Pharmaceutical giants GlaxoSmithKline (GSK) and Pfizer said today that they're creating a company dedicated to developing HIV medications. The unusual arrangement will give London-based GSK 85 percent equity and New York's Pfizer the remainder.

The companies said in a statement that the merger would "be more sustainable and broader in scope than either company's individually," giving the new partnership 19 percent of the HIV drug market through a combined portfolio of 11 already-available meds and six candidates in development. The idea is that the $2.4 billion in sales generated from the marketed drugs will keep the development pipeline moving, the companies said.

GSK's vaccine development won't be part of the new company, according to Bloomberg News.

The deal helps GSK because its HIV drugs are old (and approaching patent expiration) and its own development pipeline is thin, the Wall Street Journal notes. Pfizer, on the other hand, has more robust drug candidates but fewer meds on the market, according to the Associated Press.

Still, Gilead Sciences, based in Foster City, Calif., now dominates the HIV drug market.

"We continue expect Gilead to remain a step ahead of the competition and view the new company as another example of a pharma merger driven by weakness," M. Ian Somaiya, an analyst at Thomas Weisel Partners, wrote in a note to clients, Bloomberg News notes.

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