LONDON — AstraZeneca will buy U.S. drugmaker MedImmune for $15.6 billion (11.5 billion euros) and with it, a share of the increasingly lucrative global vaccines market.

AstraZeneca PLC, which has been looking to strengthen its pipeline of future drugs as it faces patent challenges and escalating generic competition, said Monday it will pay $58 (42.63 euros) a share for MedImmune Inc., a 21 percent premium to the stock's close on Friday.

"This acquisition represents a transformational step to deliver our biologics strategy sooner than anticipated," said AstraZeneca Chief Executive David Brennan. "It creates a leading fully integrated biologics and vaccines business with critical mass and enhances AstraZeneca's R&D science base through which we will deliver a stronger product pipeline."

The deal, which AstraZeneca hopes to close in June, will increase the company's proportion of biotechnology drugs in its pipeline from 7 percent to 27 percent and enlarge its total pipeline by 45 projects to 163 projects.

That includes two late-stage products being developed by MedImmune, based in Gaithersburg, Md., which has more than 2,500 employees in facilities across the United States, Britain and the Netherlands.

One product is a follow-up to MedImmune's flagship childhood respiratory drug Synagis. The second is a refrigerated formulation of its FluMist inhaled influenza vaccine, which will probably be launched in the coming U.S. winter season.

AstraZeneca's decision to enter the vaccines market follows the purchase of U.S. vaccines maker Chiron by Novartis AG last year. AstraZeneca's British rival, GlaxoSmithKline PLC, and Paris-based Sanofi-Aventis SA already have large vaccines businesses.

Pharmaceutical companies are turning to vaccines to grow because the market has few producers and a reduced risk of generic competition. Interest was spurred by the emergence of the deadly strain of bird flu in Asia with no vaccine to protect against it.

Brennan said that the bidding process for MedImmune was "ferociously competitive."

MedImmune had previously resisted calls for a sale from shareholders disappointed with the company's share price and setbacks on drugs like FluMist. Activist investor and billionaire Carl Icahn recently revealed that he owns just over 1 percent of MedImmune stock, and called the company's management "lackluster."

Two weeks ago, MedImmune reversed course and said it would explore a possible sale.

Wayne Hockmeyer, MedImmune's board chairman and founder, denied that the sale was driven by the shareholder pressure.

"That clearly wasn't a factor," he said. "In fact, what it (the sale) really shows is the management team here has done a fantastic job of creating remarkable value for the shareholders."

Hockmeyer said the "vast majority" of MedImmune's 2,500 employees would remain in their jobs, but added that he has not considered whether he will stay at the company he started in 1988.

Contributing: Stephen Manning