Benlysta, a new drug designed specifically to treat systemic lupus erythematosus, has passed a second clinical trial, paving the way for it to become the first new treatment approved for lupus in 50 years. Its maker, Human Genome Sciences of Rockville, Md., said it would apply to the Food and Drug Administration for approval of the drug in the first half of next year.

"This is spectacular news for the worldwide lupus community," said Dr. Tammy O. Utset of the University of Chicago. "These trials demonstrate that SLE can be studied successfully in a clinical trial setting and should encourage further interest in drug development for lupus, an area of great unmet need."

Lupus, which affects an estimated 160,000 Americans and as many as 5 million people worldwide, is an autoimmune disorder in which the body's immune system attacks its own tissues and organs, causing a host of symptoms, including arthritis, kidney damage, chest pain, fatigue and a characteristic skin rash that some early doctors thought resembled wolf bites (lupus means "wolf" in Latin). Treatments involve steroids and other drugs that alleviate symptoms but have a variety of side effects, many of them severe.

Benlysta, known generically as belimumab, is an artificially generated monoclonal antibody that inhibits the biological activity of a naturally occurring protein called B-lymphocyte stimulator, or BLyS. BLyS is required for the creation of plasma B cells that produce the antibodies that attack organs.

Results reported in July from a clinical trial involving 860 patients in Asia, South America and Eastern Europe showed that the drug reduced the severity of symptoms and the amount of steroids patients required.

The second trial, whose results were announced today by the company, involved 816 patients at 19 centers, primarily in North America and Europe. Researchers found that 43.2 percent of those taking the highest dose of Benlysta in addition to standard therapy had a significant reduction in symptoms with no worsening of the disease in organs. That compares with 33.8 percent of those taking a placebo in addition to standard care. Most of the patients were not able to reduce their steroid use, but that is not likely to prevent approval of the drug.

The company worked closely with the FDA to define the end points that would indicate successful treatment. Those end points were met in both trials.

The drug's chief drawbacks are its price and its administration by monthly infusion. The company has not said what it will charge, but monoclonal antibodies now on the market cost thousands of dollars per dose. Analysts project Benlysta's annual sales to be more than $1 billion.