“LEQEMBI® Intravenous Infusion” (Lecanemab) Approved for the Treatment ofAlzheimer’s Disease in JapanTOKYO and CAMBRIDGE, Mass., September 25, 2023 – Eisai Co., Ltd. (Headquarters: Tokyo, CEO:Haruo Naito, “Eisai”) and Biogen Inc. (Nasdaq: BIIB, Corporate headquarters: Cambridge,Massachusetts, CEO: Christopher A. Viehbacher, “Biogen”) announced today that humanized antisoluble aggregated amyloid-beta (Aβ) monoclonal antibody “LEQEMBI® Intravenous Infusion” (200 mg,500mg, lecanemab) has been approved in Japan as a treatment for slowing progression of mild
cognitive impairment (MCI) and mild dementia due to Alzheimer’s disease (AD).
LEQEMBI is a humanized immunoglobulin gamma 1 (IgG1) monoclonal antibody directed against
aggregated soluble (protofibril*) and insoluble forms of Aβ. LEQEMBI is the first and only approved
treatment shown to reduce the rate of disease progression and to slow cognitive and functional decline
by selectively binding to and eliminating the most toxic Aβ aggregates (protofibrils) that contribute toneurotoxicity in AD. In Japan, an application for marketing approval was filed and was designated forpriority review in January 2023. Japan is the second country to grant approval, following the traditional
approval in the U.S. in July 2023.
LEQEMBI’s approval is based on Phase 3 data from Eisai’s large, global Clarity AD clinical trial, in
which LEQEMBI met its primary endpoint and all key secondary endpoints with statistically significant
results and confirmed the clinical benefit of LEQEMBI. The primary endpoint was the global cognitiveand functional scale, Clinical Dementia Rating Sum of Boxes (CDR-SB). In the Clarity AD clinical trial,
treatment with LEQEMBI reduced clinical decline on CDR-SB by 27% at 18 months compared toplacebo. In addition, the secondary endpoint from the AD Cooperative Study-Activities of Daily Living
Scale for Mild Cognitive Impairment (ADCS MCI-ADL), which measures information provided by people
caring for patients with AD, noted a statistically significant benefit of 37% compared to placebo. The
ADCS MCI-ADL assesses the ability of patients to function independently, including being able to dress,
feed themselves and participate in community activities. The most common adverse events (>10%) in
the LEQEMBI group were infusion reactions, ARIA-H (combined cerebral microhemorrhages, cerebral
macrohemorrhages, and superficial siderosis), ARIA-E (edema/effusion), headache, and fall. Full
results of the Clarity AD study were presented at the Clinical Trials on Alzheimer's Disease (CTAD)
2022 conference and simultaneously published in the peer-reviewed medical journal The New England
Journal of Medicine on November 29, 2022.
“Today LEQEMBI received approval, making it the first approved anti-amyloid Alzheimer's diseasetreatment shown to reduce the rate of disease progression and to slow cognitive impairment in the earlyand mild dementia stages of the disease in Japan. We believe that we have turned a new page in thehistory of Alzheimer's disease treatment. Alzheimer's disease is a progressive and serious disease that
not only causes significant impairment and burden for the people living with it and their care partners,
but also has a tremendous impact on society as a whole,” said Haruo Naito, Chief Executive Officer atEisai. “For around 40 years since we began research on dementia at our Tsukuba ResearchLaboratories, Eisai has interacted with people with dementia and their care partners, and made efforts
to understand their sincere feelings. In response, we have been taking on the challenge to develop

1therapeutic agents that act on the underlying pathology of Alzheimer's disease. We are committed todelivering LEQEMBI to the people with early Alzheimer's disease who need it and their families as anew treatment that removes the cause of the disease. Through these efforts, we aim to create impact
on issues surrounding dementia in Japanese society.”“With this approval, alongside Eisai, we will be able to help address the devastating impact Alzheimer’s
has on people living with the condition as well the emotional, social and financial burden it places oncare partners,” said Christopher A. Viehbacher, President and Chief Executive Officer of Biogen. “Thisis a significant step in the work of Biogen and Eisai to usher in a new era of treatments for this disease
which impacts millions. We look forward to working alongside Eisai to build on the approvals in the U.S.
and now Japan to bring this option to patients and their families worldwide.”
Eisai will conduct a post-marketing special use results survey (all-case surveillance) in all patients who
are administered LEQEMBI until data from a certain number of patients are accumulated after market
launch, in accordance with an approval condition imposed by the Ministry of Health, Labour and Welfare.
In addition, the appropriate use of LEQEMBI will be promoted in accordance with the package insert
and training materials will be developed for healthcare professionals to assist the management andmonitoring of amyloid-related imaging abnormalities (ARIA).
Eisai serves as the lead of LEQEMBI development and regulatory submissions globally with both Eisaiand Biogen co-commercializing and co-promoting the product and Eisai having final decision-makingauthority. In Japan, Eisai and Biogen Japan will co-promote LEQEMBI, with Eisai distributing theproduct as the Marketing Authorization Holder.
*Protofibrils are large Aβ aggregated soluble species of 75-5000 Kd.1,2,3.

