Human Genome Sciences Inc. said yesterday that British pharmaceutical giant GlaxoSmithKline PLC has agreed to co-develop another drug with the Rockville biotech firm, this one a potential treatment for several difficult-to-treat cancers.

The drug, called HGS-ETR1, seeks out a receptor on the surface of cancer cells and provides biological instructions for the cells to die, rather than to keep growing. The drug, when used alone, has had promising results in early human testing -- for example, stabilizing lung cancer tumors in nine of 32 patients.

The announcement of Glaxo's participation comes nearly two months after the British company agreed to partner with Human Genome Sciences on its lead product, a treatment for lupus and rheumatoid arthritis. Beyond millions of dollars in development costs, the deals offer the 13-year-old biotech company confidence that it is inching closer to selling its first product.

"I think Glaxo brings credibility, capital and expertise to the development process," said David C. Stump, executive vice president of drug development for Human Genome Sciences.

Glaxo spokeswoman Gaile Renegar said, "We feel like this relationship is starting to bear fruit."

Glaxo decided to help fund both drugs under a collaboration agreement dating to 1996. The companies will equally share development costs, marketing expenses and -- if the drugs make it to market -- the profits (or losses).

In pursuing the anti-cancer drug, Human Genome Sciences and Glaxo are joining what has become the gold rush of biotech.

Genentech Inc., one of the country's most successful biotech companies, launched the cancer drug Avastin last year and had $554.5 million in sales. Rituxan, another of the company's cancer treatments, has been on the market nine years. Total sales so far: more than $7 billion.

Although cancer therapies have become financial boons, they are expensive and complicated to develop. Securing a partner with deep pockets is desirable, particularly in this case, because Human Genome Sciences is testing its drug on three types of cancers: non-Hodgkin's lymphoma, non-small-cell lung cancer and colorectal cancer.

"The potential for this drug to work in a number of cancers is what makes it so appealing," said Alexander A. Hittle, an analyst with A.G. Edwards & Sons Inc., who, like other analysts, responded positively yesterday to the deal.

Human Genome Sciences is also testing HGS-ETR1 in combination with chemotherapy, a strategy that has boosted the success of other biotech drugs in shrinking tumors.

Shares of Human Genome Sciences closed yesterday at $13.26, up 76 cents.