Development of antiviral drugs against hepatitis B virus

Abstract

Hepatitis B virus (HBV) infection is prevalent throughout the world, with approximately 300 million people chronically infected. HBV infection can cause acute and chronic liver hepatitis. Long-term chronic infection can increase the incidence of liver cirrhosis and hepatocellular carcinoma (HCC). At present, there are two types of approved drugs for chronic HBV infection: nucleos(t)ide analogs (NAs) and interferon α (IFNα), currently pegylated IFNα (PegIFNα). Although these drugs can inhibit viral replication and reduce the risk of liver cirrhosis and HCC, the limitations are obvious: HBV surface antigen (HBsAg) and covalently closed circular DNA (cccDNA) pool cannot be efficiently eliminated. To overcome these limitations, new drugs are required urgently to be designed and developed. In recent years, with the deepening of research on HBV, drugs targeting different targets of viral complex replication mechanisms and interactions with the host immune system have been developed. Drugs targeting viral complex replication mechanisms include HBV entry inhibitors, cccDNA formation and transcription inhibitors, gene editing targeting HBV DNA, HBV specific siRNAs, capsid inhibitors, polymerase inhibitors and HBV secretion inhibitors. Entry inhibitors have been used successfully in treating viral infections. Myrcludex B, a synthetic N-myristoylated lipopeptide derived from HBV preS1 protein, competes with the virus for binding to the solute carrier family 10 member 1 (SLC10A1 or NTCP). This agent prevents HBV and Hepatitis D virus (HDV) infection and in cells and animal models. cccDNA is the transcriptional template of HBV RNAs and is an ideal target for clearing HBV. DNA ligase inhibitors, which can inhibit cccDNA formation and are being developed as anticancer agents, may be tested for treatment of chronic HBV infection, if the side effects are tolerable. The genome editing tool, clustered regularly interspaced short palindromic repeats associated nuclease 9 (CRISPR/Cas9), has many innovative applications in different fields because of its potential for gene therapy. These have provided a clear proof of concept that this approach has the potential to treat or even cure patients with chronic HBV infection. RNA interference technology can be used to manipulate virus gene expression and may be developed for HBV treatment. HBV nucleocapsid formation and pgRNA packaging are critical steps of viral life cycle that may be targeted by antiviral agents. Inside the viral capsid, pgRNA is reverse transcribed into minus-strand DNA by the viral polymerase, with its terminal protein domain acting as a primer for initiation of reverse transcription. HBV polymerase is also a good drug target. Drugs targeting the host immune system include that stimulating the innate immune system, like IFNα, Toll-like receptor (TLR) agonists and stimulating cytoplasmic nucleic acid sensors, and that activating an adaptive immune response, like checkpoint inhibitors, T-cell redirection and therapeutic vaccines. IFN-based therapies are considered as immunomodulatory strategies. TLRs are expressed either at the cell surface or on endosome membranes, where they recognize molecular patterns associated with invading pathogens. Stimulation of TLR1, TLR2, TLR3, and TLR4 on hepatocytes has direct antiviral effects against HBV. The RNA helicases DExD/H-box helicase 58 (DDX58, also called RIG-I) and IFN induced with helicase C domain 1 (IFIH1, also called MDA5) are RNA sensors in the cytoplasm that induce production of cytokines by activating IRF3 and NF-kB. Activation of RIG-I induces an antiviral response against HBV. PD-1 is the inhibitory receptor expressed by most HBV specific T cells, so agents are in development to disrupt the interaction between PD-1 and its ligand, PD-L1. A therapeutic vaccine against HBV needs to overcome mechanisms of HBV-specific immune tolerance. Because natural resolution of HBV infection is accompanied by lifelong T-cell- and antibody-mediated immunity, these vaccines should induce HBV-specific B- and T-cell immune responses. Here, we review recent advances in development of directing-acting antiviral (DAA), host-targeting agents and immunomodulatory therapies. Some of which have entered clinical trials. We also discuss strategies for unbiased high-throughput screens to identify compounds that inhibit HBV and for repurposing existing drugs.