TOKYO and CAMBRIDGE, Mass. — December 4, 2025 — Leads & Copy — Eisai Co., Ltd. and Biogen Inc. announced findings on LEQEMBI and its potential to slow the progression of Alzheimer’s disease (AD) at the 18th Clinical Trials on Alzheimer’s Disease (CTAD) Conference. The latest data suggests long-term treatment with LEQEMBI could delay the progression from Mild Cognitive Impairment (MCI) to moderate Alzheimer’s by up to 8.3 years in a low-amyloid group who began treatment early.
A scientific symposium was also held regarding the subcutaneous formulation (SC-AI), approved in the U.S. for maintenance treatment in August 2025. The rolling supplemental Biologics License Application (sBLA) for initiation treatment was completed in November 2025, and the application for the subcutaneous injectable formulation in Japan was submitted in November 2025.
AD, marked by amyloid-beta (Aβ) and tau, is a progressive disease caused by neurotoxic protofibrils (PF). LEQEMBI targets both PF and amyloid plaque.
An analysis of data from the Clarity AD open-label extension (OLE) and 16 clinical studies estimated long-term AD progression over 10 years and the slowing effect of continued lecanemab treatment. Compared to natural decline based on Alzheimer’s Disease Neuroimaging Initiative (ADNI) data, early initiation and long-term lecanemab treatment may slow AD progression and help maintain cognitive function over a longer period, measured using Clinical Dementia Rating – Sum of Boxes (CDR-SB).
The analysis revealed that in the untreated group, the time to progression from MCI due to AD to mild AD was 7.2 years. With continued LEQEMBI treatment, progression to mild AD took 9.7 years, a time savings of 2.5 years.
In the low-amyloid group (patients who started treatment at an early stage with amyloid PET <60 centiloids), the time to progression from MCI to mild AD was 13.2 years with continued LEQEMBI treatment, a time savings of 6.0 years.
The time to progression from MCI due to AD to moderate AD was 10.1 years in the untreated group, whereas with continued LEQEMBI treatment it took 13.6 years, a time savings of 3.5 years.
In the low-amyloid group, the time to progression to moderate AD with continued LEQEMBI treatment was 18.4 years, a time savings of 8.3 years.
Data presented at the symposium on the subcutaneous formulation focused on treatment initiation, including results from the subcutaneous (SC) formulation subcohort (n=273) in the Clarity AD trial OLE. Weekly administration of lecanemab SC-AI at 500 mg (two 250 mg injections) demonstrated bioequivalence in drug exposure compared to intravenous (IV) dosing of 10 mg/kg every two weeks (exposure ratio: 104%, 90% CI: 99.1%–109%).
The effect on amyloid removal in the brain and safety (ARIA-E incidence) was independent of the route of administration and explained by exposure, suggesting that weekly 500 mg SC dosing provides similar efficacy and safety to biweekly 10 mg/kg IV dosing. ARIA-E incidence was also predicted to be comparable between SC and IV administration (12.4% overall, 30.9% in ApoE4 homozygotes).
In the sub-cohort which had prior exposure to lecanemab, safety evaluation showed systemic infusion reactions occurred in 0% of patients receiving 500 mg SC, all of whom had previously received IV lecanemab, and 1.4% of patients who initiated on 720 mg SC by vial, which is favorable compared to systemic infusion reactions in the IV group (26.4%). Immunogenicity assessment indicated a low incidence of anti-drug antibodies (ADA) at 1.4%.
The subcutaneous formulations of lecanemab, designed for convenience, maintains efficacy with a low incidence of systemic infusion reactions and is otherwise equivalent to conventional IV administration.
Eisai serves as the lead for lecanemab’s development and regulatory submissions globally, with Eisai and Biogen co-commercializing and co-promoting the product. Eisai has final decision-making authority.
Media Contacts:
Libby Holman, Eisai Inc. (U.S.), +1-201-753-1945, Libby_Holman@Eisai.com
Madeleine Shin, Biogen Inc., +1-781-464-3260, public.affairs@biogen.com
Investor Contacts:
Tim Power, Biogen Inc., + 1-781-464-2442, IR@biogen.com
Source: Eisai Co., Ltd.
