Simultaneous Mass Spectrometric quantification of multiple p‐tau species in plasma – findings along the Alzheimer´s disease continuum

Abstract Background Blood phosphorylated tau (p‐tau) species at positions 181, 217 and 231 have been demonstrated accurate biomarkers of Alzheimer´s disease pathology. Comparisons to assess the performance of each phosphorylation along the Alzheimer´s disease (AD) continuum rely on the use of different immunoassays. In the current study, we simultaneously quantified the plasma concentration of six different phosphorylated (p‐tau 181, 199, 202, 205, 217 and 231), and two non‐phosphorylated, tau peptides using a targeted Mass Spectrometric (MS) method. Method We analysed a total of 224 samples from two centres, the TRIAD cohort (Canada) and the BioCogBank Paris Lariboisière cohort (France). Tau enrichment prior to MS analysis was performed by immunoprecipitation, using a combination of several non‐phospho‐tau antibodies, followed by tryptic digestion. A parallel reaction monitoring method with Orbitrap MS was used to measure the levels of targeted peptides. Group‐level comparisons were carried with non‐parametric tests. Cross‐sectional associations between plasma tau biomarkers and Aβ and tau PET were investigated using the locally estimated scatterplot smoothing (LOESS) method for local polynomial regression. Voxel‐wise correlations were performed between plasma and PET biomarkers using Rminc . Result P‐tau231 levels were significantly increased in amyloid positive and tau negative (A+T‐) individuals compared to A‐T‐ (stratified by CSF biomarkers). Increases in p‐tau217 and p‐tau205 emerged later in the AD continuum, only when tau pathology was present (A+T+). Plasma p‐tau217, p‐tau231 and p‐tau205 were the site‐specific phosphorylations that showed higher correlations with Tau PET (Braak I‐VI) (R=0.7 p<0.0001; R=0.53 p<0.0001; and R=0.53 p<0.0001; respectively) and with Aβ PET (R=0.66 p<0.0001; R=0.56 p<0.0001; R=0.45 p<0.0001, respectively). LOESS analysis revealed that plasma p‐tau231 increases earlier in the disease progression, followed by p‐tau217 and p‐tau205, which showed a progressive increase along Braak stages and amyloid centiloid. Voxel‐wise analysis for tau PET were highest with p‐tau205 with associations in in the inferior, medial, and lateral temporal regions. For Aβ PET, p‐tau217 and p‐tau205 showed the highest associations with frontal, precuneus, posterior cingulate and temporal cortices. Conclusion Our results indicate that plasma p‐tau217, p‐tau231 and p‐tau205 are the site‐specific phosphorylations that better reflect amyloid and tau pathologies, although with different emergence along the AD continuum.