White matter 18F-florbetapir retention predicts cognitive decline and treatment response in Alzheimer's disease.

PURPOSE: White matter (WM) signal on ¹⁸F-florbetapir positron emission tomography (PET) is often regarded as nonspecific, yet its biological significance remains unclear. This study aimed to characterize the trajectory, clinical significance, and biomarker correlates of normal-appearing white matter (NAWM) ¹⁸F-florbetapir retention across the Alzheimer's disease (AD) continuum. METHODS: We analyzed NAWM ¹⁸F-florbetapir retention in 672 participants across the AD continuum from the Alzheimer's Disease Neuroimaging Initiative and two Chinese cohorts. Longitudinal PET, plasma, and cerebrospinal fluid (CSF) biomarkers, as well as lecanemab treatment effects, were evaluated. RESULTS: NAWM retention followed a distinct trajectory from cortical amyloid, increasing during preclinical stages and plateauing in symptomatic phases. Elevated NAWM ¹⁸F-florbetapir retention independently predicted cognitive decline, correlated with plasma p-tau217 and CSF p-tau/Aβ42 ratio, and showed significant reductions following lecanemab therapy. Combined assessment of cortical and NAWM PET improved diagnostic accuracy for amyloid positivity to 92%. NAWM retention also correlated strongly with plasma and CSF biomarkers in preclinical stages, and mediation analyses indicated that these fluid markers partly explained NAWM-cognition associations. CONCLUSION: NAWM ¹⁸F-florbetapir retention represents a biologically valid and dynamic biomarker of AD progression, with potential utility for early detection, prognostication, and therapeutic monitoring.