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Review
. 2015 Nov:123:132-7.
doi: 10.1016/j.antiviral.2015.09.011. Epub 2015 Sep 25.

Hepatitis B virus reverse transcriptase - _target of current antiviral therapy and future drug development

Affiliations
Review

Hepatitis B virus reverse transcriptase - _target of current antiviral therapy and future drug development

Daniel N Clark et al. Antiviral Res. 2015 Nov.

Abstract

Hepatitis B virus (HBV) infections rely on the proper functioning of the viral polymerase enzyme, a specialized reverse transcriptase (RT) with multiple activities. All currently approved antiviral drugs for the treatment of chronic HBV infection, except for interferon, _target the RT and belong to the same chemical class - they are all nucleoside analogs. Viral DNA synthesis is carried out by the RT enzyme in several different steps, each with distinct RT conformational requirements. In principle, each stage may be _targeted by distinct antiviral drugs. In particular, the HBV RT has the unique ability to initiate viral DNA synthesis using itself as a protein primer in a novel protein priming reaction. In order to help identify RT inhibitors and study their mechanisms of action, a number of experimental systems have been developed, each varying in its ability to dissect the protein priming stage and subsequent stages of viral DNA synthesis at the molecular level. Two of the most effective drugs to date, entecavir and tenofovir, can inhibit both the protein priming and the subsequent DNA elongation stages of HBV DNA synthesis. Interestingly, clevudine, a thymidine analog, can inhibit both protein priming and DNA elongation in a non-competitive manner and without being incorporated into the viral DNA. Thus, a nucleoside RT inhibitor (NRTI) can functionally mimic a non-NRTI (NNRTI) in its inhibition of the HBV RT. Therefore, novel NRTIs as well as NNRTIs may be developed to inhibit the DNA synthesis activity of the HBV RT. Furthermore, additional activities of the RT that are also essential to HBV replication, including specific recognition of the viral RNA and its packaging into viral nucleocapsids, may be exploited for antiviral development. To achieve a more potent inhibition of viral replication and ultimately cure chronic HBV infection, the next generation of anti-HBV therapies will likely need to include NRTIs, NNRTIs, and other agents that _target the viral RT as well as other viral and host factors in various combinations. This article forms part of a symposium in Antiviral Research on "An unfinished story: from the discovery of the Australia antigen to the development of new curative therapies for hepatitis B."

Keywords: Antiviral therapies; HBV; Protein priming; Reverse transcriptase.

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Figures

Figure 1
Figure 1. Inhibitors block the early stages of DNA synthesis by hepatitis B virus
A. The hepatitis B virus (HBV) polymerase consists of a terminal protein domain (TP), a spacer domain, the reverse transcriptase domain (RT), and an RNase H domain. DNA synthesis begins after the polymerase binds to the viral pre-genomic RNA (pgRNA), and the subsequent steps of DNA synthesis are performed inside the nucleocapsid (not shown). Also, the polymerase is also associated with host chaperone proteins (not shown). Early DNA synthesis is performed in three stages, each of which has distinct requirements for polymerase conformation and the nucleotide used. First, initiation of protein priming occurs, which attaches a deoxyguanosine nucleotide (red “G”) to a tyrosine residue (shown as an aromatic ring) on the TP domain. Next, priming continues to polymerize two deoxyadenosine nucleotides (red “AA”). Once this short oligonucleotide is synthesized, it is translocated and anneals to the 3’ end of the pgRNA at a direct repeat 1 (DR1) sequence, and the minus strand of DNA continues to elongate (red arrow). Each of these stages can be blocked by nucleoside RT inhibitors (NRTI). B. NRTIs which _target HBV polymerase are listed, along with information about their chemical structure and mode of action (De Clercq et al., 2010). * It is assumed, but has not been determined if these NRTIs act competitively and only affect the indicated stage of DNA synthesis.

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