FDA approves Hibsago, a first-of-its-kind antisense drug for hepatitis B

The Food and Drug Administration approved Hibsago, GlaxoSmithKline's bepirovirsen, on 20 October — the first antisense oligonucleotide cleared for chronic hepatitis B, and a new molecular entity that went through priority review [s1][s2]. It is a full approval, not an accelerated one [s1].

Chronic hepatitis B is usually controlled rather than cured. The standard nucleoside and nucleotide pills — collectively, NrtI therapy — suppress the virus for as long as a patient keeps taking them, often for life, but rarely clear it. Bepirovirsen takes a different approach: it is an antisense oligonucleotide, a short strand of engineered genetic material that binds the virus's messenger RNA and blocks production of viral proteins, including the surface antigen used to track the infection [s1]. The ambition behind that mechanism is a "functional cure" — sustained loss of the virus after a finite course of treatment.

Who the label is for

The indication is tightly drawn. Hibsago is approved in combination with NrtI therapy for adults with chronic HBV without cirrhosis, who have been on NrtI for at least six months with suppressed HBV DNA, and whose hepatitis B surface antigen (HBsAg) is at or below 3,000 IU/mL on an FDA-authorised test [s1]. This is a population already doing well on antivirals, with a low viral-protein burden — not a treatment for newly diagnosed or advanced disease.

The course is fixed and the schedule demanding. Hibsago is given by subcutaneous injection, 300 mg as two 150 mg shots, twice a week for the first two weeks and then once a week through Week 24 [s1]. It is always taken alongside the patient's existing antiviral.

What the trials found

Approval rests on two identically designed Phase 3 trials, B-Well 1 (NCT05630807) and B-Well 2 (NCT05630820), each randomising participants 2:1 to bepirovirsen or placebo on top of their background antiviral [s1]. B-Well 1 enrolled 981 and B-Well 2 enrolled 857; both populations were about 70% male, mostly from Asia and Europe, with a mean age around 50 and the great majority HBeAg-negative at baseline [s1].

The trial design is the key to reading the result. Participants took the drug or placebo for 24 weeks, continued their antiviral alone to Week 48, and then — if they met strict criteria, including no cirrhosis and undetectable virus — stopped the antiviral too [s1]. The primary endpoint was assessed at Week 72, a full 24 weeks after stopping all hepatitis B treatment: sustained suppression of HBV DNA with surface antigen no longer detectable [s1].

On that measure, both trials were positive. Among participants with baseline HBsAg at or below 3,000 IU/mL, 20% on Hibsago reached the endpoint in B-Well 1 and 19% in B-Well 2, against 0% of the hundreds on placebo in each trial — common rate differences of 17 and 13 percentage points, both highly significant [s1]. In the subgroup starting with HBsAg at or below 1,000 IU/mL, the response rates were higher, 25% and 28% [s1]. Response was lower in people with higher baseline surface antigen, those who were HBeAg-positive, and those aged 65 or older [s1].

The honest frame is in those numbers. A functional cure off all treatment, with none of the placebo patients achieving it, is a genuine first for a finite course. But roughly four in five treated patients — in an already-selected, well-controlled group — did not reach it. A label note points to an early signal: patients whose surface antigen fell below 1 IU/mL by treatment Week 12 did better, with 34% of them (203 of 601) reaching the primary endpoint [s1].

Safety and the catches

Hibsago is contraindicated in anyone with acute hepatic impairment or any cirrhosis, and in patients whose platelet count is below 100 × 10⁹/L before starting [s1]. The warnings are substantial: worsening kidney function, requiring eGFR monitoring; serious inflammatory and immune reactions, including vasculitis, for which the drug must be stopped permanently; and a serious injection-site reaction tied to improper technique, so the label stresses rotating sites and separating the two injections by at least two inches [s1]. Because stopping antiviral therapy can trigger a hepatitis flare, frequent monitoring is required after treatment — and people with HIV-HBV coinfection must not stop their antiviral unless an HIV clinician directs it [s1]. The most common adverse reactions, each at 10% or more, were injection-site reactions, fever, fatigue and headache [s1].

Red flags for patients: severe fatigue with yellowing of the eyes or skin, dark urine or right-upper-abdominal pain after finishing treatment can signal a hepatitis flare and warrant urgent testing; new rashes, joint pain or other signs of an immune reaction should be reported promptly.

This article describes an FDA approval and its labelling and is not medical advice. Decisions about hepatitis B treatment, and about stopping any antiviral, belong with a specialist.