Idenix Pharmaceuticals Announces Collaboration With Janssen to Initiate Phase II All-Oral Combination Studies Including IDX719,

Idenix Pharmaceuticals Announces Collaboration With Janssen to Initiate Phase
II All-Oral Combination Studies Including IDX719, Simeprevir (TMC435) and
TMC647055 for the Treatment of Hepatitis C Virus (HCV)

CAMBRIDGE, Mass., Jan. 28, 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
announced a non-exclusive collaboration with Janssen Pharmaceuticals, Inc. for
the clinical development of all-oral direct-acting antiviral (DAA) HCV
combination therapies. The collaboration will evaluate combinations including
IDX719, Idenix's once-daily pan-genotypic NS5A inhibitor, simeprevir (TMC435),
a once-daily protease inhibitor jointly developed by Janssen and Medivir AB,
and TMC647055, a once-daily non-nucleoside polymerase inhibitor, boosted with
low dose ritonavir, being developed by Janssen.

Clinical development plans include an initial drug-drug interaction study to
begin in the first quarter of 2013, followed by phase II studies as agreed
between the companies, and pending approval from regulatory authorities. The
phase II program is expected to first evaluate the two-DAA combination of
IDX719 and simeprevir plus ribavirin for 12 weeks in treatment-naïve
HCV-infected patients. Subsequently, the companies plan to evaluate a
three-DAA combination of IDX719, simeprevir and TMC647055/r, with and without
ribavirin. The clinical trials will be conducted by Idenix. Both companies
retain all rights to their respective compounds under this agreement.

"We are very pleased to be working with Janssen and look forward to initiating
a phase II study in the first quarter of this year," said Ron Renaud, Idenix's
President and Chief Executive Officer. "This will allow us to achieve a key
goal of ours for 2013, which is to advance the development of IDX719 as part
of all-oral HCV combinations in two- and three-drug regimens."

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 SIMEPREVIR (TMC435)

Simeprevir is a once-daily potent investigational hepatitis C protease
inhibitor in late Phase 3 clinical development being jointly developed
byJanssen R&D Irelandand Medivir AB to treat chronic hepatitis C virus
infections. Simeprevir is being investigated in combination with PegIFN/RBV in
Phase 3 trials and is also being evaluated with Direct-acting Antiviral (DAA)
agents in three other Phase 2 interferon-free combinations both with and
without ribavirin (RBV). For further details please
visithttp://www.medivir.com.

ABOUT TMC647055

TMC647055 is a potent non-nucleoside hepatitis C polymerase inhibitor with
broad genotypic coverage. TMC647055 is in Phase 2 clinical development and is
developed by Janssen R&D Ireland to treat chronic hepatitis C virus
infections. TMC647055 is being investigated in combination with other DAA
agents in all oral interferon-free regimens. There have been no
treatment-emergent serious adverse events reported in the program.

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 hepatitis C infection. 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, including but not limited to the statements regarding the Company's
future business and financial performance. 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," "will," and similar expressions are also intended to
identify forward-looking statements, as are expressed or implied statements
with respect to the Company's potential pipeline candidates, including any
expressed or implied statements regarding the efficacy and safety of IDX719
and the likelihood and success of any future clinical trials involving
IDX719. 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; 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 clinical
trials evaluating its product candidates; the Company's ability to obtain
additional funding required to conduct its research, development and
commercialization activities; 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. 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. 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, 2011 and
the quarterly report on Form 10-Q for the quarter ended September 30, 2012,
each 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 Contacts:
         Kelly Barry (617) 995-9033 (media)
         Teri Dahlman (617) 995-9807 (investors)
 
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