Eisai's etalanetug reduced a tau tangle-specific plasma biomarker by more than 90% at nine months in patients with dominantly inherited Alzheimer's disease, according to data presented at AAIC 2026 in London — findings that suggest plasma eMTBR-tau243 may serve as a less invasive surrogate for cerebrospinal fluid and PET-based measures of tau pathology, with potential implications for how the drug is monitored in future trials.
The data come from Phase Ib/II Study 103 (NCT04971733), an open-label study evaluating etalanetug in participants with dominantly inherited Alzheimer's disease (DIAD), a rare genetic form of the disease affecting less than 1% of the total Alzheimer's population. Etalanetug reduced CSF eMTBR-tau243 by 62% at three months and by 89% at nine months. Plasma eMTBR-tau243 reductions closely tracked those in CSF — 78% at three months and more than 90% at nine months — supporting the plasma assay as a practical readout of central tau pathology. Separately, plasma phosphorylated tau species (p-tau217, p-tau181, p-tau231) and total tau increased following etalanetug administration in both DIAD patients and healthy adults, an effect Eisai attributed to peripheral tau species being stabilized by antibody binding in plasma, thereby reducing their degradation rather than reflecting increased CNS tau burden. The company also reported the first instance of an anti-tau therapy reducing CSF p-tau205 — a marker of late-stage tau pathology under the Alzheimer's Association's revised diagnostic framework — in patients with DIAD.
Study 103 is an open-label, unblinded design, and the press release did not specify the number of participants contributing to this biomarker analysis or disclose dosing details. No safety data were reported in this readout. The data are exploratory and drawn from a genetically defined population with DIAD, which represents a distinct clinical and biological context from the broader sporadic Alzheimer's population that etalanetug is ultimately intended to reach. Cross-trial biomarker comparisons are limited by differences in patient populations, assay methods, and disease stage.
Etalanetug binds to the microtubule-binding region of tau protein, blocking the seeding and propagation of pathological tau aggregates between neurons. The drug was discovered through a collaboration between Eisai and University College London and received FDA Fast Track designation in September 2025. It is currently being evaluated in two further studies: the Tau NexGen Phase II/III trial in DIAD, conducted under the Dominantly Inherited Alzheimer Network Trials Unit and led by Washington University School of Medicine in St. Louis, and Phase II Study 202, a global randomized trial in early sporadic Alzheimer's disease — both assessing etalanetug added to lecanemab (Leqembi).