WASHINGTON — Biotech drugmaker Human Genome Sciences yesterday reported positive results for its experimental lupus drug, potentially clearing the way for the first new treatment against the inflammatory disease in a half century.

The late-stage results come after nearly a decade of research and development by the company aimed at relieving symptoms of the difficult-to-treat ailment, in which the body attacks its own tissue and organs.

Patients who took the injectable drug Benlysta plus a standard treatment for one year had reduced symptoms — including pain, rashes and infections — compared with patients taking standard treatment plus placebo. The study involved more than 860 patients in Asia, South America and Eastern Europe.

The results sent Human Genome Sciences stock surging 277 percent yesterday to close at $12.51, more than three times Friday's closing price of $3.32. The stock has ranged from 45 cents to $8 over the past year.

"We're delighted," said Executive Vice President Dr. David Stump. "It's a disease that hasn't had a new approved drug in 50 years, so it's a first in many ways."

If the next round of tests go well, HGS plans to file for Food and Drug Administration approval early next year. The drug could become available late in 2010, assuming it gets an accelerated review from the FDA. The agency gives priority to drugs that present major medical advances.

The company has not determined how much it will charge for the treatment.

Wall Street analysts believe the drug could be worth billions of dollars for the firm and its partner in developing it, GlaxoSmithKline, if Benlysta makes it to the market.

Given a history of failure in developing drugs for lupus, many Wall Street analysts had not anticipated favorable news.

The Lupus Foundation of America estimates that about 1.5 million Americans and about 5 million people worldwide have some form of the disease. Currently, doctors typically use chemotherapy and steroids for lupus, treatments that can have harsh side effects.

"This is a big deal for a lot of people," said Sandra Raymond, the organization's chief executive, speaking before the results of the tests were known. In just the past year, lupus victims have seen their hopes dashed three times as other experimental drugs failed to pass their testing processes.

Lupus is a disease that waxes and wanes unpredictably, making drug testing particularly difficult, said Dr. Gary Gilkeson, professor of medicine at the Medical University of South Carolina. "It's not like cholesterol, cancer or diabetes, where there's something easy to measure," he said.

Gilkeson, who participated in previous failed trials for experimental drugs designed to treat the disease, said there was concern in the lupus community that another failure or two in this area could discourage drugmakers from trying to develop medicines to treat the disease.

Though the disease's causes are still not entirely understood, the HGS drug is designed to target a protein that becomes overactive in lupus patients and can cause the body to attack its own organs.

For its latest round of testing, 865 patients in 13 countries outside the United States were given different doses of the drug, or a placebo, for a year. Among those given the placebo, 44 percent experienced meaningful improvement in their symptoms. But among those who took a high dose of Benlysta, 58 percent saw improvement. Fifty-two percent of those who took a low dose improved.

The results were positive enough to meet guidelines for success that HGS had established with the FDA.

HGS' president and chief executive, Thomas Watkins, said he is optimistic about the drug's chances in the next round of tests, which will involve a similar number of patients in the United States and Western Europe.

The company has spent hundreds of millions of dollars to develop the drug over the past several years.

"It's a big wager, with no guarantee of success," he said. "We're happy to make those investments, particularly when you get results like this."

Lupus is a chronic inflammatory disease that occurs when the body's immune system attacks its own tissues and organs. As many as 90 percent of victims are women, usually in their 30s and 40s when it first strikes. No two cases of lupus are identical, but symptoms can include fatigue, fever, joint pain, stiffness and swelling, rashes, skin lesions, mouth sores, hair loss and chest pain. The disease can attack many internal organs, leading eventually to death.

Only three drugs are approved for treating lupus — aspirin, the steroid prednisone and the antimalarial drug Plaquenil (hydroxychloroquine) — and all were approved during the Eisenhower administration. All can have severe side effects.