MEDIA CONTACTS
Eisai Co., Ltd.
Public Relations Department
TEL: +81 (0)3-3817-5120

Biogen Inc.
Jack Cox+ 1-781-464-3260
public.affairs@biogen.com

Eisai Inc. (U.S.)
Libby Holman+ 1-201-753-1945
Libby_Holman@eisai.com
INVESTOR CONTACTS
Eisai Co., Ltd.
Investor Relations Department
TEL: +81 (0) 3-3817-5122

Biogen Inc.
Chuck Triano+ 1-781-464-2442
IR@biogen.com

2Notes to Editors
1.

Product Outline in Japan
1) Product name
LEQEMBI® Intravenous Infusion 200mg, LEQEMBI® Intravenous Infusion 500mg
2)

Generic nameLecanemab (recombinant)3)

Indication for use
Slowing progression of mild cognitive impairment (MCI) and mild dementia due to Alzheimer’s disease.

4)

Dosage and administration
The usual dose of lecanemab (recombinant) is 10mg/kg infused intravenously over approximately 1 hour,
once every 2 weeks.

2. About LEQEMBILEQEMBI (lecanemab) is the result of a strategic research alliance between Eisai and BioArctic. LEQEMBI is ahumanized immunoglobulin gamma 1 (IgG1) monoclonal antibody directed against aggregated soluble (protofibril)and insoluble forms of amyloid-beta (Aβ). LEQEMBI is an amyloid beta-directed antibody indicated as a diseasemodifying treatment for Alzheimer’s disease (AD) in the U.S. The U.S. Food and Drug Administration (FDA) granted
LEQEMBI accelerated approval on January 6, 2023, and traditional approval on July 6, 2023. In the U.S., treatment
with LEQEMBI should be initiated in patients with mild cognitive impairment or mild dementia stage of disease, thepopulation in which treatment was initiated in clinical trials. There are no safety or effectiveness data on initiating
treatment at earlier or later stages of the disease than were studied.
Please see full U.S. Prescribing Information for LEQEMBI, including Boxed WARNING.
Eisai has also submitted applications for approval of lecanemab in EU, China, Canada, Great Britain, Australia,
Switzerland, South Korea and Israel. In China and Israel, the applications have been designated for priority review,
and in Great Britain, lecanemab has been designated for the Innovative Licensing and Access Pathway (ILAP),
which aims to reduce the time to market for innovative medicines.
Eisai has completed a lecanemab subcutaneous bioavailability study, and subcutaneous dosing is currently being
evaluated in the Clarity AD (Study 301) open-label extension (OLE). A maintenance dosing regimen has been
evaluated as part of Study 201.
Since July 2020 the Phase 3 clinical study (AHEAD 3-45) for individuals with preclinical AD, meaning they areclinically normal and have intermediate or elevated levels of amyloid in their brains, is ongoing. AHEAD 3-45 isconducted as a public-private partnership between the Alzheimer's Clinical Trial Consortium that provides theinfrastructure for academic clinical trials in AD and related dementias in the U.S, funded by the National Instituteon Aging, part of the National Institutes of Health, Eisai and Biogen.
Since January 2022, the Tau NexGen clinical study for Dominantly Inherited AD (DIAD), that is conducted byDominantly Inherited Alzheimer Network Trials Unit (DIAN-TU), led by Washington University School of Medicine
in St. Louis, is ongoing and includes lecanemab as the backbone anti-amyloid therapy.
3. About the Collaboration between Eisai and Biogen for AD
Eisai and Biogen have been collaborating on the joint development and commercialization of AD treatments since2014. Eisai serves as the lead of LEQEMBI development and regulatory submissions globally with both companies
co-commercializing and co-promoting the product and Eisai having final decision-making authority.

