NEW YORK (The Deal) -- Merck (MRK) - Get Report is making an all-cash $3.85 billion tender offer for Cambridge, Mass.-based Idenix Pharmaceuticals (IDIX) , in a strategic bid to strengthen Merck's hepatitis C drug program.

Roger Perlmutter, president, Merck Research Laboratories, laid out the strategic basis for the deal in a nutshell: "Idenix has established a promising portfolio of hepatitis C candidates based on its expertise in nucleoside/nucleotide chemistry and prodrug technologies. 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, ribavirin-free, pan-genotypic regimen that has duration of treatment as short as possible for millions of patients in need around the world."

The deal, announced Monday and already approved by the boards of Merck and Idenix, must pass muster with regulatory authorities and is expected to close in the third quarter.

Merck said it projects the HCV market to top $20 billion by 2018, based on data from research group EvaluatePharma.

Merck will pay $24.50 in cash per share of Idenix stock.

The announcement made for quite a pop in Idenix shares. They ended trading Friday at $7.23 and prices soared nearly 232%, trading at $24 in New York Monday morning.

Idenix specializes in developing nucleosides and nucleotides, or nucs. It claims a large patent estate in those compounds. "Nucs are scarce and thus perhaps quite valuable" noted ISI Group biotechnology/pharma analyst Mark Schoenebaum. "Part of Merck's purchase price almost certainly applied some kind of value to Idenix's patent portfolio," he wrote in a same-day note.

Gilead's newly approved $1,000-a-pill HCV treatment Sovalidi (sofosbuvir) is a nucleotide analog.

Idenix has three HCV drug candidates in clinical development, with the most important to Merck being IDX21437, a nucleotide prodrug. The triad includes another nucleotide prodrug, IDX21459, and an NS5a inhibitor.

HCV is an emerging epidemic, affecting 130 million to 150 million individuals across the globe, according to the World Health Organization. In the U.S., the number affected is around 3 million to 4 million. As many as half a million die each year from the disease. Because the virus can be silent for decades, a large percentage of those infected do not know they are.

Credit Suisse AG provided financial advice to Merck. The buyer turned to a team at Hughes Hubbard & Reed LLP for legal advice led by James Modlin, David Schwartz, Andrew Braiterman, Alexander Anderson, Sarah Downie, Ethan Litwin, William Kolasky, Alain Vincent, Nadine Voisin, Stefan Naumann, counsel Cyrille Gaucher and associates Julie Thorston, Justin Greenbaum, Christine Lamsvelt and Erin deCecchis.

Idenix used Centerview Partners for financial advice and Sullivan & Cromwell LLP for legal advice. The Sullivan & Cromwell team included Frank Aquila, Krishna Veeraraghavan, Ronald Creamer Jr., Matthew Friestedt, Nader Mousavi, special counsel Spencer Simon and associates Brenden Rowook Park, Mark Campbell, Julia Kim, Jeffrey Arbeil, Jeannett Braun and Jennifer Yoon.