Idenix Reports Favorable Resistance Profile for IDX719, a Potent, Pan-Genotypic HCV NS5A Inhibitor, at EASL Meeting

Idenix Reports Favorable Resistance Profile for IDX719, a Potent,
Pan-Genotypic HCV NS5A Inhibitor, at EASL Meeting

CAMBRIDGE, Mass., April 24, 2013 (GLOBE NEWSWIRE) -- Idenix Pharmaceuticals,
Inc. (Nasdaq:IDIX), a biopharmaceutical company engaged in the discovery and
development of drugs for the treatment of human viral diseases, today reported
detailed resistance data from in vitro studies and from a three-day
monotherapy clinical trial of IDX719, the Company's once-daily, potent,
pan-genotypic NS5A inhibitor for the treatment of hepatitis C virus (HCV)
infection. These data are being presented on Saturday, April 27, in a poster
session at the 48^th Annual Meeting of the European Association for the Study
of the Liver (EASL), which is being held April 24 - 28, 2013 in Amsterdam, The
Netherlands.

Data from the three-day proof-of-concept study demonstrated that IDX719 was
well-tolerated at daily doses up to 100 mg and showed potent antiviral
activity across HCV genotypes 1-4, with mean maximal viral load reductions up
to approximately 4.0 log[10] IU/mL. These data were supported by earlier in
vitro findings. Clinical plasma samples at baseline, at end of treatment and
at one week post-treatment were sequenced for mutations in NS5A at known
IDX719 resistance-associated locations.

  *The most common treatment-emergent resistant mutations were detected at
    NS5A positions 28, 31 and 93. The profile of these mutations varied among
    genotypes.
    
  *The only resistance mutation present at baseline found to negatively
    affect IDX719 response was M31 in GT2-infected patients. In contrast, all
    GT4-infected patients had virus with M31 at baseline and responded
    favorably to IDX719 treatment.
    
  *A GT1b-infected patient with a baseline Y93H mutation, which confers in
    vitro resistance to IDX719, achieved a 2.79 log[10] IU/mL viral load
    reduction after three days of once-daily 25 mg IDX719, indicating that
    IDX719 can retain activity against virus with known resistance mutations.

The poster presentation is titled, "Treatment-Emergent Variants Following 3
Days of Monotherapy with IDX719, a Potent, Pan-Genotypic NS5A Inhibitor, in
Subjects Infected with HCV Genotypes 1-4" (Abstract No. 1209).

"These additional resistance data support the promising profile of IDX719 as a
potent, pan-genotypic component of future HCV combination treatment regimens,"
commented Douglas Mayers, M.D., Chief Medical Officer of Idenix. "We look
forward to evaluating IDX719 as part of all-oral combination therapies through
our collaboration with Janssen, beginning with the initiation of a phase II
clinical trial of IDX719 and simeprevir in the first half of this year."

ABOUT IDX719

IDX719 is an NS5A inhibitor with low picomolar, pan-genotypic antiviral
activity in vitro.To date, IDX719 has been safe and well tolerated after
single and multiple doses of up to 100 mg in healthy volunteers (n=36; up to 7
days duration) and HCV-infected patients (n=69; up to 3 days duration). There
have been no treatment-emergent serious adverse events reported in the
program. IDX719 has demonstrated potent pan-genotypic antiviral activity in
HCV-infected patients with mean maximal viral load reductions up to
approximately 4.0 log[10] IU/mL across HCV genotypes 1-4 in a
proof-of-concept, three-day monotherapy study.

ABOUT HEPATITIS C

Hepatitis C virus is a common blood-borne pathogen infecting three to four
million people worldwide annually. The World Health Organization (WHO)
estimates that more than 170 million people worldwide are chronically infected
with HCV, representing a nearly 5-fold greater prevalence than human
immunodeficiency virus.

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 diseases. Idenix's current focus is on the
treatment of patients with HCV. For further information about Idenix, please
refer to www.idenix.com.

FORWARD-LOOKING STATEMENTS

This press release contains "forward-looking statements" for purposes of the
safe harbor provisions of The Private Securities Litigation Reform Act of
1995. For this purpose, any statements contained herein that are not
statements of historical fact may be deemed forward-looking statements.
Without limiting the foregoing, the words "expect," "plans," "anticipates,"
"intends," "will," and similar expressions are also intended to identify
forward-looking statements, as are any expressed or implied statements with
respect to: the Company's plans to continue to developing nucleotide
polymerase inhibitors for HCV; its clinical development plans for its uridine
nucleotide analog drug candidate and IDX719; its plans to advance other
preclinical nucleotides; and statements regarding the efficacy and safety of
its clinical compounds. Actual results may differ materially from those
indicated by such forward-looking statements as a result of risks and
uncertainties, including but not limited to the following: 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 due to numerous risks inherent in
pharmaceutical research and development; management's expectations could be
affected by unexpected regulatory actions or delays; uncertainties relating
to, or unsuccessful results of, preclinical and 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;
competition; and the Company's ability to obtain, maintain and enforce patent
and other intellectual property protection for its product candidates and its
discoveries. Such forward-looking statements involve known and unknown risks,
uncertainties and other important factors that may cause actual results to be
materially different from any future results, performance or achievements
expressed or implied by such statements. These and other risks which may
impact management's expectations are described in greater detail under the
heading "Risk Factors" in the Company's annual report on Form 10-K for the
year ended December 31, 2012, as filed with the Securities and Exchange
Commission (SEC) and in any subsequent periodic or current report that the
Company files with the SEC.

All forward-looking statements reflect the Company's estimates only as of the
date of this release (unless another date is indicated) 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. While Idenix may elect to update
these forward-looking statements at some point in the future, it specifically
disclaims any obligation to do so, even if the Company's estimates change.

CONTACT: Idenix Pharmaceuticals Contact:
         Teri Dahlman (617) 218-7987

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