Gilead Sciences Inc., one of the world's biggest biotechnology companies, said on Monday it agreed to acquire tiny Myogen Inc. for $2.5 billion to gain control of its promising hypertension drugs.

The transaction, the latest in a flurry of acquisitions in the drug and biotechnology sector, will give Gilead access to Myogen's experimental drugs ambrisentan to treat pulmonary arterial hypertension, or PAH, and darusentan for treating high blood pressure.

PAH causes extreme shortness of breath as the heart struggles to pump against high blood pressure in the lungs.

Foster City, California-based Gilead agreed to acquire all of Myogen's common stock at $52.50 per share, or a 50 percent premium to Myogen's closing stock price on Friday of $35.08. Myogen's shares rose 48 percent to $52.06 in pre-market trading.

Myogen is expected to seek approval for ambrisentan by the end of the year. The company is in a race with Encysive Pharmaceuticals Inc. to produce a drug for PAH that can compete with Tracleer, a rival made by Actelion.

Encysive received a blow in July when U.S. regulators again asked for more information about its experimental PAH drug Thelin, before agreeing to approve it.

Geoffrey Porges, an analyst at Sanford Bernstein, said he expects ambrisentan to generate more than $1 billion a year, if approved, but he questioned the price of the deal and Gilead's rationale for the acquisition.

"It's a heck of a premium," he said. "You have to worry that this is a signal that Gilead thinks they didn't have enough runway in the HIV area in particular or in infectious diseases generally."

"What I am asking myself is what do they know about cardiovascular disease that a dozen other pharmaceutical and biotech acquirers didn't know that support them paying such a high premium," he said.

The deal, which is expected to close before the end of the year, is expected to reduce Gilead's earnings in 2007 and 2008, be neutral in 2009, and add to earnings in 2010 and beyond, the companies said.

"We are excited about the potential of ambrisentan, which we believe could have important clinical benefits over existing therapies," said John Martin, Gilead's chief executive, in a statement.

Myogen markets Flolan, an intravenous treatment for primary pulmonary hypertension, in the United States through a deal with British drugmaker GlaxoSmithKline Plc.