Human Genome Sciences said yesterday that British pharmaceutical giant GlaxoSmithKline will help fund the development of LymphoStat-B, a potential drug to treat lupus and rheumatoid arthritis that could soon move into expensive advanced human trials.

Glaxo's decision to participate under the terms of a collaboration agreement that dates to 1996 means the companies will share equally further development costs, which could top $100 million, and split any profits.

It also provides the Rockville biotech firm with a major scientific endorsement for its lead product, which it has been developing since the late 1990s.

"They bring experience to the table," said David C. Stump, executive vice president of drug development for Human Genome Sciences. "They bring worldwide connections. They bring financial staying power. They enable projects to be developed right. You can't beat a great partner."

Glaxo would not comment on the terms of the deal beyond those described in a press release, which said the company would underwrite 50 percent of future drug development costs.

But "we wouldn't go forward on a deal like this if we didn't think the drug had promise," said Rick Koenig, a spokesman for Glaxo.

Analysts were generally positive about the deal but said Human Genome Sciences and Glaxo now would have several important decisions to make. Chief among them is whether to move ahead in developing the drug for rheumatoid arthritis, a crowded field with a number of treatments; to focus solely on lupus, for which there are currently no approved drugs; or to spend the money on testing for both.

Alexander A. Hittle, an analyst with A.G. Edwards & Sons Inc., said there was another wrinkle: If further test results on lupus patients are especially positive, the firms could seek an accelerated review from the Food and Drug Administration -- expediting the day when the drug can be sold.

"I don't think I've ever been in a situation where there were not different views about what to do, even internally," Stump said.

"You just have to apply good, solid consensus-seeking and conflict-resolution skills," he added.

Human Genome Sciences began developing LymphoStat-B after discovering a protein called BLyS, which is required for disease-fighting cells to mature and produce antibodies that attack viruses and bacteria.

Sometimes those disease-fighting cells instead produce cells that attack the body's tissue. Most people's immune systems kill those cells. But some don't, causing autoimmune diseases such as lupus and rheumatoid arthritis.

Research studies by Human Genome scientists and others have shown that too much BLyS actually causes so much cell activity that the malicious antibodies thrive and start attacking the body. LymphoStat-B attacks BLyS and limits the production of those damaging cells.

In early April, Human Genome Sciences said mid-stage human testing had reduced signs and symptoms of rheumatoid arthritis in 31 percent of 283 patients that tested the drug. This fall, the company is expecting test results on whether the drug helps patients with lupus, a disease that affects mostly women and causes severe joint pain and fatigue, among other symptoms.

Human Genome Sciences shares closed yesterday at $12.18, up 36 cents. Glaxo shares were up 19 cents, at $48.22.

With Glaxo's support, Human Genome Sciences Executive Vice President David C. Stump, right, and his LymphoStat-B development team are evaluating options for testing the potential drug against lupus or rhematoid arthritis, or both.