Anti-HBV Drugs in Progress

Posted by Jerry Carter on March 26th, 2021

HBV generates free covalently closed circular DNA (cccDNA) and exists in the nucleus of infected cells, which is the only known pregenomic RNA (pgRNA) template, as well as a template for viral protein transcription and viral replication. As the main reason for virus maintenance, cccDNA has a long half-life. Its transcriptional activity is under epigenetic regulation, and it can maintain chronic infection under the action of long-term polymerase inhibitors. At present, there is no method that can completely clear cccDNA from infected liver cells, which is also a major challenge for the research of new antiviral methods. The immune characteristics of HBV are also conducive to the virus chronicity that is difficult to eliminate. The replication of HBV cannot be detected by the intracellular immune system, and the high tolerance of the intrahepatic microenvironment and the characteristic of HBV to produce a large number of antigens will affect the function of T and B cells. The main viral antigen in the blood is hepatitis B e antigen (HBeAg), and other antigens include hepatitis B core antigen (HBcAg) and hepatitis B surface antigen (HBsAg), among which HBsAg is mostly used for designing non-infectious defective virus particles.

Anti-hepatitis B therapy mainly reduces inflammatory response by maximally inhibiting HBV replication and preventing liver damage. However, except for Interferon-α (IFNα), the effects of other drugs are not sustainable, including NUCs that require long-term use to prevent HBV recurrence. Not only patients with viral activity and clinical features of hepatitis (including active viral replication and alanine transferase positive), but patients with chronic hepatitis B (CHB) require treatment. The overall goal of treatment is to achieve continuous blood HBsAg reduction, liver inflammation recovery, possible reduction of liver cancer and cirrhosis, and reduction of hepatitis recurrence, which is the so-called functional cure. Current therapies still cannot effectively achieve functional cure, which is the goal of developing novel therapies.

The life cycle of HBV

HBV consists of a 3.2 kb circular DNA genome that encodes large, medium, and small capsid proteins, which can be used as L-, M- and S-HBsAg respectively, and encodes proteins such as Hepatitis B X protein (HBx). Virus invasion begins when the pre-S1 region of L-HBsAg binds to the sodium-taurocholate cotransporting polypeptide (NTCP) on the liver cell membrane, after which HBV decapsulates and releases part of double-stranded circular DNA in the nucleus. The host's cellular DNA repair machinery will convert this DNA into cccDNA and package it into viral minichromosomes. As a transcription template, the minichromosomes can be used as a viral genome library for a long time, and new viruses can be generated to infect other cells, and can also exist in progeny cells.

HBx and cccDNA's structural maintenance of chromosomes (SMC) 5 interacts with SMC6, which causes the complex to be dissociated by the proteasome and promotes cccDNA transcription. A series of viral mRNAs including pgRNA can promote transcription. In the process of virus replication, pgRNA can be reversely transcribed into negative-strand DNA, and then positive-strand DNA is synthesized. After DNA synthesis, the mature nucleocapsid can return to the nucleus to maintain the number of cccDNA, or it can be packaged with capsid protein to infect other cells. Researchers found that viral RNA found in blood samples can be used to mark the transcriptional activities of cccDNA to reflect the effect of the compound. The circular isolated DNA produced by reverse transcription of the isolated RNA can be found in patients, especially patients with advanced liver cancer.

Anti-viral drug

1. Target the way HBV enters the cell

2. Targeting cccDNA

3. Targeting the expression process of viral genes

4. The process of targeted capsid assembly

5. Targeting the release process of HBsAg

In terms of cccDNA, the overall purpose is to inhibit its transcription products, thereby indirectly affecting the liver cccDNA library, or directly eradicate or silence cccDNA. For HBsAg, its two production methods from cccDNA and integrated HBV DNA need to be considered. The combination of the above two methods could be adopted, and the synergistic use of immune stimulation methods and antiviral drugs may even be required.