GlaxoSmithKline P.L.C. and Pfizer Inc. announced yesterday an unusual deal that would combine their HIV operations into a new company that would account for almost 20 percent of sales of drugs to fight the virus.

The news arrives amid lackluster growth for the two companies' HIV drugs and increased pressure on pharmaceutical companies to boost returns on research-and-development costs for new products. In recent years, competitors, most notably Gilead Sciences Inc. and Bristol-Myers Squibb Co., have seized the lead in sales of HIV drugs.

That pressure is especially intense in the competition for new drugs to treat HIV, the virus that causes AIDS. Demand for new HIV drugs is constant because the disease changes quickly.

"There is always a need to stay one step ahead because of the issue of HIV resistance to currently available drugs," said Tim Horn, editor in chief of, which provides information for HIV patients.

While demand for HIV drugs - especially new ones - is strong, companies often cannot charge high prices for them.

Most people who have HIV live in extremely poor countries, with little or no ability to pay for drugs. Even in wealthier countries, the drugs are so crucial to survival that high prices create political furor that keeps a lid on what companies can charge. More broadly, the Obama administration is seeking to lower prescription costs for all drugs.

Horn says HIV drugs for one patient can run $12,000 a year.

Relatively poorer earnings prospects make it appealing to create a new company that will not drag down overall profit margins, said Daniel Hoffman, a pharmaceutical-company consultant in Chester County.

"Because HIV drugs are very research-intensive, and the pace of development is pretty rapid, they're not going to have enough time to recoup sunk research costs and make margins before a product has essentially obsolesced," said Hoffman, who is not working for either company. "What they're doing is moving some of these less-lucrative components off their own balance sheet."

London-based Glaxo, which employs about 4,500 people in the Philadelphia area, will own about 85 percent of the new firm. New York-based Pfizer will own the remainder.

Pfizer expects to complete the acquisition of Wyeth, which has its U.S. pharmaceutical headquarters in Collegeville, by the end of this year.

Glaxo's HIV operations are based at its U.S. headquarters in Research Triangle Park, N.C. Yesterday's announcement will have no effect on the company's Philadelphia operations, a spokesman said.

The two companies had combined HIV sales of $2.4 billion last year, including GlaxoSmithKline's top-sellers Combivir and Epzicom. Combivir, which generated $409 million in sales last year, loses its patent in 2012. The new company's franchise also will include Pfizer's HIV products, Selzentry and Celsentri and six drugs now in development.

Creating a new company is unusual in the pharmaceutical world, where firms more often merge or strike licensing deals.

"At the core of this specialist business is a broad portfolio of products and pipeline assets," GlaxoSmithKline chief executive officer Andrew Witty said in a statement, "which can be more effectively leveraged through the new company's strong revenue base and dedicated research capability."