Positive Phase I/II Data Of IDX899 Confirm Potent Antiviral Activity And Favorable Safety Profile In Treatment-Naive HIV-Infected Patients

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 reported phase I/II data for IDX899, a
non-nucleoside reverse transcriptase inhibitor (NNRTI) being developed for
the treatment of HIV-1. Patients receiving once-daily IDX899 achieved a
mean plasma viral load reduction of approximately 1.8 log(10) after seven
days of treatment in each of the 800 mg, 400 mg and 200 mg dosing cohorts.
Patients receiving placebo had a 0.05 log10 viral load increase over the
same treatment period. No treatment-related serious adverse events were
reported for any of the patients receiving IDX899 and no patients
discontinued the study. Also, there were no discernable patterns in adverse
events between treatment groups and there were no laboratory abnormalities
during the treatment period. These data demonstrate potent antiviral
activity and a favorable safety profile at all tested doses.


"The profound inhibition of HIV-1 virus replication of IDX899 at doses
of 400 and 200 mg daily confirm the potent antiviral activity previously
reported at higher doses," said Dr. Robert Murphy, John P. Phair Professor
of Infectious Diseases, Director, Global Health Research, Feinberg School
of Medicine, Northwestern University. "These early clinical data are very
encouraging, showing that IDX899 offers potent viral suppression combined
with a promising safety profile."



Study Design



The phase I/II clinical trial was designed to evaluate the safety,
tolerability, antiviral activity and pharmacokinetics of IDX899. Three
cohorts of 800 mg/day, 400 mg/day and 200 mg/day were completed with ten
HIV-1- infected treatment-na??ve patients randomized 8:2 in each cohort to
receive orally once-daily IDX899 or matching placebo, respectively, for
seven days. Based on the potent antiviral activity of IDX899 seen to date,
the study was recently amended to also evaluate a lower dose of 100 mg/day.



Study Results



Patients receiving daily oral administration of 800 mg, 400 mg and 200
mg IDX899 achieved mean viral load reductions of 1.78, 1.78, and 1.83
log(10), respectively, after seven days of treatment based on results
determined by the Roche Amplicor(R) 1.5 assay. Patients (n=6) receiving
placebo achieved mean plasma viral load increase of 0.05 log(10).



One key eligibility criterion for enrollment in the study was that
patients' CD4+ count (or T-cell count) had to be greater than or equal to
200 cells/mm3. The mean CD4+ count change from baseline increased by at
least 60 cells/mm3 for each of the 800 mg, 400 mg and 200 mg dosing cohorts
and decreased by about 80 cells/mm3 for patients receiving placebo. No
clear pharmacokinetic/pharmacodynamic relationship was demonstrated, likely
due to drug trough levels well above the EC(90) of IDX899 against wild-type
HIV-1.
















"We believe any new HIV treatment must demonstrate potent antiviral
activity at low doses to be suitable for combination therapy and possible
co-formulation with other antiretroviral drugs. We are very encouraged that
the robust antiviral activity observed at 800 mg/day was also achieved by
the 400 and 200 mg/day cohorts," said Douglas Mayers, M.D., Idenix's chief
medical officer. "We look forward to evaluating a 100 mg once-daily IDX899
dose in the upcoming weeks."



Data for patients enrolled in the study will be presented today at the
XVII International HIV Drug Resistance Workshop in Sitges, Spain.



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 idenix



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 "anticipate," "could," "may," "will," or similar
expressions, or by express or implied statements with respect to the
company's clinical development programs in HIV, or any potential pipeline
candidates for the treatment of HIV, including any expressed or implied
statement regarding the efficacy and safety of IDX899 and any future
clinical trials involving IDX899. 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 IDX899; 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 and the Quarterly Report on Form 10-Q for the
quarter ended March 31, 2008, each 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.


Amplicor(R) is a registered trademark of F. Hoffman-La Roche Ltd.


Idenix Pharmaceuticals, Inc.

idenix