LEQEMBI Shows Measurable Target Engagement with Alzheimer's Protofibrils in Cerebrospinal Fluid
Key Insights
New data from the Phase III Clarity AD study demonstrates that lecanemab (LEQEMBI) binding to amyloid-beta protofibrils can be measured in cerebrospinal fluid for the first time.
The study showed lecanemab treatment resulted in a 59% increase in CSF protofibril concentration at 12 months compared to 19% in placebo, indicating target engagement and mobilization of toxic proteins from brain tissue.
Treatment with lecanemab eliminated the correlation between protofibril levels and neurodegeneration biomarkers that was observed in placebo patients, suggesting reduced neurotoxicity.
Eisai and Biogen announced new data from the Phase III Clarity AD study demonstrating for the first time that lecanemab's binding to amyloid-beta (Aβ) protofibrils can be measured in cerebrospinal fluid (CSF), providing direct evidence of the drug's pharmacological mechanism in Alzheimer's disease patients. The findings were presented at the 18th Clinical Trials on Alzheimer's Disease (CTAD) Conference.
Protofibril Mobilization Demonstrates Target Engagement
The analysis examined a CSF sub-cohort of 410 patients from the Phase III Clarity AD study, using an ultrasensitive assay to quantify total protofibril concentration in cerebrospinal fluid. The study compared changes from baseline between placebo and lecanemab treatment groups over 18 months.
Total CSF protofibril concentration in the placebo group increased by 19% at 12 months and 29% at 18 months, representing the natural disease progression. In contrast, the lecanemab group showed a significantly higher increase of 59% at 12 months and 45% at 18 months. The difference at 12 months was statistically significant (p=0.0126).
According to the researchers, the increased protofibril levels in CSF following lecanemab treatment indicates that the drug binds to protofibrils and facilitates their mobilization from the brain parenchyma into the cerebrospinal fluid. This represents direct evidence of target engagement and the drug's pharmacodynamic effect of removing toxic proteins from brain tissue.
Reduced Neurotoxicity Correlation
The study revealed important differences in biomarker correlations between treatment groups. In the placebo group, researchers observed a statistically significant correlation between changes in CSF protofibrils and changes in neurodegeneration biomarkers including total tau and neurogranin, as well as tau pathology biomarkers p-tau181 and MTBR-tau243.
This correlation completely disappeared with lecanemab treatment, suggesting that the drug reduces neurotoxicity by binding to protofibrils and preventing their harmful effects on brain tissue. MTBR-tau243 is a novel biomarker composed of tau fragments that arise during neurofibrillary tangle formation and shows strong correlation with tau PET imaging.
Unique Dual Mechanism of Action
The findings support lecanemab's position as the only approved Alzheimer's treatment that targets both neurotoxic protofibrils and amyloid plaques. Protofibrils are considered the most toxic Aβ species that contribute to brain damage in Alzheimer's disease and play a major role in cognitive decline.
These toxic proteins can cause neuronal and synaptic damage through multiple mechanisms, not only by increasing insoluble Aβ plaque formation but also by directly damaging signaling between neurons and other cells. The dual targeting approach can impact tau pathology downstream, as evidenced by slower tau tangle accumulation on PET imaging in clinical studies.
Clinical Implications and Regulatory Status
LEQEMBI is indicated for treatment of Alzheimer's disease, with treatment initiated in patients with mild cognitive impairment or mild dementia stage of disease. The drug has been approved in 51 countries and regions including Japan, the United States, Europe, China, South Korea, Taiwan, and Saudi Arabia.
The FDA approved subcutaneous maintenance dosing with LEQEMBI IQLIK in August 2025, and a rolling Supplemental Biologics License Application for initiation treatment was completed in November 2025. In Japan, an application for subcutaneous injectable formulation was submitted in November 2025.
The drug is available as both intravenous infusion (100 mg/mL) and subcutaneous injection (200 mg/mL) formulations. Eisai serves as the lead for global development and regulatory submissions, while both Eisai and Biogen co-commercialize and co-promote the product.