The U.S. Food and Drug Administration (FDA) has approved Eisai (TYO: 4523) and Biogen's (BIIB) Leqembi (lecanemab-irmb), an Alzheimer's disease treatment, in a subcutaneous (SC) formulation called 'Leqembi IQLIK' for initiation of treatment. This approval, following the August 2025 approval for maintenance dosing, is significant as it allows patients to begin treatment at home with subcutaneous injections, eliminating the need for intravenous (IV) infusions. The previous bi-weekly IV administration required frequent hospital visits, which was burdensome for patients and caregivers. The subcutaneous formulation is expected to significantly improve patient convenience and adherence. Eisai and Biogen plan to launch this initiation formulation in the U.S. market by the end of August 2026.

From a clinical perspective, the approval of Leqembi IQLIK is based on data from the Clarity AD long-term extension (LTE) study, which demonstrated pharmacokinetic (PK) equivalence to the IV formulation. The study results showed that the weekly 500mg (250mg twice) subcutaneous initiation dose resulted in a drug exposure ratio of 104% compared to the existing IV regimen, fully meeting the equivalence range of 80-125%. Furthermore, the subcutaneous administration group showed approximately 14% higher amyloid plaque removal rate at 6 months compared to the IV administration group, demonstrating superior biomarker improvement. However, management of adverse events such as amyloid-related imaging abnormalities (ARIA) and injection site reactions remains important, and the warnings in the prescribing information must be strictly followed.

From a commercial perspective, this approval will be a crucial asset in defending against the competition from Eli Lilly's (Kisunla, donanemab). Kisunla offers monthly IV infusions and a finite dosing schedule based on amyloid removal, providing convenience. However, with Leqembi's launch of a weekly subcutaneous formulation, the market landscape is likely to change significantly. Industry experts predict that the global Alzheimer's disease treatment market will grow to approximately $17 billion to $19.3 billion by 2033, and Leqembi's global peak sales are expected to reach around $3 billion. Under the partnership agreement, Eisai and Biogen will share global revenues and losses on a 50:50 basis.

In terms of impact on the bio and healthcare ecosystem, the importance of establishing patient diagnosis and monitoring infrastructure will be further emphasized. While the subcutaneous formulation significantly reduces the number of hospital visits, diagnosis and periodic monitoring of adverse events using positron emission tomography (PET) or magnetic resonance imaging (MRI) remain essential. Therefore, a medical networking system that can handle amyloid plaque assessment and ARIA adverse events should be established quickly in small and medium-sized regional clinics to lead to actual expansion of prescriptions. In the long term, the bottleneck of patients waiting for IV infusions is expected to be resolved, further accelerating the rate of market entry for the overall Alzheimer's target treatment population.