TOKYO and CAMBRIDGE, Mass., May 15, 2024 ¨C 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 Eisai has initiated the rolling
submission of a Biologics License Application (BLA) to the U.S. Food and Drug Administration (FDA)
for lecanemab-irmb (U.S. brand name: LEQEMBI?) subcutaneous autoinjector for weekly maintenance
dosing after it was granted Fast Track designation by the FDA. LEQEMBI is indicated for the treatment
of Alzheimer¡¯s disease (AD) in patients with Mild Cognitive Impairment (MCI) or mild dementia stage of
disease (collectively referred to as early AD).

The BLA is based on data from the Clarity AD (Study 301) open-label extension (OLE) and modeling
of observed data. If approved by the FDA, the LEQEMBI autoinjector could be used to administer
LEQEMBI at home or at medical facilities. The injection process requires less time than the IV
formulation. As part of the subcutaneous autoinjector 360 mg weekly maintenance regimen under
review, patients who have completed the biweekly IV initiation phase would receive weekly doses that
maintain effective drug concentrations to sustain the clearance of highly toxic protofibrils* which can
continue to cause neuronal injury even after the amyloid-beta (A¦Â) plaque has been cleared from the
brain.

AD is an ongoing neurotoxic process that begins before and continues after plaque deposition. Data
suggest that early and continuing treatment may prolong the benefit even after plaque is cleared from
the brain. This SC autoinjector is easier for patients and their care partners to use, and may reduce the
need for hospital visits and nursing care compared to intravenous (IV) administration. In addition to
potentially maintaining the clinical and biomarker benefits, subcutaneous maintenance dosing may be
more convenient for patients and their care partners to continue the treatment.

LEQEMBI is now approved in the U.S., Japan and China, and applications have been submitted for
review in the European Union, Australia, Brazil, Canada, Hong Kong, Great Britain, India, Israel, Russia,
Saudi Arabia, South Korea, Taiwan, Singapore and Switzerland. Eisai submitted to the FDA a
Supplemental Biologics License Application (sBLA) for monthly LEQEMBI intravenous (IV)
maintenance dosing in March 2024.

Eisai serves as the lead for lecanemab¡¯s development and regulatory submissions globally with both
companies co-commercializing and co-promoting the product and Eisai having final decision-making
authority.

* Protofibrils are believed to contribute to the brain injury that occurs with AD and are considered to be
the most toxic form of A¦Â, having a primary role in the cognitive decline associated with this progressive,
debilitating condition. Protofibrils cause injury to neurons in the brain, which in turn, can negatively
impact cognitive function via multiple mechanisms, not only increasing the development of insoluble A¦Â
plaques but also increasing direct damage to brain cell membranes and the connections that transmit
signals between nerve cells or nerve cells and other cells. It is believed the reduction of protofibrils may
prevent the progression of AD by reducing damage to neurons in the brain and cognitive dysfunction.