Peptide Approaches to Hepatitis B Cure Research

Peptide Antivirals and Hepatitis

140 pM IC50

Bulevirtide, a 47-amino-acid lipopeptide derived from the HBV preS1 domain, inhibits viral entry with a half-maximal inhibitory concentration of 140 picomolar.

Liu et al., Nature Communications, 2024

Liu et al., Nature Communications, 2024

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Chronic hepatitis B virus (HBV) infection affects an estimated 296 million people worldwide, causing roughly 820,000 deaths annually from cirrhosis and liver cancer. Current nucleos(t)ide analog therapy suppresses viral replication effectively but rarely achieves functional cure, defined as sustained loss of hepatitis B surface antigen (HBsAg) after stopping treatment. Fewer than 1% of patients on standard therapy achieve that endpoint annually. Peptide-based strategies are attacking this problem from multiple angles: blocking viral entry with lipopeptides, reactivating exhausted immune responses with therapeutic peptide vaccines, and modulating immunity with peptide immunomodulators like thymosin alpha-1. For context on how peptide entry inhibitors fit into the broader landscape of antiviral peptides, see our pillar article on bulevirtide and hepatitis B/D entry.

Why hepatitis B is so hard to cure

HBV establishes a persistent infection through a mechanism that sets it apart from most viruses. After entering hepatocytes, the viral genome converts into covalently closed circular DNA (cccDNA), a stable minichromosome that persists in the nucleus independently of viral replication. Current nucleos(t)ide analogs (tenofovir, entecavir) block the viral reverse transcriptase but do not touch cccDNA. As long as cccDNA survives, the virus can reactivate.^[1]

Functional cure requires two things simultaneously: silencing or eliminating cccDNA, and restoring HBV-specific immune responses that have been exhausted by years of chronic antigen exposure. Peptide-based approaches contribute to both goals through distinct mechanisms.

Blocking entry: bulevirtide and the NTCP receptor

The most clinically advanced peptide approach to HBV is bulevirtide (formerly Myrcludex B), a synthetic lipopeptide that prevents the virus from entering hepatocytes. Bulevirtide is a 47-amino-acid sequence derived from the pre-S1 domain of the HBV large surface protein, with a myristoyl group attached at the N-terminus.^[2]

The target is NTCP (sodium taurocholate co-transporting polypeptide), a bile salt transporter on hepatocyte surfaces that both HBV and HDV (hepatitis D virus) hijack for cell entry. Ni et al. identified NTCP as the functional receptor for HBV in 2014, a discovery that transformed the field by providing a druggable target for peptide-based entry inhibition.^[3]

In 2024, Liu et al. solved the cryo-EM structure of bulevirtide bound to human NTCP, revealing exactly how the peptide blocks viral entry. Bulevirtide forms two functional domains: a "plug" lodged deep in the bile salt transport tunnel of NTCP, and a "string" that covers the receptor's extracellular surface. The N-terminal myristoyl group anchors into the lipid-exposed surface of NTCP, explaining why the lipid modification is essential for antiviral activity. The IC50 for HBV/HDV inhibition is 140 picomolar in primary human hepatocytes.^[4]

Bulevirtide received conditional approval in the EU in 2020 for treatment of chronic hepatitis D, making it the first approved therapy specifically targeting HDV.^[2] Phase 3 data from the MYR301 trial showed that bulevirtide monotherapy at 2 mg/day achieved HDV RNA negativity in 47.8% of patients at week 144, with ALT normalization in 50.4% of treated patients.^[5] An integrated safety analysis across six clinical trials confirmed that bulevirtide monotherapy was well tolerated, with asymptomatic bile salt elevation as the most common finding, consistent with NTCP inhibition.^[6]

For HBV specifically, bulevirtide is being studied in combination with other agents. Entry inhibition alone cannot clear existing cccDNA from infected hepatocytes, but by blocking new infection of uninfected cells, bulevirtide may allow the immune system to gradually eliminate the infected cell population. This same entry-blocking strategy contrasts with the direct viral protease inhibition used in HCV protease inhibitor peptides.

Therapeutic peptide vaccines: reactivating exhausted immunity

Chronic HBV infection is characterized by T-cell exhaustion. The HBV-specific CD8+ cytotoxic T lymphocytes (CTLs) and CD4+ helper T cells that are critical for viral clearance become functionally impaired after prolonged antigen exposure. Therapeutic peptide vaccines attempt to reverse this exhaustion by presenting HBV epitopes in a context that can reboot immune recognition.

