Tokyo, Japan & Cambridge, Massachusetts, USA, September 16, 2026
Eisai Co., Ltd. and Biogen Inc. announced that Japan has approved LEQEMBI® Pen, a subcutaneous formulation of LEQEMBI® (lecanemab), for slowing the progression of mild cognitive impairment (MCI) and mild dementia due to Alzheimer’s disease. The approval introduces a once-weekly subcutaneous autoinjector option alongside the existing intravenous (IV) administration given every two weeks in a hospital setting. LEQEMBI Pen contains lecanemab, a humanized IgG1 monoclonal antibody targeting aggregated soluble amyloid-beta (Aβ) protofibrils and insoluble Aβ plaques. The Japanese decision makes the subcutaneous formulation available as an additional administration option for eligible patients with early Alzheimer’s disease.
LEQEMBI Pen Enables Weekly Subcutaneous Administration
The newly approved LEQEMBI Pen is an autoinjector designed to deliver 500 mg of lecanemab once weekly, administered as two injections. The formulation is intended to allow administration by a patient or care partner at home after the required medical assessment, education and training. Treatment initiation must occur under a physician’s supervision at a medical facility, while suitability for self-administration must be carefully evaluated by the treating physician.
Compared with IV administration, which requires patients to visit a healthcare facility for infusion, the subcutaneous formulation offers a potentially more convenient treatment pathway. The companies report that each injection takes approximately 15 seconds, while home administration may reduce the frequency and time burden associated with clinic visits. The new option could also provide greater flexibility for patients and care partners when managing treatment schedules, although appropriate clinical monitoring remains necessary.
Clinical Evidence Supports Subcutaneous Formulation
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The Japanese approval was based on integrated evidence from the Phase 3 Clarity AD study, together with data from subcutaneous administration sub-studies conducted during the subsequent long-term extension. The Clarity AD trial evaluated lecanemab in people with early Alzheimer’s disease, including individuals with MCI due to Alzheimer’s disease and mild Alzheimer’s dementia. The global Phase 3 study enrolled 1,795 participants and demonstrated statistically significant results for its primary and key secondary endpoints with the approved IV formulation.
According to Eisai and Biogen, modeling and clinical pharmacology data indicated that once-weekly 500 mg subcutaneous administration provides exposure similar to IV administration every two weeks. These findings supported the expectation that the subcutaneous formulation could provide comparable therapeutic exposure to the established IV regimen. The overall safety profile observed with subcutaneous administration was generally similar to IV treatment. The companies also reported that systemic injection or infusion-related reactions occurred less frequently with subcutaneous administration in the cited clinical data, although amyloid-related imaging abnormalities (ARIA) monitoring remains an important component of treatment management.
Expanding Treatment Options for Early Alzheimer’s Disease
Lecanemab is an anti-amyloid therapy designed to target both Aβ protofibrils and amyloid plaques, two forms of amyloid associated with Alzheimer’s disease pathology. The companies are developing the treatment for use earlier in the disease process, where slowing progression is a central objective of disease-modifying treatment. Japan previously approved IV LEQEMBI in September 2023 for slowing progression of MCI and mild dementia due to Alzheimer’s disease. The approval of LEQEMBI Pen represents an expansion of the available administration options rather than a new active pharmaceutical ingredient. Eisai and Biogen stated that the subcutaneous formulation has now been approved in three countries globally, following previous approvals in the United States and China. The companies expect the availability of a weekly autoinjector option to provide additional flexibility for patients and caregivers while potentially reducing some healthcare resources associated with repeated IV infusions.
For Japanese patients receiving treatment for early Alzheimer’s disease, the regulatory decision adds a home-administration option to the existing treatment pathway, subject to medical supervision, patient or caregiver training and appropriate safety monitoring. The development marks another step in the evolution of anti-amyloid Alzheimer’s disease treatment, with research continuing to explore ways to make disease-modifying therapies more practical for long-term use.
Source: Eisai, Biogen press release
