The ratio of amyloid precursor protein (APP)669–711 (Aβ−3–40)/Aβ1–42 in blood plasma was reported to represent a novel Alzheimer’s disease biomarker. Here, we describe the characterization of two antibodies against the N-terminus of Aβ−3–x and the development and “fit-for-purpose” technical validation of a sandwich immunoassay for the measurement of Aβ−3–40. Antibody selectivity was assessed by capillary isoelectric focusing immunoassay, Western blot analysis, and immunohistochemistry. The analytical validation addressed assay range, repeatability, specificity, between-run variability, impact of pre-analytical sample handling procedures, assay interference, and analytical spike recoveries. Blood plasma was analyzed after Aβ immunoprecipitation by a two-step immunoassay procedure. Both monoclonal antibodies detected Aβ−3–40 with no appreciable cross reactivity with Aβ1–40 or N-terminally truncated Aβ variants. However, the amyloid precursor protein was also recognized. The immunoassay showed high selectivity for Aβ−3–40 with a quantitative assay range of 22 pg/mL–7.5 ng/mL. Acceptable intermediate imprecision of the complete two-step immunoassay was reached after normalization. In a small clinical sample, the measured Aβ42/Aβ−3–40 and Aβ42/Aβ40 ratios were lower in patients with dementia of the Alzheimer’s type than in other dementias. In summary, the methodological groundwork for further optimization and future studies addressing the Aβ42/Aβ−3–40 ratio as a novel biomarker candidate for Alzheimer’s disease has been set.

Systematic variation of membrane anchor, spacer and pharmacophore building blocks leads to an optimisation of the inhibitory effect of tripartite structures towards BACE1-induced cleavage of the amyloid precursor protein (APP).

In addition to the prototypic amyloid‐β (Aβ) peptides Aβ1–40 and Aβ1–42, several Aβ variants differing in their amino and carboxy termini have been described. Synthetic availability of an Aβ variant is often the key to study its role under physiological or pathological conditions. Herein, we report a protocol for the efficient solid‐phase peptide synthesis of the N‐terminally elongated Aβ‐peptides Aβ−3–38, Aβ−3–40, and Aβ−3–42. Biophysical characterization by NMR spectroscopy, CD spectroscopy, an aggregation assay, and electron microscopy revealed that all three peptides were prone to aggregation into amyloid fibrils. Immunoprecipitation, followed by mass spectrometry, indicated that Aβ−3–38 and Aβ−3–40 are generated by transfected cells even in the presence of a tripartite β‐site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitor. The elongated Aβ peptides starting at Val(−3) can be separated from N‐terminally‐truncated Aβ forms by high‐resolution isoelectric‐focusing techniques, despite virtually identical isoelectric points. The synthetic Aβ variants and the methods presented here are providing tools to advance our understanding of the potential roles of N‐terminally elongated Aβ variants in Alzheimer's disease.