, while the nucleoside analogue alone did not. ASO treatment has pan-genotypic antiviral activity in the hydrodynamic transfection system. Finally, cccDNA-driven HBV gene expression is ASO sensitive in HBV infected cells in vitro.

CONCLUSION

Our results demonstrate in a preclinical setting the efficacy of an antisense approach against HBV by efficiently reducing serum HBsAg (as well as viremia) across different genotypes alone or in combination with standard nucleoside therapy. Since the applied antisense technology is already in clinical use, a lead compound can be rapidly validated in a clinical setting and thus, constitutes a novel therapeutic approach targeting chronic HBV infection.

Drug targets and molecular mechanisms of drug resistance in chronic hepatitis B

Chronic hepatitis B continues to be a major cause of end-stage liver disease and hepatocellular carcinoma worldwide. Nucleos(t)ide analogues have proven to be effective in controlling the disease and perhaps decreasing the incidence of hepatocellular carcinoma. However, development of drug resistance is a major limitation to their long-term effectiveness. Understanding the mechanisms of drug resistance are important for designing new agents and devising strategies to manage and prevent the development of antiviral drug resistance. The development of resistance is determined by an interplay of viral, host, and drug characteristics Homology of the HBV polymerase to the human immunodeficiency virus-1 reverse transcriptase has allowed predictions to be made on the effect mutations have on HBV polymerase structure. In vitro functional studies provide complementary information. Several broad principles on the mechanism of resistance have emerged from these studies. First, most of the primary mutations cluster in the vicinity of the incoming nucleotide and act by directly affecting the position or stability of the bound substrate, template, or primer. In contrast, secondary mutations tend to occur away from the nucleotide-binding pocket. Finally, the structural and functional consequences of mutations are quite variable among the different agents. This paper reviews the key mutations and mechanisms associated with resistance to the nucleos(t)ide analogues approved for clinical use and discuss new targets for drug development.

Hepatobiliary elimination of bile acid-modified oligodeoxynucleotides in Wistar and TR- rats: evidence for mrp2 as carrier for oligodeoxynucleotides

As therapeutic antisense tools, oligonucleotides (ODNs) must enter cells to bind to their target structures. ODNs distribute in nearly each tissue with relatively high concentrations in kidney and liver from where excretion into urine and bile occurs. To investigate mechanisms involved in hepatic ODN transport, normal mixed backbone phosphodiester/phosphorothioate ODNs (n-ODN) and two different bile acid-conjugated mixed backbone ODNs (1BA-ODN and 2BA-ODN) were applied to two different rat strains, normal Wistar rats and Wistar TR- rats. In normal Wistar rats, concentration-dependent hepatobiliary elimination of the ODNs was observed with a remarkable increase of excretion of the cholic acid BA-ODN conjugates. In contrast to normal Wistar rats, n-ODN excretion into bile by TR- rats, a mutant Wistar rat strain lacking a functional multidrug resistance-associated protein 2 (mrp2) at the canalicular membrane, was strongly diminished, whereas these rats excreted an ODN conjugated with two cholic acid molecules (2BA-ODN) into bile. Concomitant application of substrates transported by mrp2 such as bromosulfophthalein (BSP) or the synthetic chlorogenic acid derivative S 3025 significantly reduced the biliary appearance of normal ODN and 2BA-ODN in Wistar rats and also in TR- rats. To inhibit the expression of cRNA derived from the Na+ -dependent taurocholate cotransporting polypeptide (Ntcp), antisense ODNs were constructed which fully retained the antisense properties when coupled with two bile acid molecules. The results indicate that ODNs are secreted via the mrp2 into bile. In the absence of mrp2, further excretory transport systems with affinity for bile acids seem to be relevant for their excretion. The results further indicate that bile acid tagged ODNs are useful tools for liver specific antisense therapy.

