CAMBRIDGE, Mass., July 29 /PRNewswire-FirstCall/ -- 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 initiated a first-in-man study of IDX184 under a United States investigational new drug (IND) application. IDX184 is a once-daily, oral nucleotide prodrug polymerase inhibitor for the treatment of chronic hepatitis C. Today, Idenix also announced that is has selected a lead clinical candidate (IDX375) from its HCV non-nucleoside polymerase inhibitor discovery program and has advanced IDX375 into IND-enabling pharmacokinetic and toxicology studies. Idenix has also advanced two protease inhibitor drug candidates (IDX136 and IDX316) into IND-enabling pharmacokinetic and toxicology studies.

"We are pleased with the progress we have made in our hepatitis C discovery program in the past few months," said Jean-Pierre Sommadossi, Ph.D., chief executive officer of Idenix. "With IDX184 entering human testing and clinical candidates undergoing late-stage preclinical testing from our HCV non-nucleoside and protease inhibitor programs, we are progressing toward our ultimate goal of developing a proprietary combination of direct-acting antivirals for the treatment of hepatitis C."

IDX184 Nucleotide Prodrug Polymerase Inhibitor

IDX184 is a once-daily, oral nucleotide prodrug candidate based on Idenix's proprietary liver-targeting technology that has demonstrated HCV antiviral activity in both in vitro and in vivo preclinical models. Pre-clinical testing suggests that this technology enables the delivery of high levels of active nucleoside triphosphate into the liver, the site of primary HCV infection. In HCV genotype-1 infected chimpanzees, once-daily oral administration of 10 mg/kg of IDX184 produced a mean viral load reduction of 2.3 log10 after four days of dosing.

"The in vitro antiviral activity of IDX184 combined with the marked viral load reductions observed in HCV-infected chimpanzees support the potential for once-a-day, low milligram dosing of IDX184 in HCV-infected patients," said David Standring, Ph.D., executive vice president of biology for Idenix.

The company has initiated a first-in-man study of IDX184 under a U.S. IND. The study design is a double-blind, placebo-controlled, single dose-escalation study to evaluate the safety and pharmacokinetic activity of IDX184 in healthy volunteers. This study will evaluate six single rising doses of IDX184, ranging from 5 mg to 100 mg once-per-day. Each cohort of the study will evaluate eight volunteers randomized six to IDX184 and two to placebo. This study will be followed by a phase I/II proof-of-concept study in treatment/naive, HCV genotype-1 infected patients.

IDX375 Non-Nucleoside Polymerase Inhibitor

Idenix has selected IDX375 as its lead clinical candidate from its HCV non-nucleoside polymerase inhibitor discovery program. Preclinical testing demonstrated that IDX375 targets the palm non-nucleoside pocket of HCV polymerase. IDX375 has exhibited single nanomolar in vitro potency against HCV genotype 1b replicon (EC50 = 2 nM) and against HCV genotype 1a and 1b polymerases. Additionally, cellular cytotoxicity testing in Huh-7 cells demonstrated that IDX375 is not cytotoxic (CC50 >100 micrometers), resulting in a selectivity index >33,000 for IDX375. In preclinical in vitro studies, IDX375 did not inhibit human cellular DNA polymerases alpha, beta and gamma (IC50 >100 micrometers), demonstrating selectivity for the HCV 1a and 1b polymerases. After oral administration in monkeys, bioavailability of IDX375 was approximately 30%. Based on monkey plasma drug exposure levels, IDX375 has the potential for once-daily dosing in man.

IDX136 and IDX316 Macrocyclic Protease Inhibitors

Idenix has scaled up manufacturing of two clinical candidates, IDX136 and IDX316, from its HCV protease inhibitor discovery program to support IND-enabling pharmacology and toxicology studies. Both IDX136 and IDX316 are based on Idenix's proprietary scaffold B and were developed through SAR (structural activity relationship) approaches aided by high-resolution co-crystal structures with the HCV protease. IDX136 and IDX316 have demonstrated single nanomolar potency against HCV genotype 1a and 1b purified proteases and nanomolar potency against HCV genotype 1b replicon (EC50 = 4 to 10 nM). Additionally, these compounds are highly selective, binding tightly to the HCV protease and demonstrating no activity against eight human cellular proteases. Both drug candidates appear to have a differentiated resistance profile when compared to other macrocyclic protease inhibitors in development. Favorable pharmacokinetic properties of IDX136 and IDX316 in non-human primates suggest the potential for once- or twice-daily dosing in man.

Conference Call Information

Idenix will hold a conference call and webcast today at 8:30 a.m. ET. To access the call please dial 800-471-3635 U.S./Canada or 706-758-9475 International and enter passcode 56201724 or to listen to and view the live webcast of the call, go to "Calendar of Events" in the Idenix Investor Center at www.idenix.com. A replay of the call will also be available from 11:30 a.m. ET on July 29, 2008 until 11:59 p.m. ET on August 12, 2008. To access the replay, please dial 800-642-1687 U.S./Canada or 706-645-9291 International and enter passcode 56201724. An archived webcast will also be available for two weeks after the call on the Idenix website.

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 "may," "plans," "anticipates," "will," or similar expressions, or by express or implied statements with respect to the company's discovery and development programs or commercialization activities in hepatitis C, or any other potential pipeline candidate. 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, clinical trials, including additional data relating to the ongoing clinical trials evaluating its product candidates; 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 Quarterly Report on Form 10-Q for the quarter ended March 31, 2008, 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:

Amy Sullivan: 617-995-9838

SOURCE Idenix Pharmaceuticals, Inc.