Activist hedge fund manager Barry Rosenstein of Jana Partners is turning up the heat on Charles River Laboratories.

Rosenstein, who owns about 7 percent of the shares, thinks the drug research company will short change investors if it completes its announced plans to purchase Chinese rival WuXi PharmaTech (Cayman) for $1.6 billion in cash and stock, or $21.25 a share. On Friday, Rosenstein fired off at least his third lengthy letter to management hoping it would change its mind or to convince shareholders to oppose the deal.

In his latest salvo, Rosenstein asserts that Charler River’s recent investor presentation is flawed and would hurt investors. He insists Charles River claims that the transaction will generate at least $75 million to $100 million of revenue synergies on an annual basis by 2013 are speculative at best and run counter to industry perceptions of the transaction as well as practical industry dynamics. ....

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