Restoration of correct splicing of thalassemic beta-globin pre-mRNA by modified U1 snRNAs

The T-->G mutation at nucleotide 705 in the second intron of the beta-globin gene creates an aberrant 5' splice site and activates a 3' cryptic splice site upstream from the mutation. As a result, the IVS2-705 pre-mRNA is spliced via the aberrant splice sites leading to a deficiency of beta-globin mRNA and protein and to the genetic blood disorder thalassemia. We have shown previously that in cell culture models of thalassemia, aberrant splicing of beta-thalassemic IVS2-705 pre-mRNA was permanently corrected by a modified murine U7 snRNA that incorporated sequences antisense to the splice sites activated by the mutation. To explore the possibility of using other snRNAs as vectors for antisense sequences, U1 snRNA was modified in a similar manner. Replacement of the U1 9-nucleotide 5' splice site recognition sequence with nucleotides complementary to the aberrant 5' splice site failed to correct splicing of IVS2-705 pre-mRNA. In contrast, U1 snRNA targeted to the cryptic 3' splice site was effective. A hybrid with a modified U7 snRNA gene under the control of the U1 promoter and terminator sequences resulted in the highest levels of correction (up to 70%) in transiently and stably transfected target cells.

Hammerhead ribozyme-mediated inhibition of hepatitis B virus X gene expression in cultured cells

BACKGROUND/AIMS

Chronic infection with hepatitis B virus (HBV) is endemic to sub-Saharan Africa and parts of Asia. Common complications of HBV persistence include cirrhosis and hepatocellular carcinoma (HCC). Present treatment of chronic HBV infection is usually ineffective and novel therapeutic approaches are an important objective. The HBV X protein (HBx) is a transcriptional activator that is required for the establishment of HBV infection and is implicated in hepatocarcinogenesis. The aim of this study was to assess the ability of two endogenously expressed hammerhead ribozymes to inhibit expression of HBV genes in transfected cultured cells.

METHODS

Eukaryotic expression plasmids producing two ribozymes targeted to the HBx open reading frame, as well as their catalytically inactive homologues, were generated. Established cell lines and a primary culture of malignant hepatocytes were transfected to assess ribozyme effects on HBx expression and HBV replication.

RESULTS

The ribozyme-expressing vectors inhibit expression of functional HBx protein and decrease HBV mRNA encoding surface and HBx sequences in transfected cells. Moreover, decreased HBsAg and HBeAg secretion from cells transfected with the ribozymes and an HBV replication competent plasmid provide evidence for an antireplicative effect of the ribozymes. However, the data do not exclude a dominant antisense effect that inhibits HBV gene expression.

CONCLUSIONS

Inactivation of HBx, a sequence that is conserved in mammalian hepadnaviruses and found in all HBV transcripts, has potential for the treatment of chronic HBV infection.

Masked antisense: a molecular configuration for discriminating similar RNA targets

Antisense technology has great potential for the control of RNA expression, but there remain few successful applications of the technology. Expressed antisense RNA can effectively down-regulate expression of a gene over long periods, but cannot differentiate partly identical sequences, such as the mRNA of fusion genes or those with point mutants. We have designed a structured form of expressed antisense, which can discriminate between highly similar mRNA molecules. These 'masked' antisense RNAs have most of the antisense sequence sequestered within duplex elements, leaving a short single-stranded region to initiate binding to target RNA. After contacting the correct target, the structured RNA can unravel, releasing the masked antisense region to form a stable duplex with the mRNA. We demonstrate that suitable masked antisense RNA can discriminate between the two forms of BCR-ABL mRNA that result from the Philadelphia chromosomal translocations, as well as discriminating the normal BCR and ABL mRNA.

In situ demonstration of inhibitory effects of hammerhead ribozymes that are targeted to the hepatitis Bx sequence in cultured cells

Chronic hepatitis B virus (HBV) infection is endemic to several populous areas of the world and is frequently complicated by hepatocellular carcinoma. Ribozymes can be designed to cleave target RNA sequences specifically and show promise for the treatment of HBV infection. Demonstration of intracellular inhibition of HBV gene expression, essential to developing therapeutic ribozymes, has been the aim of this investigation. We generated two vectors encoding hammerhead ribozymes that target the HBx region of HBV. Plasmids containing intact HBV sequences or a modification in which the preS2/S region was replaced by DNA encoding enhanced green fluorescent protein (EGFP) were used to test ribozyme action in transfected cells. Both ribozymes inhibited surface antigen secretion and EGFP expression similarly. The measurement of EGFP expression is convenient to assess ribozyme action in situ and effective targeting of HBV sequences that are common to all HBV transcripts is potentially useful to develop strategies to counter HBV infection.

