Biogen Idec Inc. : New Data Presented at 64th AAN Annual Meeting Highlights Biogen Idec's Commitment to Deliver Promising Therapies for Unmet Needs in Neurodegenerative Diseases

Biogen Idec (NASDAQ: BIIB) will highlight its commitment to using novel science to address unmet medical needs in neurology at the upcoming 64^th American Academy of Neurology (AAN) Annual Meeting. The meeting, held in New Orleans April 21-28, 2012, features 49 company-sponsored platform and poster presentations that demonstrate how, through focused research and development programs, Biogen Idec is pursuing treatments for neurological diseases with high unmet need.

In addition to presenting data from three marketed products in multiple sclerosis (MS) - TYSABRI^® (natalizumab), AVONEX^® (interferon beta-1a), and FAMPYRA^® (prolonged-release fampridine tablets) - the company will present results from investigational trials of its late-stage pipeline, including: BG-12, PEGylated interferon beta-1a, and daclizumab high-yield process (DAC HYP) for MS; and dexpramipexole, a potential treatment for amyotrophic lateral sclerosis (ALS).

"We are committed to using novel science to tackle the unmet medical needs of those living with chronic and debilitating neurologic conditions," said Douglas E. Williams, Ph.D., executive vice president, Research and Development at Biogen Idec. "Our focus on research and development of MS therapies has resulted in numerous product offerings and an extensive pipeline addressing the individual needs of many MS patients, both now and in the future. As we expand this expertise into other diseases areas such as ALS, we will continue to make progress toward addressing significant unmet medical needs in neurology."

Key study results to be presented include:

• BG-12: Detailed results from CONFIRM (Comparator and an Oral Fumarate in RRMS), the second pivotal Phase 3 study evaluating the investigational oral compound in people with relapsing-remitting MS (RRMS), will be presented for the first time.
• TYSABRI: Important new data from two studies:
  • The ongoing TYSABRI Observational Program (TOP) assessing long-term outcomes in RRMS patients in the postmarketing setting
  • TYNERGY, a multicenter one-year clinical follow-up study conducted to evaluate the effect of TYSABRI on MS-related fatigue
• DEXPRAMIPEXOLE: Design, methodology and baseline features of EMPOWER, the largest randomized, placebo-controlled, Phase 3 clinical trial conducted in patients with ALS to date

Additional presentations include long-term data for AVONEX; data highlighting FAMPYRA, the first oral formulation indicated for the improvement of walking in adult MS patients with walking disability; and full data from the DAC HYP Phase 2b SELECT trial.

Notable data from Biogen Idec at AAN 2012:

BG-12

• Clinical Efficacy of BG-12 in Relapsing-Remitting Multiple Sclerosis (RRMS): Data From the Phase 3 CONFIRM Study - Platform S01.003
• Safety and Tolerability of BG-12 in Patients With Relapsing-Remitting Multiple Sclerosis (RRMS): Analyses From the CONFIRM Study - Platform S41.005

TYSABRI

• Updated Incidence of Progressive Multifocal Leukoencephalopathy in Natalizumab-Treated Multiple Sclerosis Patients Stratified by Established Risk Factors - Platform S41.001
• Anti-JCV Antibody Prevalence in Patients with Relapsing Multiple Sclerosis Receiving or Considering Treatment with Natalizumab: Baseline Results of STRATIFY-2 - Platform S41.002
• Long-Term Safety and Efficacy and Association Between Baseline Treatment History and Postbaseline Relapses in Multiple Sclerosis Patients Treated with Natalizumab in the TYSABRI Observational Program (TOP) - Poster P04.134
• Natalizumab Reduces Fatigue as Measured by the Fatigue Scale for Motor and Cognitive Functions (FSMC) - First Results from the TYNERGY Trial - Poster P07.081

AVONEX

• An Early Disease Activity Composite Can Predict Long-Term Disease Outcome in CHAMPIONS - Discussed Poster PD5.010
• Does Change in Patient-Reported QOL Correlate with Change in Other Clinical and MRI Measures in Early MS? Analysis of the 10-Year CHAMPIONS Cohort - Poster P07.100

FAMPYRA

• Relationship between Heat Intolerance and Response to Prolonged-Release Fampridine in Patients with Multiple Sclerosis - Poster P07.078
• Response to Prolonged-Release Fampridine in Multiple Sclerosis Patients with Various Walking-Related MS Symptoms - Poster P07.079

