WASHINGTON (AP) — A first-of-its-kind drug for hepatitis B is letting some patients stop treatment without showing signs of the dangerous liver virus, what's called a "functional cure," researchers reported Thursday.

In two international studies, about 1 in 5 patients given the experimental drug saw their virus reduced to levels low enough for the immune system to keep in check.

"We have not had a treatment which has come to this level of cure," Dr. Seng Gee Lim of the National University Health System of Singapore, who helped lead the GSK-funded studies, told reporters before presenting the findings at a scientific meeting in Barcelona, Spain.

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The data also was published Thursday in the New England Journal of Medicine.

Chronic hepatitis B can cause liver cancer or liver failure, and kills about 1.1 million people around the world each year. Improvements to today's lifelong therapy, which can be hard to stick with or to access in some countries, have been sought for decades.

The new findings "represent a major step," Dr. Anna Lok, a hepatitis expert at the University of Michigan who wasn't involved in the research, wrote in the journal. But she cautioned that more study is needed to see how long that remission-like state lasts.

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The drug is bepirovirsen, nicknamed "bepi" and developed by GSK and Ionis Pharmaceuticals. It is under fast-track review by the U.S. Food and Drug Administration, with a decision expected in October. Regulators in Japan, China and Europe also are considering the drug.

Hepatitis B is a serious liver infection spread through contact with blood or other bodily fluids, including childbirth. A highly effective vaccine can prevent it. For people who are infected, many have an "acute" illness that lasts several months. But for some — about 1.7 million people in the U.S. and more than 250 million worldwide — it becomes a chronic form that gradually damages the liver.

Standard treatments, including daily pills, reduce levels of the virus and prevent liver damage. But a true cure is elusive because hepatitis B has an unusual ability to hide in the body, ready to rebound if therapy stops.

The new drug attacks hepatitis B by binding to its genetic components, suppressing viral replication as well as a key protein, the "S" or surface protein, and stimulates the immune system, said GSK vice president Melanie Paff.

The trials included 1,838 patients assigned to get either a bepi shot or a dummy shot weekly for six months, in addition to their regular pills. If the virus was undetectable for six months after stopping the shots, they could stop their regular pills, too. In about 20% of the bepi recipients, the virus remained undetectable for six more months after they stopped all treatment — that "functional cure" — something no patients given the dummy shots achieved, the researchers reported.

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Bepi recipients who started the study with lower levels of that S protein were slightly more likely to achieve a functional cure, Lim said. He is doing additional research to try to determine why only some people respond.

As for how long the functional cure lasts, GSK has tracked a small number of patients from earlier-stage studies and found most still faring well up to three years later, Paff said.

Lim said side effects included mild injection-site redness or pain and a temporary rise in enzymes that can indicate liver stress.

Lok, the Michigan hepatitis expert, noted the trials didn't include patients with cirrhosis, high S protein levels or other complicating factors.

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