First Clinical Evidence of Gene Editing, Epigenetic Silencing of HBV cccDNA

Two late-breaker posters presented at the European Association for the Study of the Liver (EASL) Congress 2026 provided the first clinical evidence that both epigenetic silencing and gene editing can directly target and suppress covalently closed circular DNA (cccDNA), the viral reservoir that has made hepatitis B virus (HBV) so difficult to cure.

Epigenetic Silencing of HBV From the Inside Out

Tune-401 (Tune Therapeutics) takes an approach to HBV fundamentally different from any currently approved therapy.¹ Rather than suppressing viral replication or degrading viral RNA, Tune-401 is designed to epigenetically silence the HBV genome itself, introducing repressive chromatin modifications directly onto cccDNA and integrated HBV DNA sequences to shut down their transcriptional activity without altering the underlying DNA sequence. The therapeutic cargo, messenger RNA (mRNA) encoding an epigenetic silencing protein together with a guide RNA targeting the HBV genome, is delivered intravenously to hepatocytes via lipid nanoparticles (LNPs), a delivery platform with established clinical precedent from mRNA vaccine and RNA interference programs.

Twenty-one patients were enrolled across 4 single-dose cohorts (0.2, 0.45, 0.65, and 0.85 mg/kg; n = 2/6/3/3) and 1 multidose cohort receiving 3 doses of 0.65 mg/kg at approximately 4-week intervals (n = 7). The enrolled population included 12 hepatitis B e-antigen (HBeAg)–negative and 9 HBeAg-positive participants. Primary end points were safety and tolerability, with antiviral pharmacodynamic assessment via quantitative serum hepatitis B surface antigen (HBsAg), pregenomic RNA (pgRNA), HBeAg, and hepatitis B core-related antigen (HBcrAg).

The antiviral results were striking and dose-dependent. At 20 weeks following a single dose, mean HBsAg reductions from predose baseline were 29.6% in the 0.45 mg/kg cohort and 54.0% in the 0.65 mg/kg cohort, substantial reductions achieved without any change in NA backbone and persisting months after a single administration.

Additionally, multiple patients achieved durable reductions of more than 95% in pgRNA levels and more than 99% in HBcrAg levels, indicating profound and sustained cccDNA silencing. Three of 10 patients in the 0.65 mg/kg cohorts achieved complete pgRNA and HBcrAg biomarker negativity, an unprecedented finding with a single-agent, single-dose approach. In the 0.85 mg/kg single-dose cohort, 2 patients achieved HBeAg loss, with 1 progressing to seroconversion.

Preliminary results from the multidose cohort indicated that additional doses increased antiviral efficacy without any new safety signals, suggesting that the silencing effect is additive and titratable.

Tune-401 was generally safe and well tolerated across all cohorts. The most common adverse events were infusion-related reactions, with grade 1, 2, and 3 events occurring in 14, 6, and 2 patients, respectively. Transient alanine aminotransferase/aspartate aminotransferase elevations were predominantly mild, with grade 2 and 3 events in 5 and 3 patients, respectively. Most adverse events were transient and resolved within 2 weeks.

Biopsy Confirmation of cccDNA Elimination and Inactivation

PBGENE-HBV (Precision BioSciences) approaches the cccDNA problem through permanent gene editing using the company's ARCUS nuclease platform, a single-protein gene editing system that differs from CRISPR-Cas9 in its compact size, simpler structure, and potential for more precisely defined cuts at a single target site. PBGENE-HBV received FDA Fast Track Designation in April 2025, reflecting the agency's recognition of the unmet need and the program's potential.²

The late-breaker poster reported liver biopsy data from 2 voluntary participants in cohort 2 (0.4 mg/kg) of the ongoing phase 1 ELIMINATE-B trial.³ In the first participant, paired biopsies were collected before treatment and after the second dose. In the second participant, a posttreatment biopsy only was collected after the third and final dose. Both short- and long-read sequencing techniques were applied to evaluate editing of cccDNA and integrated HBV DNA at the DNA and RNA level.

The biopsy data from participant 1 provided the first paired, pre- and posttreatment evidence of cccDNA editing and elimination in a human patient. Analysis showed a 10-fold (1-_log) decrease in the percentage of cccDNA-derived RNA transcripts after 2 administrations of PBGENE-HBV, directly supporting cccDNA elimination. Among the 0.16% of cccDNA-derived transcripts that remained, edits were present in 23% of sequences, consistent with inactivation of residual cccDNA that had not been eliminated outright.

Participant 2, whose posttreatment biopsy was collected after 3 doses, showed edits in 80% of remaining cccDNA transcripts, demonstrating a cumulative effect of repeat administrations. This patient also had detectable pgRNA in blood at baseline that became and remained undetectable after dosing, with a corresponding absence of pgRNA in the biopsy tissue, confirming that the molecular editing observed in the liver translated into functional viral suppression.

Sequencing confirmed editing at both the DNA and RNA levels with a high degree of concordance across both participants, and analysis of HBsAg transcripts confirmed that residual surface antigen was expressed predominantly from integrated HBV DNA rather than cccDNA, a meaningful finding indicating that the active, episomal viral reservoir had been effectively silenced or eliminated. Durable HBsAg declines in the blood were observed in both participants 9 months after initial dosing.

Taken together, these 2 late-breaker presentations represent a meaningful inflection point for the chronic HPV field. What both posters share is a common and historic claim: direct, biopsy-confirmed evidence that the cccDNA reservoir—long considered untouchable—can now be targeted and suppressed in human patients.

References

1. Gane EJ, Jucov A, Congdon K, et al. Tune-401: a first-in-class epigenetic silencer of HBV demonstrates deep and durable antiviral activity in the phase 1b/2a proof of concept Tune-401-001 study. Presented at: EASL Congress; May 27-30; Barcelona, Spain.
2. Precision BioSciences receives US FDA Fast Track Designation for PBGENE-HBV, a first-in-class gene editing therapy designed to eliminate the root cause of chronic hepatitis B. Precision Biosciences. News release. April 15, 2025. Accessed June 5, 2026. https://investor.precisionbiosciences.com/news-releases/news-release-details/precision-biosciences-receives-us-fda-fast-track-designation
3. Yuen MF, Jucov A, Harrison E, et al. First evidence of elimination and inactivation of cccDNA in liver biopsies collected from patients with chronic hepatitis B treated with PBGENE-HBV. Presented at: EASL Congress; May 27-30; Barcelona, Spain.