AstraZeneca’s acquisition of MedImmune, the US biotechnology group, for $15.6 billion (£7.8 billion) is under threat from an American class-action lawsuit.

The lawsuit, filed on behalf of MedImmune investors, claims that the sale was tailored to benefit senior MedImmune executives and AstraZeneca, rather than MedImmune shareholders. It also raises questions about big payouts that MedImmune directors are set to receive if the deal proceeds.

Chris Larson, the investor who filed the lawsuit, claimed in it that company executives “will reap disproportionate benefits to the exclusion of maximising stockholder value”.

The lawsuit names as defendants David Mott, MedImmune’s chief executive and president, who is to receive more than $145 million in cash as a one-off payment after the deal, and Wayne Hockmeyer, the company’s founder and chairman, who will receive $59.7 million.

Mr Larson said: “Instead of attempting to obtain the highest value reasonably available for the company’s stockholders, defendants spent a substantial effort tailoring the acquisition to meet the specific needs of AstraZeneca.”

An AstraZeneca spokesman played down the lawsuit’s significance. He said: “This is a matter for MedImmune as AstraZeneca is not party to the suit. However, MedImmune’s [US Securities and Exchange Commission] filing states that it does not believe the suit has any merit. Therefore, we are confident that the deal will go ahead.”

Althought the lawsuit was filed at Montgomery County Circuit Court in Maryland on April 25 – two days after AstraZeneca announced the deal – it did not emerge until a MedImmune regulatory filing with the Securities and Exchange Commission was made public on Tuesday night.

Mr Larson has asked the court to stop the merger, which AstraZeneca hopes to complete next month, and to ensure that the highest possible value is obtained for shareholders.

AstraZeneca saw off several rival bidders for MedImmune by raising its offer to $58 a share – a 53 per cent premium to MedImmune’s share price before its auction, which was handled by Goldman Sachs.

The British group won the support of senior MedImmune executives after striking last-minute deals by which they are to receive huge one-off retention payments on condition that they remain with the new company for a set period.

The all-cash deal is AstraZeneca’s biggest acquisition since its creation in 1999.

Medicine chest

MedImmune
–– World’s seventh-largest biotechnology company
–– 2006 sales: $1.2 billion
–– 2,500 employees
–– Based in Maryland, US
–– Produces Synagis (respiratory drug), FluMist (influenza vaccine)
AstraZeneca
–– Britain’s second-largest drugmaker
–– 2006 sales: $26.5 billion
–– 66,000 employees
–– Based in London
–– Produces Nexium (stomach acid treatment)

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