Interim IDX184 Phase IIb Data and Development Pipeline Update

Interim IDX184 Phase IIb Data and Development Pipeline Update Conference Call January 9, 2012

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Safe Harbor Statement

This presentation includes forward-looking statements about Idenix and its business, including without limitation, statements regarding drug discovery, research and clinical development, regulatory approval processes and market opportunities. These forward-looking statements are subject to risks and uncertainties that may cause actual events or results to differ materially from our current expectations. These risks and uncertainties are detailed in our filings with the Securities and Exchange Commission. All forward-looking statements speak only as of the date of this presentation and, except as required by law, we undertake no obligation to update such statements.

IDX184 Phase IIb Interim Results

IDX184 Phase IIb Status and Next Steps

• DSMB concluded that there was no evidence of hepatoxicity and recommended that the study continue after review of the interim data of first 31 patients
• Interim data of 31 patients and DSMB’s recommendations were submitted to FDA in January 2012 to support potential removal of partial clinical hold
• Additional future phase IIb clinical studies also proposed in FDA submission

HCV Development Pipeline Update

NS5A Inhibitor IDX719 Promising Profile for Combination Therapy

• Clean preclinical safety profile to date
• Potential for low mg doses and QD dosing in humans
• No in vitro interaction with 7 human CYP 450 enzymes at 10 μM (well above physiologic concentrations)
• No significant interaction with human transporters at physiologic concentrations
• Additive antiviral effects with other HCV DAAs (e.g., PIs and IDX184)
• No in vitro DDIs with common HBV and HIV therapeutic agents

Novel Nucleotide Prodrug Program

• Intensive program in place
  - Robust synthetic and screening efforts ongoing, focus on single diastereomers
• Diverse spectrum of nucleotides
  -Purines and pyrimidines
  -Known prodrugs and novel prodrugs
  - 2’ Me sugars and some novel sugars
• Identify promising compounds in vitro and in mouse and monkey
  - Level of triphosphate production, kinetics of metabolism, cytotoxicity, etc
  - Levels of triphosphate in the liver after oral administration in vivo
• Lead nucleotide inhibitor candidates, IDX19368 and IDX19370, selected
  - IND-enabling studies underway with IND filings expected mid-year
  - In preclinical studies, IDX19368 generates high triphosphate levels
• Many new potential clinical candidates currently being evaluated

2012: An Eventful Year Expected to Create Value

• IDX184: Nucleotide HCV Polymerase Inhibitor
  - Potential removal of the partial clinical hold
  - Combine with one or more DAAs for combination regimen and initiate broad Phase IIb trials
  - Establish non-exclusive/exclusive collaboration
• IDX719: HCV NS5A Program
  - Successful completion of Phase I and proof-of-concept studies including evaluation in multiple genotypes
• Next-Generation Nucleotide HCV Polymerase Inhibitors
  - File INDs for lead candidates, IDX19368 and IDX19370, and initiate Phase I and proof-of-concept studies
  - Continue robust nucleotide prodrug discovery efforts

We believe we are well positioned to play a major role in the evolving HCV field.

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