The Evidence Pack: How a Novel Antisense Oligonucleotide Achieves a Functional Cure in Chronic Hepatitis B

For decades, the standard of care for chronic hepatitis B has been a lifelong commitment to daily pills that suppress the virus but rarely eliminate it. Now, peer-reviewed data from two massive Phase 3 clinical trials suggest a finite, curative approach is within reach. Published in The New England Journal of Medicine, the B-Well 1 and B-Well 2 trials demonstrated that an investigational antisense oligonucleotide called bepirovirsen achieved a "functional cure" in up to 26% of selected patients.[1][8]

The results, simultaneously presented at the European Association for the Study of the Liver (EASL) congress, represent a watershed moment in hepatology. Across the two global trials encompassing over 1,800 patients in 29 countries, a 24-week course of bepirovirsen resulted in a 19% functional cure rate in the overall study population. In the placebo groups, the cure rate was exactly zero.[1][2][4][5][7]

The efficacy was even more pronounced in a specific subgroup of patients. Among participants who entered the trial with lower baseline levels of hepatitis B surface antigen (HBsAg)—specifically, 1,000 IU/mL or less—the functional cure rate climbed to 26%. This subgroup is not a niche population; it represents approximately 45% of all diagnosed chronic hepatitis B cases globally, making the 26% clearance rate highly relevant to real-world clinical practice.[1][2][3][4]

To understand the magnitude of this shift, it is necessary to examine the current standard of care. Today, patients are typically prescribed nucleoside or nucleotide analogues (NAs). These daily oral medications are highly effective at suppressing viral replication, keeping the amount of circulating virus low. However, they almost never clear the virus entirely; functional cure rates on NAs hover between 1% and 5%. Consequently, patients must remain on the drugs for life to prevent viral rebound and liver damage.[3][4][6]

Chronic hepatitis B is not a marginal public health issue. The virus affects an estimated 240 to 300 million people worldwide. Because the virus chronically inflames the liver, it is a primary driver of cirrhosis and accounts for more than half of all global liver cancer cases. Shifting from lifelong viral suppression to a finite, curative treatment could fundamentally alter the trajectory of the disease for millions.[2][4][5][7]

The medical community defines a "functional cure" through strict virological criteria. It requires the sustained loss of detectable hepatitis B surface antigen (HBsAg) and undetectable hepatitis B virus (HBV) DNA in the blood for at least six months after all treatment has been stopped. Achieving this state indicates that the patient's own immune system has successfully regained control over the virus without the need for ongoing medication.[1][2][4][5]

The clinical stakes of achieving a functional cure are profound. According to epidemiological data, the sustained loss of HBsAg is associated with an 89% reduction in the risk of developing liver cancer. Furthermore, it corresponds to a 62% reduction in the risk of all-cause mortality. By clearing the antigen, the therapy effectively halts the chronic inflammatory cycle that leads to end-stage liver disease.[2][7][8]

Bepirovirsen achieves these results through a novel mechanism of action. It is an antisense oligonucleotide (ASO)—a synthetic strand of nucleic acids designed to bind precisely to the messenger RNA (mRNA) of the hepatitis B virus. Once bound, it triggers the degradation of the viral RNA, effectively shutting down the virus's ability to manufacture viral proteins and surface antigens.[2][5][6]

This mechanism addresses a critical blind spot in traditional NA therapies. While NAs stop the virus from copying its DNA, they do not stop the infected liver cells from pumping out viral surface antigens. These circulating antigens act as a decoy, overwhelming and exhausting the patient's immune system so that it cannot mount an effective response against the infected cells.[4][8]

By drastically reducing the production of these antigens, bepirovirsen removes the immunological smokescreen. The therapy not only starves the virus of its necessary proteins but also stimulates the host's immune system, allowing it to "wake up" and clear the remaining infected cells. This dual action—antiviral suppression combined with immune reactivation—is what enables the sustained functional cure after the drug is discontinued.[2][6]

It is important to distinguish a "functional cure" from a "sterilizing cure." Hepatitis B is notoriously difficult to eradicate entirely because it hides a specialized form of genetic material, called covalently closed circular DNA (cccDNA), deep inside the nucleus of the liver cells. Bepirovirsen does not eliminate this hidden cccDNA reservoir. Instead, it suppresses the virus so thoroughly that the immune system can keep the cccDNA permanently dormant, mirroring the state of a patient who naturally cleared an acute infection.[6][8]

Beyond the primary endpoint, the B-Well trials yielded promising exploratory data. Nearly half (49%) of the patients treated with bepirovirsen achieved HBsAg levels of 100 IU/mL or less one year after completing treatment. Hepatologists recognize this specific threshold as a strong marker for improved long-term immune control, suggesting that even patients who missed the strict criteria for a functional cure still derived significant, durable immunological benefits from the finite treatment course.[2][4]

The safety profile of the drug was broadly consistent across both massive trials. While adverse events were reported, serious adverse events were relatively low. Notably, researchers observed that patients who experienced significant increases in alanine aminotransferase (ALT)—an enzyme that spikes during liver inflammation—were actually more likely to achieve a cure. In the context of this therapy, an ALT flare often signals that the immune system is actively destroying infected liver cells, a necessary step for viral clearance.[6][7][8]

The regulatory pathway for bepirovirsen is already in motion. The U.S. Food and Drug Administration (FDA) has granted the therapy both Fast Track and Breakthrough Therapy designations, recognizing the urgent unmet medical need for a finite HBV treatment. GSK has submitted applications to regulatory agencies in the US, Europe, Japan, and China, with the first decisions anticipated in the third quarter of 2026. If approved, industry analysts project the drug could reach peak global sales of $1.5 billion.[3][4][5]

Looking ahead, the 26% cure rate in the low-antigen subgroup provides a clear roadmap for future treatment protocols. Researchers are already exploring combination therapies—such as pairing bepirovirsen with small interfering RNA (siRNA) drugs—designed to artificially lower a patient's antigen levels into the "sweet spot" before administering the ASO. By driving down the viral load first, clinicians hope to expand the 26% functional cure rate to a much broader swath of the 240 million people living with the virus today.[3][8]