34. About the Collaboration between Eisai and BioArctic for ADSince 2005, Eisai and BioArctic have had a long-term collaboration regarding the development andcommercialization of AD treatments. Eisai obtained the global rights to study, develop, manufacture and market
LEQEMBI for the treatment of AD pursuant to an agreement with BioArctic in December 2007. The development
and commercialization agreement on the antibody LEQEMBI back-up was signed in May 2015.
5. About Eisai Co., Ltd.
Eisai's Corporate Concept is "to give first thought to patients and people in the daily living domain, and to increase
the benefits that health care provides." Under this Concept (also known as human health care (hhc) Concept), we
aim to effectively achieve social good in the form of relieving anxiety over health and reducing health disparities.
With a global network of R&D facilities, manufacturing sites and marketing subsidiaries, we strive to create anddeliver innovative products to target diseases with high unmet medical needs, with a particular focus in our strategicareas of Neurology and Oncology.
In addition, we demonstrate our commitment to the elimination of neglected tropical diseases (NTDs), which is atarget (3.3) of the United Nations Sustainable Development Goals (SDGs), by working on various activities together
with global partners.
For more information about Eisai, please visit www.eisai.com (for global headquarters: Eisai Co., Ltd.), and connect
with us on Twitter, LinkedIn and Facebook.
6. About Biogen
Founded in 1978, Biogen is a leading global biotechnology company that has pioneered multiple breakthroughinnovations including a broad portfolio of medicines to treat multiple sclerosis, the first approved treatment for spinal
muscular atrophy, and two co-developed treatments to address a defining pathology of Alzheimer’s disease.
Biogen is advancing a pipeline of potential novel therapies across neurology, neuropsychiatry, specialized
immunology and rare diseases and remains acutely focused on its purpose of serving humanity through science
while advancing a healthier, more sustainable and equitable world.
The company routinely posts information that may be important to investors on its website at www.biogen.com.
Follow Biogen on social media – Twitter, LinkedIn, Facebook, YouTube.

Biogen Safe Harbor
This news release contains forward-looking statements, including statements made pursuant to the safe harborprovisions of the Private Securities Litigation Reform Act of 1995, about the potential clinical effects of LEQEMBI;
the potential benefits, safety and efficacy of LEQEMBI; potential regulatory discussions, submissions and approvals
and the timing thereof; the treatment of Alzheimer's disease; the anticipated benefits and potential of Biogen'scollaboration arrangements with Eisai; the potential of Biogen's commercial business and pipeline programs,including LEQEMBI; and risks and uncertainties associated with drug development and commercialization. Thesestatements may be identified by words such as "aim," "anticipate," "believe," "could," "estimate," "expect,""forecast," "intend," "may," "plan," "possible," "potential," "will," "would" and other words and terms of similar
meaning. Drug development and commercialization involve a high degree of risk, and only a small number of
research and development programs result in commercialization of a product. Results in early-stage clinical studies
may not be indicative of full results or results from later stage or larger scale clinical studies and do not ensure
regulatory approval. You should not place undue reliance on these statements or the scientific data presented.
These statements involve risks and uncertainties that could cause actual results to differ materially from thosereflected in such statements, including without limitation unexpected concerns that may arise from additional data,
analysis or results obtained during clinical studies, including the Clarity AD clinical trial and AHEAD 3-45 study; theoccurrence of adverse safety events; risks of unexpected costs or delays; the risk of other unexpected hurdles;
regulatory submissions may take longer or be more difficult to complete than expected; regulatory authorities may

4require additional information or further studies, or may fail or refuse to approve or may delay approval of Biogen's
drug candidates, including LEQEMBI; actual timing and content of submissions to and decisions made by theregulatory authorities regarding LEQEMBI; uncertainty of success in the development and potentialcommercialization of LEQEMBI; failure to protect and enforce Biogen's data, intellectual property and otherproprietary rights and uncertainties relating to intellectual property claims and challenges; product liability claims;
third party collaboration risks; and the direct and indirect impacts of the ongoing COVID-19 pandemic on Biogen'sbusiness, results of operations and financial condition. The foregoing sets forth many, but not all, of the factors that
could cause actual results to differ from Biogen's expectations in any forward-looking statement. Investors should
consider this cautionary statement as well as the risk factors identified in Biogen's most recent annual or quarterly
report and in other reports Biogen has filed with the U.S. Securities and Exchange Commission. These statements
are based on Biogen's current beliefs and expectations and speak only as of the date of this news release. Biogen
does not undertake any obligation to publicly update any forward-looking statements, whether as a result of new
information, future developments or otherwise.
References
1. https://www.alzforum.org/news/conference-coverage/lecanemab-sweeps-toxic-av-protofibrils-catches-eyestrialists
2. Sehlin D, Englund H, Simu B, Karlsson M, Ingelsson M, Nikolajeff F, Lannfelt L, Pettersson FE. Large
aggregates are the major soluble Aβ species in AD brain fractionated with density gradient ultracentrifugation.
PLoS One. 2012;7(2):e32014. https://doi.org/10.1371/journal.pone.0032014 Epub 2012 Feb 15. PMID:
22355408; PMCID: PMC3280222.
3. Söderberg, L., Johannesson, M., Nygren, P. et al. Lecanemab, Aducanumab, and Gantenerumab — Binding
Profiles to Different Forms of Amyloid-Beta Might Explain Efficacy and Side Effects in Clinical Trials for
Alzheimer’s Disease. Neurotherapeutics. 2023;20:195-206. https://doi.org/10.1007/s13311-022-01308-6

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