The Food and Drug Administration approved the first antisense treatment for chronic hepatitis B on Tuesday after two Phase 3 trials found that 19% of selected patients maintained undetectable viral DNA and hepatitis B surface antigen for 24 weeks after stopping all treatment. The new drug, Hibsago, or bepirovirsen, creates a finite treatment path for some adults who otherwise may remain on antiviral pills indefinitely, according to the FDA decision and the pivotal trial report.

The approval is narrower than the headline idea of a hepatitis B cure. Hibsago is indicated only for adults without cirrhosis who have taken nucleos(t)ide analogue therapy for at least six months, already have suppressed hepatitis B virus DNA, and have a hepatitis B surface antigen level no higher than 3,000 international units per milliliter on an authorized test. It is added to existing therapy for 24 weeks; only patients meeting laboratory and clinical criteria may later stop the background antiviral drug.

Even within that selected population, about four in five Hibsago recipients did not reach the trial’s functional-cure endpoint. The drug also carries boxed warnings for potentially serious liver reactions and thrombocytopenia, a decline in platelets that can cause life-threatening bleeding. The approval therefore represents a meaningful shift in what is possible for chronic hepatitis B, but not a universal or risk-free replacement for long-term suppression.

A finite treatment changes the goal

Standard nucleos(t)ide analogues such as tenofovir and entecavir suppress viral replication effectively and reduce the risk of cirrhosis and liver cancer, but they rarely remove hepatitis B surface antigen, or HBsAg, from the blood. Because the virus can persist in liver cells even when circulating DNA is controlled, treatment often continues for years. The 2025 AASLD guideline conditionally advises against withdrawing these drugs until HBsAg loss, reflecting the danger of viral rebound and liver inflammation after therapy ends.

Hibsago changes the strategy by targeting viral RNA, the genetic messages used to make viral proteins. As an antisense oligonucleotide, it binds to complementary RNA and promotes its degradation, lowering both viral replication and production of HBsAg. The manufacturer says the reduction in antigen may also help the immune system regain control, but that immune contribution is harder to isolate clinically from the drug’s direct molecular effect.

A functional cure is not the same as eradicating every trace of hepatitis B from the liver. In the trials it meant undetectable HBV DNA and HBsAg for at least 24 weeks after all treatment stopped. That endpoint is clinically important because HBsAg loss is associated with better long-term outcomes, but the trials were not designed or followed long enough to show reductions in liver failure, liver cancer or death. Nor do 24 weeks of treatment-free control establish lifelong durability.

What the Phase 3 trials found

B-Well 1 and B-Well 2 were replicate, randomized, double-blind, placebo-controlled trials conducted across 29 countries. They enrolled 1,838 adults with noncirrhotic chronic hepatitis B who were already receiving stable nucleos(t)ide analogue therapy and had HBsAg levels of 3,000 IU/mL or less. The study designs are recorded separately for B-Well 1 and B-Well 2; both were funded by GSK, and company scientists participated in the research and writing.

Participants received weekly injections of bepirovirsen or placebo for 24 weeks while continuing their existing antiviral medication. They remained on background therapy for another 24 weeks. At week 48, those who had achieved the required virologic and antigen response could stop the oral drug and enter a treatment-free observation period.

At week 72, functional cure occurred in 127 of 650 bepirovirsen recipients, or 20%, in B-Well 1, compared with none of 328 placebo recipients. B-Well 2 produced a closely matching result: 106 of 570 recipients, or 19%, reached the endpoint, compared with none of 286 controls. The replication across two large trials strengthens confidence that the difference was caused by treatment rather than chance. An independent CIDRAP review also noted that 24% of treated participants qualified to stop background therapy, although not all maintained the full endpoint afterward.

Baseline antigen level was an important predictor. Functional cure reached 25% and 28% in the two trials among patients starting at or below 1,000 IU/mL, but fell to 10% and 5% among those between 1,000 and 3,000 IU/mL. That gradient suggests the authorized HBsAg test is not simply administrative screening: it identifies a biologically more responsive subgroup and will influence how clinicians discuss the likelihood of benefit.

Safety monitoring is central to treatment

The efficacy results came with clinically significant toxicity. In the pooled trial analysis, adverse events occurred in 91% of bepirovirsen recipients and 73% of placebo recipients; serious adverse events occurred in 7% and 4%, respectively. Grade 3 or higher events were reported in 16% of treated patients and 3% of controls, with elevated alanine aminotransferase, a marker of liver injury or inflammation, the most frequent severe laboratory event.

The prescribing information requires liver tests before treatment and frequent monitoring during therapy because hepatic flares may reach more than 20 to 40 times the upper limit of normal. Platelet counts must be checked at least weekly, and the drug is contraindicated when the pretreatment count is below 100 billion per liter. Monitoring is also required for declining kidney function and inflammatory or immune reactions, including vasculitis.

Stopping background antiviral therapy introduces another risk. Severe hepatitis exacerbations can occur after anti-HBV treatment ends, so the label calls for frequent follow-up for at least a year after discontinuation. Patients treated for both HIV and hepatitis B must not stop a shared antiviral regimen without direction from an HIV clinician, and Hibsago is not recommended with hepatitis D coinfection because liver inflammation may be amplified. These safeguards make the treatment a specialist-managed pathway rather than a simple six-month substitution.

The eligible population remains limited

The trials excluded several groups with high medical need, including people with cirrhosis and those with HBsAg levels above 3,000 IU/mL. The FDA indication also does not cover children, and available data are insufficient for pregnancy, breastfeeding, moderate or severe kidney impairment and end-stage kidney disease. Longer follow-up is needed to determine how often the functional-cure response persists beyond the trial window and whether viral control translates into fewer cancers or liver-related deaths.

Those limitations matter because hepatitis B remains a major global disease. The WHO estimates that chronic infection caused about 1.1 million deaths in 2024, mostly from cirrhosis and hepatocellular carcinoma. In the United States, the CDC reports marked disparities in hepatitis B mortality, with 2023 death rates among Asian and Pacific Islander people more than eight times the rate among White people and rates among Black people nearly three times as high.

Access will also depend on more than drug authorization. Patients need quantitative HBsAg testing to establish eligibility, regular laboratory monitoring during therapy, and reliable follow-up after treatment withdrawal. The FDA simultaneously authorized a Roche quantitative HBsAg companion diagnostic for selection and monitoring. Pricing and the timing of broad U.S. availability were not disclosed when the approval was announced, Reuters reported, leaving insurers and health systems without enough information to assess cost-effectiveness or coverage requirements.

What the approval establishes

Hibsago establishes that a finite antisense regimen can produce sustained treatment-free viral control in a reproducible minority of carefully selected adults. That is a clinical outcome beyond simple drug adoption or short-term viral suppression, and the two placebo-controlled trials provide stronger evidence than a single uncontrolled study would. The synchronized results also offer a measurable way to identify patients most likely to benefit.

The approval does not establish eradication of hepatitis B, benefit for patients with cirrhosis or high antigen levels, or protection against liver cancer and mortality. It also leaves a demanding safety and monitoring burden. The next evidence will need to show how durable the response remains over several years, whether real-world patients complete monitoring safely, how payers handle the drug and required testing, and whether future combinations can extend functional cure beyond the roughly one in five patients reached in the pivotal trials.