Antisense oligonucleotides as therapeutic agents

Antisense oligonucleotides can block the expression of specific target genes involved in the development of human diseases. Therapeutic applications of antisense techniques are currently under investigation in many different fields. The use of antisense molecules to modify gene expression is variable in its efficacy and reliability, raising objections about their use as therapeutic agents. However, preliminary results of several clinical studies demonstrated the safety and to some extent the efficacy of antisense oligodeoxynucleotides (ODNs) in patients with malignant diseases. Clinical response was observed in some patients suffering from ovarian cancer who were treated with antisense targeted against the gene encoding for the protein kinase C-alpha. Some hematological diseases treated with antisense oligos targeted against the bcr/abl and the bcl2 mRNAs have shown promising clinical response. Antisense therapy has been useful in the treatment of cardiovascular disorders such as restenosis after angioplasty, vascular bypass graft occlusion, and transplant coronary vasculopathy. Antisense oligonucleotides also have shown promise as antiviral agents. Several investigators are performing trials with oligonucleotides targeted against the human immunodeficiency virus-1 (HIV-1) and hepatitis viruses. Phosphorothioate ODNs now have reached phase I and II in clinical trials for the treatment of cancer and viral infections, so far demonstrating an acceptable safety and pharmacokinetic profile for continuing their development. The new drug Vitravene, based on a phosphorothioate oligonucleotide designed to inhibit the human cytomegalovirus (CMV), promises that some substantial successes can be reached with the antisense technique.

Antisense oligonucleotides as therapeutic agents

Antisense oligonucleotides can block the expression of specific target genes involved in the development of human diseases. Therapeutic applications of antisense techniques are currently under investigation in many different fields. The use of antisense molecules to modify gene expression is variable in its efficacy and reliability, raising objections about their use as therapeutic agents. However, preliminary results of several clinical studies demonstrated the safety and to some extent the efficacy of antisense oligodeoxynucleotides (ODNs) in patients with malignant diseases. Clinical response was observed in some patients suffering from ovarian cancer who were treated with antisense targeted against the gene encoding for the protein kinase C-alpha. Some hematological diseases treated with antisense oligos targeted against the bcr/abl and the bcl2 mRNAs have shown promising clinical response. Antisense therapy has been useful in the treatment of cardiovascular disorders such as restenosis after angioplasty, vascular bypass graft occlusion, and transplant coronary vasculopathy. Antisense oligonucleotides also have shown promise as antiviral agents. Several investigators are performing trials with oligonucleotides targeted against the human immunodeficiency virus-1 (HIV-1) and hepatitis viruses. Phosphorothioate ODNs now have reached phase I and II in clinical trials for the treatment of cancer and viral infections, so far demonstrating an acceptable safety and pharmacokinetic profile for continuing their development. The new drug Vitravene, based on a phosphorothioate oligonucleotide designed to inhibit the human cytomegalovirus (CMV), promises that some substantial successes can be reached with the antisense technique.

Replication of hepatitis B virus which carries foreign DNA in vitro

Targeting a specific DNA sequence to the desired tissues is an important step in gene therapy. The hepatitis B virus (HBV) is the only DNA virus that has hepatocyte specificity. We attempted to construct an HBV-based vector for targeting the liver. We observed the replication and secretion of virus particles in an HBV construct that lacks X gene and carries an extra 63 bp DNA fragment in vitro. Replication was observed in the cell line HuH-7 but not HepG2. From this construct, we designed an HBV-based vector that could carry foreign DNA. HBV based vectors provide for the possibilities of generating therapeutic agents for individual patients. Our host vector system may be used to clear out the HBV from the HBV carrier or chronic hepatitis B patients by introducing a genetically engineered HBV into these patients.