Toward a new era of hepatitis B virus therapeutics: The pursuit of a functional cure

Abstract

Hepatitis B virus (HBV) infection, although preventable by vaccination, remains a global health problem and a major cause of chronic liver disease. Although current treatment strategies suppress viral replication very efficiently, the optimal endpoint of hepatitis B surface antigen (HBsAg) clearance is rarely achieved. Moreover, the thorny problems of persistent chromatin-like covalently closed circular DNA and the presence of integrated HBV DNA in the host genome are ignored. Therefore, the scientific community has focused on developing innovative therapeutic approaches to achieve a functional cure of HBV, defined as undetectable HBV DNA and HBsAg loss over a limited treatment period. A deeper understanding of the HBV life cycle has led to the introduction of novel direct-acting antivirals that exert their function through multiple mechanisms, including inhibition of viral entry, transcriptional silencing, epigenetic manipulation, interference with capsid assembly, and disruption of HBsAg release. In parallel, another category of new drugs aims to restore dysregulated immune function in chronic hepatitis B accompanied by lethargic cellular and humoral responses. Stimulation of innate immunity by pattern-recognition receptor agonists leads to upregulation of antiviral cytokine expression and appears to contribute to HBV containment. Immune checkpoint inhibitors and adoptive transfer of genetically engineered T cells are breakthrough technologies currently being explored that may elicit potent HBV-specific T-cell responses. In addition, several clinical trials are attempting to clarify the role of therapeutic vaccination in this setting. Ultimately, it is increasingly recognized that elimination of HBV requires a treatment regimen based on a combination of multiple drugs. This review describes the rationale for progressive therapeutic interventions and discusses the latest findings in the field of HBV therapeutics.

Keywords: Chronic hepatitis B; Direct-acting antivirals; Functional cure; Gene silencing; Immunotherapy; Therapeutic vaccination.

Figure 1

Schematic representation of the hepatitis B virus life cycle and the targets of direct-acting antivirals. Viral entry can be prevented, either by disrupting viral interaction with heparan sulfate proteoglycans or by inhibiting high-affinity binding to the sodium taurocholate cotransporting polypeptide receptor. Strategies targeting covalently closed circular DNA (cccDNA) include: Prevention of cccDNA formation, cccDNA degradation or destabilization, gene editing tools to cause sequence-specific damage, and epigenetic manipulations to functionally silence cccDNA. Inhibition of the interplay between hepatitis B virus (HBV) X protein and host proteins leads to transcriptional silencing. Therapeutics based on RNA interference target viral transcripts and block HBV protein expression. Nucleoside or nucleotide drugs are approved inhibitors of reverse transcriptase. Core protein allosteric modulators interfere with the kinetics of nucleocapsid assembly/disassembly and can affect the various functions of the core protein. Hepatitis B surface antigen (HBsAg) release inhibitors limit the circulating HBsAg load. cccDNA: Covalently closed circular DNA; CpAMs: Core protein allosteric modulators; ER: Endoplasmic reticulum; ESCRT: Endosomal sorting complexes required for transport; HBeAg: Hepatitis B e antigen; HBsAg: Hepatitis B surface antigen; HBV: Hepatitis B virus; HBx: Hepatitis B virus X protein; HSPG: Heparan sulfate proteoglycans; NTCP: Sodium taurocholate cotransporting polypeptide; NUCs: Nucleot(s)ide analogues; rcDNA: Relaxed circular DNA.

Figure 2

Immunotherapeutic interventions to revive host immunity in chronic hepatitis B virus infection. Pattern-recognition receptors (PRRs), including toll-like receptors, retinoic acid-inducible gene-I-like receptors, nucleotide-binding oligomerization domain-like receptors, stimulator of interferon genes, are key players in innate immunity and the first line of defense that recognizes pathogen-associated molecular patterns/damage-associated molecular patterns. Activation of PRRs by their corresponding agonists triggers transduction signals and transcription factors [nuclear factor-κB, interferon regulatory factor (IRF) 3, IRF 7, signal transducer and activator of transcription 6], which in turn upregulate type interferons, inflammatory cytokines and chemokines, leading to well-orchestrated immune cell differentiation. Immune checkpoint inhibitors aim to restore T-cell function by inhibiting negative regulators of T-cell activation (programmed cell death protein 1, cytotoxic T-lymphocyte-associated protein 4, T-cell immunoglobulin and mucin domain-3). Adoptive transfer of genetically engineered T -cells is an alternative strategy to elicit potent hepatitis B virus - specific T-cell responses. The role of therapeutic vaccination in overcoming exhausted cellular and humoral responses is currently under investigation. An efficient vaccine repairs function and induces antigen- presenting cells to activate the two arms of adaptive immunity: polyclonal and multispecific CD4+ and CD8+ T-cell responses, and well-regulated B cells that differentiate into plasma cells and secrete neutralizing antibodies. IFNs: Interferons; IRF: Interferon regulatory factor; NF-κΒ: Nuclear factor-κB; NLRs: Nucleotide-binding oligomerization domain-like receptors; PAMPS: Pathogen-associated molecular patterns; PD-1: Programmed cell death protein 1; PRRs: Pattern-recognition receptors; RLRs: Retinoic acid-inducible gene-I-like receptors; STAT6: Signal transducer and activator of transcription 6; STING: Stimulator of interferon genes; TIM3: T-cell immunoglobulin and mucin domain-3; TLRs: Toll-like receptors.