Idenix Pharmaceuticals announces significant progress in HCV development programs

Idenix Pharmaceuticals, Inc. (Nasdaq: IDIX), a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral diseases, today announced significant progress in three HCV programs. Idenix will provide a business update on these three HCV development programs and on its partnered programs during a presentation by Jean-Pierre Sommadossi, Ph.D., chairman and chief executive officer, to financial analysts and investors at the 28th Annual J.P. Morgan Healthcare Conference in San Francisco on Thursday, January 14 at 8:00 a.m. PST.

IDX184: Nucleotide HCV Polymerase Inhibitor

The phase II clinical trial, initiated in the fourth quarter of 2009, is a randomized, double-blind, placebo-controlled, sequential dose-escalation study evaluating the safety, tolerability, pharmacokinetics and antiviral activity of IDX184 in combination with pegylated interferon and ribavirin in treatment-naive HCV genotype 1-infected patients. Patients will receive a daily dose of IDX184 or placebo plus pegylated interferon and ribavirin for 14 days and then continue on pegylated interferon and ribavirin for an additional 14 days. Antiviral activity will be assessed at the 14-day and 28-day timepoints. Four dosing regimens of IDX184 ranging from 50 to 200 mg per day will be evaluated. In the 100 mg and 200 mg cohorts, QD and BID regimens will be compared. Each cohort includes 20 patients randomized 4:1, IDX184:placebo. This study is being conducted at multiple centers in the United States and Argentina.

Interim analysis of the first 10 patients randomized into the first cohort (50 mg QD):

Patients with Undetectable Median Change in HCV RNA Viral Load at Day 14 Cohort (log10) at Day 14 (<15 IU/mL) ------ ------------------------ ----------------------------- 50 mg/day IDX184 + PegIFN/Ribavirin>Median ALT and AST levels, markers of liver injury, improved during treatment. There were no serious adverse events on treatment, no treatment discontinuations and laboratory profiles were comparable to standard PegIFN/Ribavirin treatment.

"We are very encouraged by these interim data for IDX184 combined with pegylated interferon and ribavirin and look forward to seeing additional data as the study progresses," said Douglas Mayers, M.D., Idenix's executive vice president and chief medical officer.

IDX375: Non-Nucleoside HCV Polymerase Inhibitor

A Phase I single ascending dose study evaluating the safety, tolerability and pharmacokinetics of IDX375 in healthy volunteers is ongoing. The first five cohorts (25 mg QD, 50 mg QD, 100 mg QD, 200 mg QD and 200 mg BID; 6 active:2 placebo) in this double-blind, placebo-controlled study have been completed. Data suggest favorable plasma exposure of IDX375 with a long elimination half-life of 32-40 hours demonstrating the potential for once- or twice-daily dosing in patients. IDX375 was generally safe and well tolerated. There were no significant lab abnormalities. The most common adverse event was mild diarrhea (3/30 subjects). Additional cohorts with higher single and multiple doses are planned.

IDX320: HCV Protease Inhibitor

A Clinical Trial Application for a new protease inhibitor clinical candidate, IDX320, was filed in December 2009. IDX320 is a non-covalent macrocyclic inhibitor with nanomolar potency, broad genotypic coverage and a favorable preclinical pharmacokinetic profile supporting the potential for once-daily dosing in man.

"We are pleased with these early data from our clinical programs and look forward to their continued advancement throughout the year," said Jean-Pierre Sommadossi, Ph.D., chairman and chief executive officer of Idenix. "In addition, we are excited about our new protease inhibitor clinical candidate, IDX320, and expect to begin a phase I clinical trial soon. With three promising clinical programs that span major HCV drug classes, Idenix is well positioned to play an important role in the transformation of the HCV treatment paradigm."

2009 Financial Guidance

Idenix today reported that it ended 2009 with approximately $48.1 million of cash, cash equivalents and marketable securities. The company's 2009 financial results have not yet been audited.