Idenix Pharmaceuticals Halts Hepatitis C Drug Development in U.S.

Idenix Pharmaceuticals today announced that after discussions with the United States Food and Drug Administration (FDA) the development program of valopicitabine (NM283) for the treatment of hepatitis C has been placed on clinical hold in the United States based on the overall risk/benefit profile observed to date in clinical testing.

"We are disappointed with the FDA's perspective on the program and are working with Novartis to evaluate our options for valopicitabine," said Jean- Pierre Sommadossi, Ph.D., chairman and chief executive officer of Idenix. "We remain committed to building a leading antiviral franchise and will continue to focus on ensuring a successful launch of Tyzeka®/Sebivo® and on advancing our pipeline. We have a novel non-nucleoside reverse transcriptase inhibitor being evaluated in phase I clinical testing for the treatment of HIV. Additionally, we have a comprehensive HCV discovery effort, which includes a second-generation nucleoside polymerase inhibitor that is being evaluated in IND-enabling preclinical testing and novel HCV non-nucleoside polymerase inhibitor and HCV protease inhibitor programs."

As of June 30, 2007, Idenix had approximately $160 million of cash, cash equivalents and marketable securities.

"Over the next few weeks, we will be taking a critical look at our expenses with the goal of investing in programs that we believe will create shareholder value," said Ronald Renaud, chief financial officer of Idenix. "Our balance sheet is strong and we believe that we have enough cash to fund early clinical development of the pipeline."

Conference Call Information

Idenix will hold a conference call today at 8:00 a.m. EDT. To access the call please dial (800) 774-5358 U.S./Canada or (706) 758-9475 International and enter passcode 7177935 or to listen to a live webcast of the call, go to "Calendar of Events" in the Idenix Investor Center at www.idenix.com. Please log in approximately 10 minutes before the call to ensure a timely connection. A replay of the conference call and webcast will be available until July 30, 2007. To access the replay, please dial (800) 642-1687 U.S./Canada or (706) 645-9291 International and enter the passcode 7177935.

About Idenix

Idenix Pharmaceuticals, Inc., headquartered in Cambridge, Massachusetts, is a biopharmaceutical company engaged in the discovery, development and commercialization of drugs for the treatment of human viral and other infectious diseases. Idenix's current focus is on the treatment of infections caused by hepatitis B virus, hepatitis C virus and HIV. For further information about Idenix, please refer to www.idenix.com.