DENVER — Gilead Sciences Inc., a leading producer of HIV drugs, said Monday it plans to acquire biotech Myogen Inc. for about $2.5 billion in cash, signaling a surprising expansion into treatments for pulmonary diseases.

Gilead agreed to pay Myogen shareholders $52.50 per share, a premium of 50 percent over the stock’s closing price Friday of $35.08 a share. Myogen’s stock hit a 52-week high while Gilead’s shares dropped 5 percent in afternoon trading Monday.

The announcement caught many off-guard, not only for the acquisition price but because it represented a shift for Gilead out of its area of expertise in infectious diseases.

Sanford C. Bernstein & Co. analyst Geoff Porges said the offer indicates a possible bidding war for Myogen and also reflects the scarcity of late-stage drugs in the pulmonary disease category. “It’s a very generous offer,” he said.

During a conference call with analysts, John C. Martin, Gilead’s president and chief executive officer, said the acquisition will help Gilead build on its $365 million buyout of privately held Corus Pharma in August.

“Our planned acquisition of Myogen represents both a scientific and strategic step,” Martin said.

Gilead, a Foster City-based biopharmaceutical company with a market value of $29.2 billion, is known for its series of HIV drugs. It manufactures two of the three drugs in a once-a-day pill called Atripla that was approved last summer for HIV and AIDS treatment.

The company also makes the Tamiflu flu treatment.

Gilead posted second-quarter net income of $265.2 million, or 56 cents per share, up 34 percent from $196 million, or 41 cents per share, in the previous second quarter. The company credited the jump to strong sales of its HIV treatments.

Myogen, a 10-year-old biopharm based in the Denver suburb of Westminster, had a market value of about $2.2 billion as of Monday. It posted a net loss of $18.5 million, or 44 cents a share, in the second quarter compared with a net loss of 21.7 million, or 61 cents a share, in the second quarter of 2005.

Myogen is developing a drug called ambrisentan to treat pulmonary arterial hypertension. It believes information developed from two studies will support a new drug application that it expects to file with the Food and Drug Administration by year’s end.

It also has a drug for treatment of primary pulmonary hypertension and a late-stage study for a drug that could treat resistant hypertension.

Argus Research analyst David Coleman, who follows Myogen, said Gilead likely will be able to capitalize on the drug better than Myogen.

“I do believe it is a very good deal for Myogen shareholders,” he said.

In a research note to clients, Piper Jaffray & Co. analyst Thomas Wei said Gilead will be able to draw on its marketing expertise to build buyers for ambrisentan.

“We think this company’s prior success in competitive marketing will be an asset to the ambrisentan launch, and a successful launch in this area will create a strong pulmonary franchise for future opportunities,” Wei wrote.

The deal is structured in two phases, with Myogen slated to become a wholly owned subsidiary of Gilead. Myogen’s board approved the deal, which is expected to close by the end of the year, pending regulatory and other standard approvals.

Gilead said the deal will boost earnings in 2007 and 2008. It does not expect Myogen to add to 2009 profit.

Myogen stock rose $16.28, or 46 percent, to close at $51.36 a share in trading on the Nasdaq Stock Market. It has traded between $17.96 a share and $42.27 a share in the past year.

Shares of Gilead fell $3.78, or 5.5 percent, to close at $64.99 in trading on the Nasdaq. In the past year, it has traded between $44.73 a share and $68.77 a share.