Theradigm-HBV: the first HBV peptide vaccine

The earliest clinical attempt was Theradigm-HBV (CY-1899), a lipopeptide vaccine consisting of three components: an HBV core antigen CTL epitope (HBcAg 18-27), a tetanus toxoid-derived T helper epitope (peptide 830-843), and two palmitic acid molecules to enhance uptake by antigen-presenting cells.^[7]

In a Phase 1 dose-escalation trial in 26 healthy volunteers, Vitiello et al. demonstrated that 500-microgram doses induced HBV-specific CTL responses comparable in magnitude to those seen in patients who naturally clear HBV infection. Five of five subjects at the highest dose developed measurable CTL activity. The CTL precursor frequencies were similar to influenza-specific memory responses, suggesting the vaccine could generate clinically meaningful immunity.^[7]

However, when tested in chronic HBV patients, the results were less encouraging. In a pilot trial of 90 chronically infected patients, CTL responses were low across all dose groups, and peak responses never exceeded 10 lytic units regardless of vaccine dose. The difference between healthy volunteers and chronic patients highlighted a fundamental challenge: the exhausted immune system in chronic HBV is not merely ignorant of viral antigens but actively suppressed. This challenge is not unique to HBV peptide vaccines; similar immune escape mechanisms affect peptide cancer vaccines.

Synthetic long peptide vaccines

More recent approaches have shifted from short peptide epitopes to synthetic long peptides (SLPs) of 25 to 35 amino acids. SLPs require processing by antigen-presenting cells before presentation on MHC molecules, which improves CD4+ and CD8+ T cell activation compared to minimal epitope peptides that can bind directly to MHC on any cell surface (potentially inducing tolerance rather than immunity).

ISA104, developed by ISA Pharmaceuticals, uses SLP technology incorporating multiple HBV-derived epitopes. Phase 1 data showed that the vaccine induced both CD4+ and CD8+ T cell responses in chronic HBV patients with suppressed viral loads on nucleos(t)ide therapy. The combination of antiviral suppression plus therapeutic vaccination represents a sequential approach: suppress the virus first, then vaccinate to restore immunity before attempting treatment withdrawal.

HepTcell

HepTcell (Altimmune) combined nine synthetic HBV peptides with IC31, a TLR9 agonist adjuvant. Phase 1 results in 60 HBeAg-negative patients on nucleos(t)ide therapy confirmed safety and demonstrated activation of HBV-specific cellular immune responses. A Phase 2 trial followed, but in June 2024, Altimmune discontinued the program after failing to meet efficacy endpoints. The program illustrates a recurring pattern in HBV therapeutic vaccine development: immunogenicity is achievable, but translating immune responses into functional cure remains difficult.

Peptide immunomodulators: thymosin alpha-1

Thymosin alpha-1 (Ta1), a 28-amino-acid peptide originally isolated from thymus tissue, has been used as an immunomodulator in chronic HBV treatment, primarily in Asia. Ta1 enhances dendritic cell maturation, natural killer cell activity, and T cell function through multiple pathways including toll-like receptor signaling.^[8]

Naylor et al. reviewed the clinical evidence for Ta1 in hepatitis B in the context of newer direct-acting antiviral strategies. In combination with entecavir, Ta1 increased HBeAg seroconversion rates to 35.1% compared to 16.2% for entecavir alone, a statistically significant difference that persisted through follow-up. While Ta1 alone did not achieve functional cure rates, its ability to enhance immune responses when combined with antiviral suppression positioned it as a potential component of combination cure strategies.^[8] For a deeper look at this peptide's mechanism, see our article on thymosin alpha-1 for post-viral immune recovery.

Combination strategies: the path to functional cure

The emerging consensus in HBV cure research is that no single agent will achieve functional cure alone. The most promising approaches combine multiple mechanisms:

Entry inhibition + immune restoration. Bulevirtide blocks new infections while therapeutic vaccines or checkpoint inhibitors restore the immune system's ability to clear infected cells. By preventing spread to new hepatocytes, entry inhibitors create a declining population of infected cells that the immune system can manage.

Antiviral suppression + therapeutic vaccination. Nucleos(t)ide analogs reduce viral load and antigen levels, partially reversing immune exhaustion. Therapeutic peptide vaccines administered during viral suppression may encounter a more responsive immune environment than vaccination during active viremia.

Direct cccDNA targeting + peptide immunomodulation. Emerging technologies like siRNA (bepirovirsen) or CRISPR-based approaches that directly target cccDNA could be combined with peptide immunomodulators like thymosin alpha-1 to both eliminate the viral reservoir and sustain immune control after treatment withdrawal.

Current limitations of peptide approaches

Bulevirtide's scope. Entry inhibition prevents new infection but cannot eliminate cccDNA already present in hepatocytes. In HBV monoinfection (without HDV), bulevirtide alone is unlikely to achieve functional cure. Its primary approved use remains HDV, where it has clearer standalone efficacy.^[9]

Vaccine immunogenicity vs. efficacy. Every HBV therapeutic peptide vaccine tested has demonstrated immunogenicity (the ability to generate measurable immune responses). None has yet demonstrated that these immune responses translate into HBsAg loss at rates superior to current therapy. The gap between immune activation and functional cure remains the central problem.

