Chronic hepatitis B is one of those diseases that quietly grinds people down. It's a long-term liver infection that raises the risk of cirrhosis and liver cancer, and for decades the standard playbook has been antiviral pills that suppress the virus without ever really evicting it. On Tuesday, the U.S. Food and Drug Administration approved something new for that fight: GSK plc (GSK)'s Hibsago (bepirovirsen) injection, cleared for adult chronic hepatitis B patients without cirrhosis.

A First-of-Its-Kind Mechanism

Hibsago is a first-in-class antisense oligonucleotide, which is a fancy way of saying it disables the genetic instructions the hepatitis B virus uses to replicate and churn out viral proteins. Fewer instructions means lower circulating virus and antigen levels. It's given as a weekly subcutaneous injection.

The approval came under FDA priority review and Breakthrough Therapy Designation. The label covers adults who have received antiviral therapy for at least six months, maintain suppressed viral DNA, and show surface antigen (HBsAg) levels of 3,000 IU/mL or less. Regulators also cleared the Roche Elecsys HBsAg II quant II RxDx assay as a companion diagnostic to help select and monitor patients.

The Numbers Behind the Green Light

The approval rests on data from the B-Well 1 and B-Well 2 phase 3 trials. Across those studies, a 24-week Hibsago regimen alongside existing antiviral medicine let 24% of participants stop their baseline medication. In the placebo group, that number was zero.

Twenty-four weeks after halting all therapies, 19% of Hibsago patients (233 of 1,220) sustained a functional cure, meaning viral DNA and HBsAg remained undetectable. Placebo achieved zero responses, while standard care alone typically achieves less than 1% after one year.

Among participants with baseline antigen levels at or below 1,000 IU/mL, a group representing about 45% of global cases, functional cure rates reached 26% (200 of 768). Exploratory data at week 72 showed 49% of overall participants reduced surface antigen levels to 100 IU/mL or less.

The Fine Print

This is not a risk-free wonder drug. Hibsago carries a boxed warning for serious liver reactions and low platelet counts that increase bleeding risks. Regulators also flagged kidney decline, inflammatory reactions like vasculitis, and risks tied to improper administration.

GSK Price Action: GSK shares were down 0.75% at $50.25 at the time of publication on Wednesday, according to market data.