DAC HYP

• A Randomized, Double-Blind, Placebo-Controlled Study to Evaluate the Safety and Efficacy of Daclizumab HYP Monotherapy in Relapsing-Remitting Multiple Sclerosis: Primary Results of the SELECT Trial - Platform S01.005
• CD56^bright Natural Killer Cell Expansion Predicts Response to Daclizumab HYP Treatment in RRMS: Results of the SELECT Trial - Platform S31.004

PEGYLATED INTERFERON BETA-1A

• ADVANCE Phase 3 Study of PEGylated Interferon Beta-1a for Relapsing Multiple Sclerosis: Patient Baseline Characteristics - Poster P01.133
• PEGylated Interferon Beta-1a Pharmacokinetics, Pharmacodynamics and Safety in Subjects with Normal or Impaired Renal Function - Poster P06.165

DEXPRAMIPEXOLE

• The EMPOWER Study: Design, Methodology and Baseline Features of the First Phase 3 Clinical Trial of Dexpramipexole for Patients with ALS - Platform S25.004

Full session details and data presentation listings for the 2012 Annual Meeting can be found through the AAN website (www.aan.com/go/am12).

About Biogen Idec

Through cutting-edge science and medicine, Biogen Idec discovers, develops and delivers to patients worldwide innovative therapies for the treatment of neurodegenerative diseases, hemophilia and autoimmune disorders. Founded in 1978, Biogen Idec is the world's oldest independent biotechnology company. Patients worldwide benefit from its leading multiple sclerosis therapies, and the company generates more than $5 billion in annual revenues. For product labeling, press releases and additional information about the company, please visit www.biogenidec.com.

About TYSABRI

TYSABRI is approved in more than 65 countries. TYSABRI is approved in the United States as a monotherapy for relapsing forms of MS, generally for patients who have had an inadequate response to, or are unable to tolerate, an alternative MS therapy. In the European Union, it is approved for highly active RRMS in adult patients who have failed to respond to beta interferon or have rapidly evolving, severe RRMS.

TYSABRI has advanced the treatment of MS patients with its established efficacy. Data from the Phase 3 AFFIRM trial, which was published in the New England Journal of Medicine, showed that after two years, TYSABRI treatment led to a 68 percent relative reduction (p<0.001) in the annualized relapse rate when compared with placebo and reduced the relative risk of disability progression by 42-54 percent (p<0.001).

TYSABRI increases the risk of progressive multifocal leukoencephalopathy (PML), an opportunistic viral infection of the brain which usually leads to death or severe disability. Infection by the JC virus (JCV) is required for the development of PML and patients who are anti-JCV antibody positive have a higher risk of developing PML. Factors that increase the risk of PML are presence of anti-JCV antibodies, prior immunosuppressant use, and longer TYSABRI treatment duration. Patients who have all three risk factors have the highest risk of developing PML. Other serious adverse events that have occurred in TYSABRI-treated patients include hypersensitivity reactions (e.g., anaphylaxis) and infections, including opportunistic and other atypical infections. Clinically significant liver injury has also been reported in the post-marketing setting. A list of adverse events can be found in the full TYSABRI product labeling for each country where it is approved.

TYSABRI is marketed and distributed by Biogen Idec Inc. and Elan Corporation, plc. For full prescribing information and more information about TYSABRI, please visit www.biogenidec.com or www.elan.com.

About AVONEX

AVONEX is one of the most prescribed treatments for relapsing forms of MS worldwide. It has been approved for use in the United States and European Union for more than 15 years. AVONEX is indicated for the treatment of patients with relapsing forms of MS to slow the accumulation of physical disability and decrease the frequency of clinical exacerbations. Patients with MS in whom efficacy has been demonstrated include patients who have experienced a first clinical episode and have MRI features consistent with MS.

AVONEX should be used with caution in patients with depression or other mood disorders and in patients with seizure disorders. AVONEX should not be used by pregnant women. Patients should also be monitored for signs of hepatic injury. Rare cases of anaphylaxis have been reported. Patients with cardiac disease should be closely monitored. Routine periodic blood chemistry and hematology tests are recommended during treatment with AVONEX.

The most common side effects associated with AVONEX treatment are flu-like symptoms, including chills, fever, myalgia, and asthenia.

