US pharmaceutical giant Merck today said it was buying Idenix Pharmaceuticals to expand its portfolio of hepatitis C drugs in a cash deal valued at $3.85 billion. 

Idenix is a biopharmaceutical company that specialises in the treatment of human viral diseases, and which is currently focused on oral antiviral therapeutics to treat hepatitis C virus infection.

"Idenix's investigational hepatitis C candidates complement our promising therapies in development and will help advance our work to develop a highly effective, once-daily, all oral regimen that has a duration of treatment as short as possible for millions of patients in need around the world," said Roger Perlmutter, the head of Merck Research Laboratories.

Merck is offering $24.50 per Idinex share, more than three times Idenix's closing price Friday of $7.23. In pre-market trading today the company's shares jumped to $24.24. 

The acquisition, approved by the boards of directors of both companies, is expected to be concluded in the third quarter, the companies said in a statement. 

Last month Merck agreed to sell its over-the-counter, or non-prescription, consumer care business to German giant Bayer for $14.2 billion.