• 100 mg cohort demonstrates potent antiviral activity and favorable safety profile

Cambridge, Mass., September 4, 2008 -- Idenix Pharmaceuticals, Inc. (Nasdaq: IDIX), a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral and other infectious diseases, today announced that it has completed the proof-of-concept study evaluating IDX899, a non-nucleoside reverse transcriptase inhibitor (NNRTI) being developed for the treatment of HIV-1.

Data from the study demonstrate that the 100 mg/day cohort achieved a mean plasma viral load reduction of 1.87 log10 after seven days of treatment, similar to the potency observed with the other evaluated doses of 800 mg, 400 mg, and 200 mg/day in this study. As with the other cohorts, no treatment-related serious adverse events were reported for any of the patients receiving 100 mg/day of IDX899 and no patients discontinued the study.

"We are pleased that HIV-1-infected patients receiving IDX899 in this trial achieved potent viral suppression at all doses tested," said Douglas Mayers, MD, Idenix's chief medical officer. "With the promising antiviral activity and safety profile seen to date, especially at low doses, we believe that IDX899 could become an important part of combination antiretroviral therapy."

The phase I/II clinical trial was designed to evaluate the safety, tolerability, antiviral activity and pharmacokinetics of IDX899. Four cohorts of 800 mg/day, 400 mg/day, 200 mg/day, and 100 mg/day were completed with ten HIV-1-infected treatment-naive patients randomized 8:2 in each cohort to receive oral once-daily IDX899 or matching placebo, respectively, for seven days.

Patients (n=32) receiving once-daily oral administration of 800 mg, 400 mg, 200 mg, and 100 mg of IDX899 achieved mean viral load reductions of 1.78, 1.78, 1.84, and 1.87 log10, respectively, after seven days of treatment as tested with the Roche Amplicor1.5 assay. Patients (n=8) receiving placebo achieved a mean plasma viral load increase of 0.10 log10. As with IDX899-treated patients in the 800 mg, 400 mg, and 200 mg cohorts, all patients receiving 100 mg/day of IDX899 showed a consistent response with all patients exhibiting a one log or greater (range: 1.3 - 2.4 log10) reduction in viral load after seven days of treatment.

No treatment-related serious or non-serious adverse events were reported and no patients discontinued the study. The most common adverse events observed were dyspepsia, headache, and nausea; the rate of these events was similar between IDX899-treated patients and those receiving placebo. Additionally, no patterns in laboratory abnormalities between treatment groups were observed during the treatment period.

Two additional studies were also completed that evaluated the potential for a pharmacokinetic drug-drug interaction between IDX899 and other drugs for the treatment of HIV-1, as well as the safety and tolerability of IDX899 when administered in combination with those drugs. These separate studies were conducted using Reyataz [atazanavir] and Truvada [tenofovir + emtricitabine] in combination with IDX899 and concluded that the study drugs were well tolerated and there were no clinically relevant drug-drug interactions.

About Idenix

Idenix Pharmaceuticals, Inc., headquartered in Cambridge, Massachusetts, is a biopharmaceutical company engaged in the discovery and development 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 C virus and HIV.

For further information about Idenix, please refer to www.idenix.com.