Lecanemab is a monoclonal antibody targeting amyloid-beta protofibrils, indicated for patients with mild cognitive impairment (MCI) and mild dementia due to Alzheimer’s disease.

This study reports interim findings of an ongoing, multicenter, prospective, observational post-marketing study for all patients treated with lecanemab in routine clinical practice in Japan, focusing on amyloid-related imaging abnormalities (ARIAs) and infusion-related reactions primarily observed during up to 28 weeks after treatment initiation.

Patients treated with lecanemab at any medical institutions across Japan are included in the study. Data are collected using an electronic data capture system via standardized case report forms (CRFs). Study items included the incidence of ARIA, ARIA-edema or effusion (-E), ARIA-hemorrhage (-H: cerebral microhemorrhages, superficial siderosis, and macrohemorrhage), and infusion-related reactions, reported as adverse drug reactions.

As of July 5, 2025, CRFs from 2,675 patients were collected, of whom 2,672 had data available for the interim analysis. The median age was 76.0 years, and 62.6% (1,672/2,672) of patients were diagnosed with MCI. At Week 28, 7.3% (195/2,672) of patients discontinued treatment, with a mean treatment duration of 189.6 ± 34.4 days. Among 2,634 patients confirmed to have undergone MRI scans after treatment initiation, ARIA was observed in 7.1% (188/2,634) of patients, ARIA-E in 3.0% (78/2,634), and ARIA-H in 5.2% (137/2,634). Serious ARIA-H (macrohemorrhage) occurred in two patients (0.1%). Infusion-related reactions were observed in 17.0% (455/2,672), including 0.7% (18/2,672) serious cases. The proportion of patients who experienced ARIA was highest in patients with apolipoprotein E ( APOE ) ε4 homozygotes.

This interim analysis represents one of the largest real-world lecanemab cohorts reported globally to date. Although absolute rates are not directly comparable with those from clinical trials, the trends in ARIA distributions across APOE genotypes and infusion-related reactions were comparable to those observed in clinical trials.