HLA restriction. Peptide vaccines targeting specific MHC-restricted epitopes work only in patients expressing the relevant HLA alleles. Theradigm-HBV was restricted to HLA-A2+ individuals, limiting the eligible patient population to roughly 40-50% of patients in most ethnic groups. Multi-epitope vaccines like HepTcell attempt to address this, but broader coverage adds manufacturing complexity.

T-cell exhaustion is deep. Chronic HBV infection produces multiple layers of immune suppression: T-cell exhaustion, regulatory T-cell expansion, myeloid-derived suppressor cell accumulation, and hepatic immune tolerance. Peptide vaccines alone may not overcome all of these barriers, which is why combination with checkpoint inhibitors or other immunomodulators is being explored.

Delivery route matters. Most peptide vaccines require injection, but the mucosal immune system in the liver is distinct from systemic immunity. Nasal formulations like NASVAC (containing both HBsAg and HBcAg) have shown promise in early trials, suggesting that mucosal delivery of HBV antigens may engage liver-resident immune populations more effectively.

The Bottom Line

Peptide-based approaches to hepatitis B cure span three categories: entry inhibitors (bulevirtide, approved for HDV with IC50 of 140 pM), therapeutic peptide vaccines (Theradigm-HBV, HepTcell, ISA104), and immunomodulators (thymosin alpha-1). Bulevirtide has the strongest clinical evidence, with Phase 3 data showing HDV RNA negativity in nearly half of treated patients. Therapeutic peptide vaccines have consistently demonstrated immunogenicity but have not yet translated that into functional cure. The consensus direction is combination therapy targeting multiple aspects of the virus-host interaction simultaneously.

Frequently Asked Questions

Can hepatitis B be cured with peptides?

No peptide-based therapy has achieved hepatitis B functional cure as a standalone treatment. Bulevirtide, a 47-amino-acid lipopeptide, blocks viral entry and is approved for hepatitis D but is being studied in HBV combination regimens. Therapeutic peptide vaccines have shown the ability to generate immune responses against HBV but have not yet demonstrated HBsAg loss rates superior to existing therapy. Combination approaches pairing peptides with other drug classes are the most actively pursued strategy.

What is bulevirtide and how does it work?

Bulevirtide is a synthetic lipopeptide of 47 amino acids derived from the pre-S1 domain of the hepatitis B virus surface protein. It blocks HBV and HDV entry into liver cells by binding to the NTCP receptor on hepatocyte surfaces with an IC50 of 140 picomolar. It received conditional EU approval in 2020 for chronic hepatitis D treatment. The peptide works by lodging into the bile salt transport tunnel of NTCP and covering the receptor's surface, physically preventing viral attachment.

What is a therapeutic hepatitis B vaccine?

A therapeutic hepatitis B vaccine is designed to treat existing chronic infection, unlike preventive vaccines given before exposure. These vaccines present HBV protein fragments (peptide epitopes) to the immune system to reactivate T cells that have become exhausted during chronic infection. Several peptide-based therapeutic vaccines have been tested clinically, including Theradigm-HBV (a lipopeptide) and HepTcell (a multi-peptide formulation), but none has achieved functional cure in clinical trials.

What is functional cure for hepatitis B?

Functional cure for hepatitis B is defined as sustained loss of hepatitis B surface antigen (HBsAg) in blood, with undetectable HBV DNA, maintained for at least 24 weeks after stopping all treatment. This does not mean complete viral eradication because cccDNA may persist in hepatocyte nuclei at low levels. Fewer than 1% of patients on current nucleos(t)ide analog therapy achieve functional cure annually.

Does thymosin alpha-1 help with hepatitis B?

Thymosin alpha-1, a 28-amino-acid immunomodulatory peptide, has been studied as an adjunct to standard hepatitis B treatment. When combined with entecavir, it increased HBeAg seroconversion rates to 35.1% versus 16.2% for entecavir alone. It works by enhancing dendritic cell maturation and T cell function rather than directly targeting the virus. Its role is as a combination therapy component, not a standalone cure.

Why is hepatitis B so hard to cure?

Hepatitis B persists because the virus converts its genome into covalently closed circular DNA (cccDNA), a stable minichromosome in the hepatocyte nucleus that current antiviral drugs cannot reach. Additionally, chronic infection drives deep T-cell exhaustion, suppressing the immune responses needed to clear infected cells. Curing HBV requires both eliminating or silencing cccDNA and restoring functional HBV-specific immunity, which is why combination strategies are considered necessary.