Accuracy of Tau Positron Emission Tomography as a Prognostic Marker in Preclinical and Prodromal Alzheimer Disease

Importance: Tau positron emission tomography (PET) tracers have proven useful for the differential diagnosis of dementia, but their utility for predicting cognitive change is unclear. Objective: To examine the prognostic accuracy of baseline fluorine 18 (18F)-flortaucipir and 18FRO948 (tau) PET in individuals across the Alzheimer disease (AD) clinical spectrum and to perform a head-to-head comparison against established magnetic resonance imaging (MRI) and amyloid PET markers. Design, Setting, and Participants: This prognostic study collected data from 8 cohorts in South Korea, Sweden, and the US from June 1, 2014, to February 28, 2021, with a mean (SD) follow-up of 1.9 (0.8) years. A total of 1431 participants were recruited from memory clinics, clinical trials, or cohort studies; 673 were cognitively unimpaired (CU group; 253 37.6% positive for amyloid-β Aβ), 443 had mild cognitive impairment (MCI group; 271 61.2% positive for Aβ), and 315 had a clinical diagnosis of AD dementia (315 100% positive for Aβ). Exposures: 18FFlortaucipir PET in the discovery cohort (n = 1135) or 18FRO948 PET in the replication cohort (n = 296), T1-weighted MRI (n = 1431), and amyloid PET (n = 1329) at baseline and repeated Mini-Mental State Examination (MMSE) evaluation. Main Outcomes and Measures: Baseline 18Fflortaucipir/18FRO948 PET retention within a temporal region of interest, MRI-based AD-signature cortical thickness, and amyloid PET Centiloids were used to predict changes in MMSE using linear mixed-effects models adjusted for age, sex, education, and cohort. Mediation/interaction analyses tested whether associations between baseline tau PET and cognitive change were mediated by baseline MRI measures and whether age, sex, and APOE genotype modified these associations. Results: Among 1431 participants, the mean (SD) age was 71.2 (8.8) years; 751 (52.5%) were male. Findings for 18Fflortaucipir PET predicted longitudinal changes in MMSE, and effect sizes were stronger than for AD-signature cortical thickness and amyloid PET across all participants (R2, 0.35 tau PET vs 0.24 MRI vs 0.17 amyloid PET; P < .001, bootstrapped for difference) in the Aβ-positive MCI group (R2, 0.25 tau PET vs 0.15 MRI vs 0.07 amyloid PET; P < .001, bootstrapped for difference) and in the Aβ-positive CU group (R2, 0.16 tau PET vs 0.08 MRI vs 0.08 amyloid PET; P < .001, bootstrapped for difference). These findings were replicated in the 18FRO948 PET cohort. MRI mediated the association between 18Fflortaucipir PET and MMSE in the groups with AD dementia (33.4% 95% CI, 15.5%-60.0% of the total effect) and Aβ-positive MCI (13.6% 95% CI, 0.0%-28.0% of the total effect), but not the Aβ-positive CU group (3.7% 95% CI, -17.5% to 39.0%; P = .71). Age (t = -2.28; P = .02), but not sex (t = 0.92; P = .36) or APOE genotype (t = 1.06; P = .29) modified the association between baseline 18Fflortaucipir PET and cognitive change, such that older individuals showed faster cognitive decline at similar tau PET levels. Conclusions and Relevance: The findings of this prognostic study suggest that tau PET is a promising tool for predicting cognitive change that is superior to amyloid PET and MRI and may support the prognostic process in preclinical and prodromal stages of AD.