Qualitative and quantitative characterization of the amyloid β peptide (Aβ) population in biological matrices using an immunoprecipitation–LC/MS assay

Targeting the metabolism of amyloid β peptides (Aβ) is currently the leading experimental approach to treatment of Alzheimer's disease (AD). Described here is an immunoprecipitation–liquid chromatography/mass spectrometry (ip–LC/MS) assay to simultaneously characterize and quantitate different forms of Aβ in biological samples. The 4G8 antibody, specific for the 17-24 amino acid epitope of Aβ was employed to selectively isolate Aβ from in vitro samples for subsequent LC–MS analysis. A high resolution accurate mass hybrid linear ion trap-Orbitrap, LTQ-Orbitrap mass spectrometer was used to identify forms of 12 Aβ in H4-APP751 swe cell extracts based on ab initio calculations, accurate mass measurements, isotopic modeling and by de novo peptide sequencing using tandem mass spectrometry. The quantitative LC–MS analysis was performed on a linear ion trap mass spectrometer, LTQ, in full scan mode, this mode of operation enables sensitive detection levels and post-acquisition data mining for different forms of Aβ for quantitative assessment. Dosing studies with three known inhibitors of Aβ production, sulindac sulfide (SSide), BMS-299897 (‘897) and compound W (CW) are reported to demonstrate the utility and analytical characteristics of the assay. This assay has the potential to provide insight into the formation of Aβ; increase understanding of drug mechanisms; and to contribute to drug efficacy studies.