Head-to-head comparison between plasma ptau-217 and Flortaucipir-PET in amyloid-positive patients with cognitive impairment

Background: Plasma measurements of phosphorylated tau (ptau) have emerged as promising biomarkers for Alzheimer’s disease (AD). Studies have shown strong associations between ptau and tau-PET that are mainly driven by the difference between amyloid-positive and amyloid-negative patients. However, the relationship between ptau and tau-PET is not well characterized within patients with AD. We conducted a head-to-head comparison between plasma ptau-217 and tau-PET in patients at the clinical stage of AD and assessed biomarker relationships with demographic and clinical variables. Methods: Eighty-seven amyloid-positive patients (age=66±10, 48% women) with mild cognitive impairment (n=53) or dementia (n=34) underwent structural MRI, amyloid-PET (11C-PIB), tau-PET (18F-flortaucipir, FTP), Mini-Mental State Examination (MMSE, with 28 patients having longitudinal measures 1.5±0.7 years after baseline) and blood draw within a year. Mean cortical PET-Standardized Uptake Value Ratio (SUVR) were extracted using FreeSurfer5.3. Plasma ptau-217 concentrations were measured using an electrochemiluminescence-based assay on the Meso Scale Discovery platform. Results: Ptau-217 and cortical FTP-SUVR were strongly correlated (r=0.61, p0.25). In the whole group, ptau-217 was associated with FTP-SUVR in temporo-parietal and dorsal prefrontal cortices (Figure 3A). However, ptau-217-FTP association patterns varied with amyloid burden: in patients with moderate amyloid (