Objective: To investigate the clinical effectiveness and safety of lecanemab in patients with early-onset Alzheimer's disease (EOAD), thereby providing a reference for the clinical application of disease-modifying treatment (DMT).

Methods: The diagnosis and treatment process of a 54-year-old male patient with EOAD were reported, and the disease modifying effect of lecanemab was analyzed based on clinical manifestations, imaging features, treatment response and relevant literature.

Results: After receiving traditional anti-dementia drug treatment, the patient initiated treatment with lecanemab (10 mg/kg body weight, once every 2 weeks) and was followed up for 9 months. Before treatment, the Mini-mental State Examination (MMSE) scale score was 20, Montreal Cognitive Assessment (MoCA) scale score was 12, amyloid β-protein (Aβ)- PET showed a standardized uptake value ratio (SUVR) of 1.325 (positive), and the Centiloid value was 51.073. After 9 months of treatment, β -PET indicated a reduction in SUVR to 1.015 (below the negative threshold of 1.10) and a decrease of 55.688 units in Centiloid value; Tau-PET showed no further progression of pathological deposits. Clinical assessments revealed improvement of 1 point in MMSE score (to 21 points), and stable scores in MoCA and Boston Naming Test (BNT). No adverse effects, such as cerebral microhemorrhages or edema, were observed during treatment.

Conclusion: This case report shows that lecanemab can effectively clear the deposition of Aβ in the brain, delay the pathological progression of Tau protein, improve the function in the patient with EOAD, and has good safety. This case provides clinical evidence for early DMT intervention in EOAD.