Analytical validation of plasma amyloid β measuring system by immunoprecipitation‐mass spectrometry

Abstract Background With the increasing number of patients with Alzheimer's disease, construction of inspection system for blood biomarkers to replace or complement amyloid PET and CSF biomarkers has been required. Since our first report in 2014 (Kaneko et al., Proc Jpn Acad Ser B Phys Biol Sci. 2014), we have been focusing on three amyloid peptides, Aβ1‐40, Aβ1‐42, and APP669‐711 in human plasma, and found that the combination of peptide ratios APP669‐711/Aβ1‐42 and Aβ1‐40/Aβ1‐42 became a biomarker for amyloid peptide accumulation in human brain (Nakamura et al., Nature 2018). Here we describe the analytical validation of plasma amyloid β measuring system by immunoprecipitation‐matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry (IP‐MALDI‐MS). Method Commercially available human plasma and artificial plasma were immunoprecipitated with anti‐Aβ antibody (clone 6E10) covalently bound to the magnetic beads, then measured by MALDI‐TOF‐MS. The peak intensity of the Aβ‐related peptides was normalized by the internal standard peak, and biomarker values were calculated from those intensities. A method validation including experiments of calibration range, sensitivity, specificity, reproducibility, interfering substances, dilution linearity, recovery, and stability was performed. Result The system showed good linearity over the range from 15 to 150 pM for Aβ1‐40, and 3 to 15 pM for Aβ1‐42 and APP669‐711. The trueness was within 100 ± 25% at the lower limit of quantification and within 100 ± 20% at all other concentrations. The precision of the intra‐ and inter‐assay was 1% to 13% as a CV value. Recovery from spiked human plasma ranged from 83% to 101% at low, middle, and high concentrations. Stability testing has shown that the antibody beads are stable for up to 6 months in the refrigerator so far. The addition of the interfering substances, such as anticoagulants, Alzheimer's disease drugs, etc., had only a slight effect on the biomarker values showing 10% or less difference relative to the control. Further results will be reported in the presentation. Conclusion The results demonstrate that the plasma amyloid β measuring system by IP‐MALDI‐MS is an accurate, reliable, and robust assay that would be most suitable for screening subjects in clinical trials.