MedImmune Inc., which officially put itself up for sale less than two weeks ago, agreed Monday to be acquired by AstraZeneca plc for $15.2 billion.

The definitive agreement calls for AstraZeneca to pay $58 in cash for each MedImmune share, a significant premium to the already-increased price since April 12, when MedImmune said it was exploring the sale of the company. The stock gained another $8.56 Monday, or 17.8 percent - on top of the $10.17 it already had gained since April 11 - to close at $56.57.

MedImmune shares (NASDAQ:MEDI) have risen 49 percent since the $37.84 close on April 11; AstraZeneca's offer represents a 53 percent premium to that price.

Even disgruntled shareholders who led the effort to get MedImmune to sell were not expecting that kind of premium. Nor were analysts who follow MedImmune.

"The price was higher than I even imagined," Joel Sendek, a managing director and senior biotechnology analyst at Lazard Capital Markets, told BioWorld Today. "That leads to two conclusions: One, we were valuing MedImmune differently than a strategic partner would; and two, what we might not ever know is how competitive the bidding process was.

"Clearly it seems to me there were multiple bidders," Sendek said. "When that's the situation, you often have a premium price and the company that wants it the most is the victor."

Eun Yang, a managing director and analyst at Jefferies & Co., told BioWorld Today her firm estimated a fair takeout price would be about $43 per share, an estimate made after the initial MedImmune announcement that it was up for sale, when the stock had been trading in the low- to the mid-$30 range.

Both Yang and Sendek, who had a similar estimate of the value of MedImmune, said at the time, however, that a bidding war or other factors could move the value higher.

"What happened was there were several pharmaceutical companies interested," Yang said. "The competition may have driven the premium. AstraZeneca said there were three other companies" involved in the bidding process.

MedImmune, a 19-year-old company with about 2,500 employees worldwide and the blockbuster product Synagis and other drugs on the market, is the seventh largest biotechnology company in the U.S. in terms of market capitalization. It previously estimated revenues this year would be about $1.5 billion.

AstraZeneca, of London, said the acquisition of Gaithersburg, Md.-based MedImmune "significantly accelerates" its biologics strategy. Last year AstraZeneca acquired the UK-based firm Cambridge Antibody Technology Group plc for about $1.1 billion. It cited in particular its interest in MedImmune's biologics manufacturing capabilities and its vaccine technologies, a new area at AstraZeneca.

AstraZeneca said the MedImmune purchase would increase its pipeline by 45 projects, to 163, and increase the proportion of biologics in its pipeline to 27 percent from 7 percent.

The $15.2 billion agreement represents a price-to-revenue multiple of 10 to 11, which Sendek said marked a significant premium to the less-than-5 multiple paid in the acquisitions of Chiron Corp. and Serono SA, similarly sized biotechnology companies bought last year by Novartis AG and Merck KGaA, respectively.

"I think it's on the high end," Sendek said, "but it could signal a new benchmark valuation for profitable companies that have the whole package, which includes not only blockbuster products but also a broad pipeline, a major manufacturing facility and long-term intellectual property."

Sendek added, however, that "there are very few assets like that around," so the price of MedImmune might have gone up due to "scarcity value."

"It's telling they would pay this kind of multiple for a company with revenue growth of" 11 percent to 12 percent, Sendek said. "They are paying a lot and not getting very fast growth, which tells you something about the state of the pharmaceutical business."

Pressure on MedImmune to sell had been building for months, as shareholders complained the company was not delivering the kind of growth its products, pipeline and technologies should have warranted. Among the most active dissenters was David Katz, president and chief investment officer of New York-based Matrix Asset Advisors Inc., which held about 1.8 million MedImmune shares. The stock gained about 15.3 percent April 12, after MedImmune said it was accepting offers, to close at $43.63.

Katz told BioWorld Today at the time that he was pleased with the news and the stock's rise that day to nearly the mid-$40s. He also said he had heard rumors there had been expressions of interest higher than $45 per share.

That the offer and definitive agreement came in so quickly and at $58 per share is rather stunning.

"Here's an example of shareholder activism yielding a pretty quick return," Sendek said. "On the other hand, this could very well be a unique situation. That the premium was so much higher than we thought it would be is good overall for the biotechnology sector, especially for profitable companies."

AstraZeneca said it would draw from a committed banking facility to pay $15 billion of the purchase price. It said the move would not preclude it from making other transactions. It also said the deal would not affect its planned $4 billion stock buyback program this year. The MedImmune acquisition is expected to close in June.

MedImmune in 2006 had revenues of about $1.28 billion, primarily on $1.1 billion in sales of Synagis (palivizumab), a monoclonal antibody indicated for the prevention of serious lower respiratory tract disease caused by respiratory syncytial virus in at-risk pediatric patients. Revenues for Ethyol, a product to reduce dry mouth in patients undergoing chemotherapy or radiation therapy, were $87 million. And revenues for FluMist, a nasal-spray influenza vaccine, were $56 million. MedImmune had about $1.5 billion in cash as of Dec. 31.

A near-term event at MedImmune is the May 28 Prescription Drug User Fee Act date for a label expansion for CAIT-V (FluMist), a refrigerator-stable formulation of FluMist that was approved by the FDA in January for prevention of influenza infection in healthy children and adults ages 5 to 49. MedImmune is seeking to expand the label to include children younger than 5. It also plans to file a biologics license application by the end of the year for Numax, an antibody distinct from Synagis but also targeting the respiratory syncytial virus.

David Mott, president and CEO of MedImmune, and James Young, president, research and development, committed to remain with the company, and it is expected other top officials will, too, AstraZeneca said. It also said it wants to retain employees and maintain the culture at MedImmune, and is offering one-time retention grants.

The price being paid for MedImmune is in the range of the market caps of two other biotech companies, Genzyme Corp. and Biogen Idec Inc., both of which saw nice gains in their shares Monday.

Two bigger companies, Celgene Corp. ($22 billion market cap) and Gilead Sciences Inc. ($39 billion), also saw shares rise. Amgen Inc. and Genentech Inc., the longtime leaders in the industry, are worth as much as most big pharma companies, and less likely to be subject to buyout speculation.

No other biotech company was valued higher than $6 billion, according to BioWorld Snapshots.