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TOKYO and CAMBRIDGE, Mass. – March 5, 2014 – °®¶¹´«Ã½.(TSE: 4523) and (NASDAQ: BIIB) announced today that they have entered into a collaboration to develop and commercialize two of Eisai's clinical candidates for Alzheimer's disease (AD), E2609 and BAN2401. The agreement also provides Eisai with an option to jointly develop and commercialize two of Biogen Idec's candidates for AD, the anti-amyloid beta (Aβ) antibody BIIB037 and an anti-tau monoclonal antibody.

The collaboration initially will be centered on the co-development and co-commercialization of Eisai's two candidates: E2609, a β-site amyloid precursor protein cleaving enzyme (BACE) inhibitor, and BAN2401, an anti-Aβ antibody. These candidates have the potential to reduce Aβ plaques that form in the brains of patients with AD and to stop the formation of new plaques, potentially improving symptoms and suppressing disease progression.

Eisai will serve as the operational and regulatory lead in the co-development of E2609 and BAN2401 and will pursue marketing authorizations for both compounds worldwide. In major markets, such as the United States and the European Union, Eisai and Biogen Idec will also co-promote the products following marketing approval. Both companies will share overall costs, including research and development expenses, with Eisai booking all sales for E2609 and BAN2401 and with profits to be split between the companies.

Biogen Idec will provide Eisai with an upfront payment and a fixed amount of development, approval and commercial milestone payments. The agreement also includes options for Eisai to receive an additional one-time payment from Biogen Idec related to joint development and commercialization activities in Japan.

“This collaboration is a natural fit with our mission to develop therapies for patients with severe neurodegenerative diseases. Eisai's candidates have demonstrated compelling early data and complement our AD research while extending our pipeline in this critical area,” said George A. Scangos, Ph.D., chief executive officer of Biogen Idec. “Eisai is a pioneer in successfully developing and commercializing AD treatments. This history, combined with their strong scientific heritage, geographical reach and unwavering commitment to the AD community, makes Eisai an excellent collaboration partner to help drive our mission.”

°®¶¹´«Ã½. President & CEO Haruo Naito added, “There exists an urgent need to develop AD therapies that suppress disease progression in order to effectively alleviate both the emotional burden of suffering and uncertainty experienced by patients and their families, as well as the financial burden on overall society in terms of the huge costs of nursing and patient care. Eisai remains deeply focused on the development of such therapies based on the knowledge and experience it has accumulated through its development of the anti-AD agent Aricept®. Through our collaboration with Biogen Idec, a company that specializes in neurodegenerative diseases, I believe we will be able to further enhance our existing R&D capacities for developing next-generation AD treatments, thereby accelerating the development of promising therapies and increasing the benefits provided to patients with AD worldwide.”

E2609, discovered in-house by Eisai, is an investigational next-generation oral candidate for the treatment of AD that is believed to inhibit BACE, a key enzyme in the production of Aβ. By inhibiting BACE, E2609 decreases Aβ proteins in the brain, potentially improving symptoms and slowing disease progression. Currently, E2609 is undergoing preparations to enter Phase Ⅱ clinical trials.

BAN2401 is a humanized monoclonal antibody that is the result of a strategic research alliance between Eisai and BioArctic Neuroscience AB to identify a potential immunotherapy for AD. BAN2401 is believed to selectively bind to, neutralize and eliminate soluble, toxic Aβ aggregates that are thought to contribute to the neurodegenerative process in AD. As such, BAN2401 has the potential to have an immunomodulatory effect that may suppress the progression of the disease. Eisai obtained the global rights to study, develop, manufacture and market BAN2401 for the treatment of AD pursuant to an agreement concluded with BioArctic Neuroscience AB in December 2007. Currently, the compound is undergoing Phase Ⅱ clinical trials.

BIIB037 is an anti-Aβ human monoclonal antibody that is currently under investigation by Biogen Idec as a treatment for AD under a license from Neurimmune. It is believed that BIIB037 binds to and eliminates the toxic amyloid plaques that form in the brains of patients with AD, thereby potentially suppressing the progression of the disease. BIIB037 is currently undergoing a Phase Ib clinical trial.

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. For product labeling, press releases and additional information about the Company, please visit .

This press release contains forward-looking statements, including statements about Biogen Idec's expectations and mission to develop treatments for people with Alzheimer's disease, through its collaboration with Eisai. These forward-looking statements may be accompanied by such words as “anticipate,” “believe,” “estimate,” “expect,” “forecast,” “intend,” “may,” “plan,” “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 risks and uncertainties associated with drug development and commercialization, Biogen Idec's dependence on third parties over which it may not always have full control, and the other risks and uncertainties that are described in the Risk Factors section of Biogen Idec's most recent annual or quarterly report filed with the Securities and Exchange Commission. These statements are based on current beliefs and expectations and speak only as of the date of this press release. Biogen Idec does not undertake any obligation to publicly update any forward-looking statements.

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