Idenix Pharmaceuticals, Inc. (Nasdaq: IDIX) today announced the approval of TYZEKA(TM) (telbivudine) by the U.S. Food and Drug Administration (FDA) as a new once-a-day oral treatment, taken with or without food, for patients with chronic hepatitis B (CHB). TYZEKA rapidly and profoundly(1) suppresses the hepatitis B virus (HBV) in adult patients with evidence of viral replication and either evidence of persistent elevations in serum aminotransferases (ALT or AST) or histologically active disease.

"The FDA approval of TYZEKA is a major milestone for Idenix as it is the first Idenix drug to receive U.S. approval," said Jean-Pierre Sommadossi, Ph.D., chairman and chief executive officer at Idenix Pharmaceuticals, Inc. "Receiving approval just six years after TYZEKA entered clinical development is a tremendous accomplishment and demonstrates Idenix's commitment to discovering and developing new treatment options for patients affected by devastating viral diseases."

"Profound suppression of the hepatitis B virus is associated with improved outcomes and is a primary treatment goal," said Adrian M. Di Bisceglie, MD, Professor of Medicine and Chief of Hepatology, Division of Gastroenterology and Hepatology, at Saint Louis University, and Co-Director, Saint Louis University Liver Center. "TYZEKA's ability to provide rapid viral suppression in the first 24 weeks of treatment, along with its safety and tolerability profile, make it a promising treatment option for appropriate patients."

Data from the pivotal phase III clinical trial, known as the GLOBE study, compared TYZEKA to lamivudine in 1,367 patients. The primary efficacy endpoint of the GLOBE study was therapeutic response at one year, a composite endpoint coupling viral suppression (serum HBV DNA suppression below 100,000 copies/mL) with either improved liver disease markers (ALT normalization) or loss of detectable hepatitis B e-antigen (HBeAg). In HBeAg-positive patients, therapeutic response was 75 percent among patients treated with TYZEKA and 67 percent for those patients treated with lamivudine, while the response for HBeAg-negative patients after one year was 75 percent vs. 77 percent, respectively. In the GLOBE study, patients who achieved non-detectable HBV DNA levels at 24 weeks were more likely to undergo e-antigen seroconversion, achieve undetectable levels of HBV DNA, normalize ALT, and minimize resistance at one year.

In clinical studies TYZEKA was generally well tolerated with most adverse experiences classified as mild or moderate in severity. Frequently occurring adverse events (> 5%) were upper respiratory tract infection (14%), fatigue and malaise (12%), abdominal pain (12%), nasopharyngitis (11%), headache (11%), blood CPK increased (9%), cough (7%), nausea and vomiting (7%), influenza and influenza-like symptoms (7%), post-procedural pain (7%), diarrhea and loose stools (7%), pharyngolaryngeal pain (5%). Please see Important Safety Information.

"The approval of TYZEKA is based primarily on the efficacy and safety data from the GLOBE study, the largest worldwide registration trial ever conducted to date in patients with chronic hepatitis B," said Nathaniel Brown, MD, chief medical officer of Idenix Pharmaceuticals, Inc. "We believe that TYZEKA will be an important new treatment option for patients with hepatitis B."

About TYZEKA

TYZEKA (telbivudine) is indicated for the treatment of chronic hepatitis B in adult patients with evidence of viral replication and either evidence of persistent elevations in serum aminotransferases (ALT or AST) or histologically active disease.

This indication is based on virologic, serologic, biochemical and histologic responses after one year of treatment in nucleoside-treatment-naïve adult patients with HBeAg-positive and HBeAg-negative chronic hepatitis B with compensated liver disease.

Already approved in Switzerland, telbivudine will be marketed as SEBIVO(R) outside the United States. Applications for approval were filed with the European Medicines Agency (EMEA) and the Chinese health authority in the first quarter of 2006.

Important Safety Information About TYZEKA

-- Lactic acidosis and severe hepatomegaly with steatosis, including fatal cases, have been reported with the use of nucleoside analogues alone or in combination with antiretrovirals.

-- Severe acute exacerbations of hepatitis B have been reported in patients who have discontinued anti-hepatitis B therapy, including TYZEKA. Hepatic function should be monitored closely with both clinical and laboratory follow-up for at least several months in patients who discontinue anti-hepatitis B therapy. If appropriate, resumption of anti-hepatitis B therapy may be warranted.

-- Cases of myopathy have been reported with TYZEKA use several weeks to months after starting therapy. Myopathy has also been reported with some other drugs in this class. Physicians considering concomitant treatment with these or other agents associated with myopathy should weigh carefully the potential benefits and risks and should monitor and advise patients to report any signs or symptoms of unexplained muscle pain, tenderness or weakness, particularly during periods of upward dosage titration. TYZEKA therapy should be interrupted if myopathy is suspected, and discontinued if myopathy is diagnosed.

-- Because TYZEKA is eliminated primarily by renal excretion, co-administration of TYZEKA with drugs that affect renal function may alter plasma concentrations of TYZEKA and/or the co-administered drug. Dose interval adjustment is recommended in patients with creatinine clearance < 50mL/min.

-- The safety and efficacy of TYZEKA in liver transplant recipients are unknown. If TYZEKA treatment is determined to be necessary for a liver transplant recipient who has received or is receiving an immunosuppressant that may affect renal function, such as cyclosporine or tacrolimus, renal function should be monitored both before and during treatment with TYZEKA.

-- Patients should be advised that treatment with TYZEKA has not been shown to reduce the risk of transmission of HBV to others through sexual contact or blood contamination.

-- Safety and effectiveness of TYZEKA in pediatric patients under the age of 16 years have not been established.

-- Selected treatment-emergent clinical adverse events of moderate to severe intensity (Grade 2-4) reported in the GLOBE study with Tyzeka were: muscle-related symptoms 2%; fatigue/malaise 1%; headache 1%; pyrexia 1%; abdominal pain