LONDON, June 25 (Reuters) - An interim analysis of GSK’s experimental hepatitis B therapy bepirovirsen in a mid-stage study has raised hopes for a functional cure for millions suffering from a chronic infection, the British drugmaker said on Saturday.

If the results are reinforced in later-stage trials, the British drugmaker has estimated the drug could generate peak sales of more than 2 billion pounds ($2.5 billion) annually.

Although there are safe and effective vaccines for the hepatitis B virus - including shots made by GSK – nearly 300 million people globally have chronic hepatitis B (CHB), a long-lasting infection that occurs when the body is unable to fend off the virus and it persists in the blood and liver, according to the World Health Organization (WHO).

The mid-stage study is evaluating the impact of administering bepirovirsen in patients who were either on or off existing standard CHB therapy.

In the group of 227 patients on standard therapy, 24 weeks of treatment with bepirovirsen resulted in 28% of patients with sharp reductions in two key biomarkers of CHB such that a test would not be able to detect them.

Meanwhile, in the cohort of 230 patients who were not already on standard therapy, 24 weeks of treatment with bepirovirsen resulted in 29% of patients with corresponding decreases in the same biomarkers.

How durable these responses are remains to be seen.

Patients must be off therapy for typically six months or more and show no evidence of recurrence of the virus to achieve a functional cure, Christopher Corsico, GSK’s head of development, told Reuters.

But this data suggests that a functional cure is possible, Corsico said.

"The fact that you have such a high number of responses, just even when you stop the therapy is unprecedented ... compared to other drugs that are out there being tested."

Existing CHB medicines, such as nucleoside/nucleotide analogues (NA), are often taken for life because they suppress but rarely clear the virus. Despite their use, patients can develop serious complications, including liver cancer. In 2019, hepatitis B resulted in some 820,000 deaths, according to WHO estimates.

GSK, led by CEO Emma Walmsley, licensed bepirovirsen from Ionis Pharmaceuticals in 2019. The drug is designed to combat the disease on three fronts: by inhibiting viral replication, by suppressing the production of viral proteins associated with hepatitis B virus infection, and by stimulating the immune system.

($1 = 0.8155 pounds) (Reporting by Natalie Grover in London; Editing by David Holmes)