Idenix Pharmaceuticals, Inc. Presents Positive Preclinical Data on HCV Programs

CAMBRIDGE, Mass., and MILAN, Italy, April 23 /PRNewswire-FirstCall/ -- Idenix Pharmaceuticals, Inc. (Nasdaq: IDIX - News), a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral and other infectious diseases, today announced preclinical data from the company’s polymerase and protease inhibitor programs for the treatment of hepatitis C. These data are being presented at the Annual Meeting of the European Association for the Study of the Liver (EASL) being held this week in Milan, Italy.

IDX184 is a nucleotide prodrug derived from Idenix’s proprietary liver- targeting technology. Results from in vitro and in vivo preclinical studies confirm that this technology preferentially activates IDX184 in the liver, potentially enhancing the antiviral activity of the drug and limiting systemic side effects. In an HCV replicon model, IDX184 exhibited 10 times greater potency than the first-generation HCV nucleoside clinical drug candidates currently in development. In HCV genotype-1 infected chimpanzees, once-daily oral administration of 10 mg/kg of IDX184 resulted in a mean viral load reduction of 2.3 log10 (n=5) after four days of dosing. No toxicities, including gastrointestinal or hematological, or abnormal blood chemistry were observed at oral doses greater than or equal to 600 mg/kg/day in a supporting seven-day toxicology study in cynomolgus monkeys.

IDX184 demonstrated additive antiviral activity in the HCV replicon in combination with HCV protease inhibitors and interferon and synergistic activity in combination with ribavirin. IDX184 also remained fully active in vitro against HCV containing known protease and non-nucleoside inhibitor drug resistance mutations.

Data from this program are being presented by David Standring, Ph.D., executive vice president, biology, Idenix Pharmaceuticals, in an oral presentation session at 5:30 p.m. CET on Friday, April 25, 2008, and by Erika Cretton-Scott, Ph.D., director, preclinical pharmacology at Idenix Pharmaceuticals, in a poster session beginning on Thursday, April 24, 2008.

Novel Macrocyclic Protease Inhibitor Program

Based on two novel macrocyclic scaffolds, Idenix has discovered compounds that demonstrated potency with subnanomolar antiviral activity against the HCV genotype 1b NS3/4A protease target and single nanomolar activity in the HCV replicon. These compounds also displayed high selectivity in vitro with no inhibition of selected human cellular proteases. In vitro, the Idenix protease inhibitors exhibited antiviral activity against certain HCV resistant mutants associated with first-generation protease inhibitors currently in clinical development. Pharmacokinetic studies demonstrated adequate oral bioavailability and sustained liver concentrations suggesting the potential for once-daily or twice-daily dosing. Data from this program are being presented by Dr. Standring in an oral presentation session at 6:15 p.m. CET on Thursday, April 24, 2008.

“The preclinical profiles emerging in our HCV nucleoside and protease inhibitor programs are very encouraging when compared to first-generation drug candidates from both of these drug classes,” said David Standring, Ph.D., executive vice president, biology, of Idenix. “We plan to have product candidates from both of these programs in clinical development within the next 12 months with the goal to evaluate our own novel combinations thereafter.”

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.

Forward-looking Statements

This press release contains “forward-looking statements” within the meaning of The Private Securities Litigation Reform Act of 1995. Such forward- looking statements can be identified by the use of forward-looking terminology such as “could,” “may,” “plans,” “will,” or similar expressions, or by express or implied statements with respect to the company’s clinical development programs in hepatitis C, or any potential pipeline candidates for the treatment of hepatitis C. Such forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause actual results to be materially different from any future results, performance or achievements expressed or implied by such statements. There can be no guarantees that the company will advance any clinical product candidate or other component of its potential pipeline to the clinic, to the regulatory process or to commercialization. In particular, management’s expectations could be affected by unexpected regulatory actions or delays; uncertainties relating to, or unsuccessful results of, pre-clinical studies and/or clinical trials, including additional data relating to the ongoing pre-clinical studies and/or clinical trials evaluating its product candidates, including IDX 184 and novel protease inhibitors; the company’s ability to obtain additional funding required to conduct its research, development and commercialization activities; the company’s dependence on its collaboration with Novartis Pharma AG; changes in the company’s business plan or objectives; the ability of the company to attract and retain qualified personnel; competition in general; and the company’s ability to obtain, maintain and enforce patent and other intellectual property protection for its product candidates and its discoveries. These and other risks which may impact management’s expectations are described in greater detail under the caption “Risk Factors” in the company’s annual report on Form 10-K for the year ended December 31, 2007 as filed with the Securities and Exchange Commission (SEC) and other filings that the company makes with the SEC.

All forward-looking statements reflect the company’s expectations only as of the date of this release and should not be relied upon as reflecting the company’s views, expectations or beliefs at any date subsequent to the date of this release. Idenix anticipates that subsequent events and developments may cause these views, expectations and beliefs to change. However, while Idenix may elect to update these forward-looking statements at some point in the future, it specifically disclaims any obligation to do so.

Idenix Pharmaceuticals Contact:

Teri Dahlman (617) 995-9905

Source: Idenix Pharmaceuticals, Inc.

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