Seth Klarman’s Baupost Group becomes largest investor in Idenix, a hepatitis drug developer

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The largest hedge fund firm in Boston, Baupost Group, has bought up more than one-quarter of the stock of a Cambridge biotech firm working on drugs for hepatitis C.

Seth Klarman’s Baupost Group held 27.6 percent of Idenix Pharmaceuticals Inc. at the end of September, according to a Nov. 1 filing with securities regulators, up from 18.5 percent in April. Baupost is now the company’s largest shareholder, edging ahead of drug giant Novartis Pharma AG.

Shares of Idenix soared 40 percent on Friday, two days after the company said one of its antiviral drugs was going into clinical trials in Belgium and Canada. Idenix is one of several companies competing to get a drug on the market to treat the blood-borne virus that affects an estimated 150 million people globally.

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