Merck & Co. Inc. said it would buy Idenix Pharmaceuticals Inc. in a deal valued at about $3.85 billion to complement its hepatitis C portfolio.

Merck said it would pay $24.50 per share, more than three times Idenix's Friday closing price of $7.23.

Idenix's stock soared almost 240 percent to $24.35 before the bell on Monday.

Cambridge, Massachusetts-based Idenix, which focuses on the treatment of human viral diseases, has three drugs in development to treat hepatitis C.

The companies said they expect the transaction to close in the third quarter.

Credit Suisse acted as financial adviser to Merck, while Hughes Hubbard & Reed LLP was its legal adviser.

Centerview Partners acted as financial adviser to Idenix and Sullivan & Cromwell acted as its legal adviser.

Merck shares were down slightly in light trading before the bell.