Human Genome Sciences Inc said its experimental lupus drug Benlysta was successful in a late-stage clinical trial, paving the way for approval of the first new treatment for the disease in 50 years.

The news sent Human Genome shares up 35 percent in early trading.

The test results, announced early Monday, showed patients who took a high dose of the drug Benlysta, in combination with standard therapies, experienced a statistically significant improvement in symptoms compared with those taking standard treatments in combination with a placebo.

The results met the main goal of the trial, which was the second

of two needed for the drug to be considered for approval by the U.S. Food and Drug Administration. If approved, analysts expect the drug to generate annual sales of at least $1 billion and potentially much more. Human Genome will split profits from the drug with its partner, GlaxoSmithKline Plc.

Lupus, a disease affecting an estimated 5 million people worldwide, causes the immune system to attack the body's own tissue and organs.

This is a pivotal moment in lupus research, said Margaret Dowd, president of the Lupus Research Institute, an organization that funds lupus research but did not fund Human Genome's trial.

Data from a composite of three measures in the latest trial showed that after 52 weeks, 43.2 percent of patients taking 10 milligrams of Benlysta in combination with standard therapies, achieved an improvement in symptoms, with no significant worsening of the disease in individual organs.

That compared with 33.8 percent of patients taking a placebo plus standard care.

Human Genome said it expects to file for approval for the drug in the United States in the first quarter of 2010. It could be on the market by the end of next year.

Still, results of the trial, known as BLISS-76, were not as robust as those from an earlier late-stage trial known as BLISS-52. The drug did not work to a significant extent at a lower dose, and it did not significantly reduce the need for patients to take steroids, which are a mainstay of current treatment that can cause serious side effects at high doses.

These were secondary goals of the trial and not required to be successful in order for the drug to be approved.

The fact that BLISS-76 missed key secondary endpoints is not a major concern to us, said Cory Kasimov, an analyst at J.P. Morgan. Indeed, we believe it reinforces just how difficult it is to crack lupus, and highlights Benlysta's strong competitive position, which could border on monopoly-like status for a significant period of time.

Part of the reason the results were slightly weaker in the latest trial is because patients were treated in different geographies, Barry Labinger, Human Genome's chief commercial officer, said in an interview.

The latest trial involved patients in the United States and Europe. BLISS-52 involved patients in Latin America, Asia and Eastern Europe.

Patients in both trials had slightly different levels of background therapy, which could have influenced results, Labinger said.

The BLISS-52 trial showed 57.6 percent of patients taking a high dose of Benlysta in combination with standard care showed an improvement in symptoms, compared with 43.6 percent of patients taking standard care plus a placebo.

Andrew Baum of Morgan Stanley said the smaller benefit seen in the new trial might make it hard to achieve the expected high price for Benlysta -- $20,000 to $30,000 per patient annually.

In the latest trial, serious side effects were reported in 26.8 percent of patients taking Benlysta, compared with 24 percent of patients taking a placebo.

Lupus causes the immune system to attack the body's own tissue and organs, including the joints, kidneys, heart, lungs, brain, blood and skin. It can cause arthritis, kidney damage, chest pain and skin rash, among other disorders.

Lupus affects about 1.5 million people in the United States and 5 million worldwide, according to the Lupus Foundation of America.

Labinger said the number of lupus patients under the care of a rheumatologist is about 325,000. Of those, some 200,000 have moderate to severe form of the disease. This is the patient population that will initially be targeted.

Human Genome shares rose $6.60 to $25.29 in early Nasdaq trade.

(Reporting by Toni Clarke and Ben Hirschler; Editing by Hans Peters, David Holmes and John Wallace)