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, 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.



Second Generation Nucleoside Polymerase Inhibitor Program



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 http://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, Inc.

http://www.idenix.com