About FAMPYRA

FAMPYRA is a prolonged-release (sustained release) tablet formulation of the drug fampridine (4-aminopyridine, 4-AP or dalfampridine). FAMPYRA has been developed to improve walking in adult patients with MS. In MS, damaged myelin exposes channels in the membrane of axons allowing potassium ions to leak, weakening the electrical current sent through nerves. Studies have shown that fampridine can increase conduction along damaged nerves, which may result in improved walking ability. This prolonged-release formulation was developed and is being commercialized in the United States by Acorda Therapeutics, Inc. (NASDAQ: ACOR) under the trade name AMPYRA^® (dalfampridine) Extended Release Tablets, 10 mg. Biogen Idec licensed rights from Acorda to develop and commercialize fampridine in all markets outside the United States.

About BG-12

BG-12 (dimethyl fumarate) is an investigational oral therapy for which two large pivotal Phase 3 clinical studies for the treatment of RRMS, the most common form of MS, have been completed. Biogen Idec has recently filed regulatory submissions for BG-12 in the United States and European Union. BG-12 is the only currently known investigational compound for the treatment of RRMS that has experimentally demonstrated activation of the Nrf-2 pathway. In 2011, Biogen Idec announced positive data from DEFINE and CONFIRM, two global, placebo-controlled Phase 3 clinical trials that evaluated 240 mg of BG-12, administered either twice a day or three times a day, for two years.

About Daclizumab High-Yield Process

Daclizumab high-yield process (DAC HYP) is a subcutaneous formulation of daclizumab in late-stage clinical development for the treatment of RRMS, the most common form of MS. DAC HYP is a humanized monoclonal antibody that binds to CD25, a receptor subunit that is expressed at high levels on T-cells that are thought to become abnormally activated in autoimmune conditions such as MS. Data from previous clinical trials showed that DAC HYP increases CD56^bright Natural Killer cells, which target the activated immune cells that can play a key role in MS without causing general immune cell depletion. DAC HYP is currently being studied in the DECIDE Phase 3 clinical trial, which is evaluating the efficacy and safety of once-monthly subcutaneous DAC HYP as a monotherapy compared to interferon beta 1-a therapy.

About PEGylated Interferon Beta-1a

PEGylated interferon beta-1a is currently under investigation in a Phase 3 clinical trial for the treatment of relapsing MS. PEGylated interferon beta-1a, administered via subcutaneous injection, is being evaluated for its ability to last longer in a patient's system, potentially leading to an MS treatment that would require fewer injections.

About Dexpramipexole

Dexpramipexole is a novel, orally administered compound under development for the treatment of ALS. It has shown neuroprotective properties in both in vitro assays and in vivo ALS models. In a Phase 2 study, dexpramipexole achieved its primary objective evaluating safety and tolerability and also showed a trend toward dose-related slowing of functional decline and a trend toward extending survival at the highest dose (150 mg twice daily). Dexpramipexole has been granted Fast Track status by the U.S. Food and Drug Administration (FDA), which may result in an expedited review, and has received orphan drug designation for the treatment of ALS from both the FDA and the European Medicines Agency. Biogen Idec and Knopp Neurosciences have an exclusive, worldwide license agreement for dexpramipexole.

Safe Harbor

This press release contains forward-looking statements, including statements about the anticipated development, timing and therapeutic scope of programs in our clinical pipeline. These forward-looking statements may be accompanied by such words as "anticipate," "believe," "estimate," "expect," "forecast," "intend," "may," "plan," "project," "target," "will" and other words and terms of similar meaning. You should not place undue reliance on these statements.

These statements involve risks and uncertainties that could cause actual results to differ materially from those reflected in such statements, including the risk that unexpected concerns and adverse safety events may arise during clinical trials, regulatory authorities may require additional information or may fail to approve any potential new therapy, competition, the ability to protect intellectual property rights and the cost of doing so, uncertainty of success in commercializing products, changes in the availability of product reimbursement, failure to comply with government regulation and possible adverse impact of changes in such regulation, product liability claims, and the other risks and uncertainties that are described in the Risk Factors section of Biogen Idec Inc.'s most recent annual or quarterly report and in other reports Biogen Idec Inc. has filed with the U.S. Securities and Exchange Commission. These statements are based on current beliefs and expectations and speak only as of the date of this press release. No obligation to publicly update any forward-looking statements is undertaken.

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