Comparison of Two Different Methods for Measurement of Amyloid-β Peptides in Cerebrospinal Fluid after BACE1 Inhibition in a Dog Model

Beta-secretase is the first cleavage enzyme of amyloid-beta protein precursor (A beta PP) in the amyloidogenic pathway, leading to the formation of the plaque forming Amyloid-beta (A beta)(1-42) peptide. BACE(beta-site A beta PP cleaving enzyme) 1 inhibition is therefore considered to be a promising disease modifying therapy for Alzheimer's disease. An early assessment of the in vivo activity of BACE inhibitors was done in dogs since A beta PP processing is the same as in humans and this species easily enables longitudinal cerebrospinal fluid (CSF) sampling. A beta changes in CSF compared to baseline are used to evaluate target engagement of the compounds. Levels of A beta(1-37), A beta(1-38), A beta(1-40), and A beta(1-42) in CSF are measured with immunoassay (Mesoscale electrochemiluminescence technology) and with an ultra high-performance liquid chromatography mass spectrometry (UPLC-MS/MS). Two experimental BACE inhibitors were evaluated. With the immunoassay, a dose dependent decrease is observed for all four A beta peptides. Measurements with the UPLC-MS/MS are in line with the immunoassay for A beta(1-37), A beta(1-38), and A beta(1-40,) however, for A beta(1-42), differences are sometimes observed when comparing to changes seen in the other peptides with UPLC-MS/MS and with immunoassay results. Generally lower concentrations are measured with immunoassay. The reason for these differences is still unknown. A beta(1-42) is more prone to form aggregates compared to the other peptides. One hypothesis could be that while the immunoassay only measures free A beta, bound and aggregated A beta peptides are at least partially dissolved with the UPLC-MS/MS method, since acetonitrile is added to the CSF samples. This increases variability in the concentration of A beta peptide measured with UPLC-MS/MS, especially for A beta(1-42), potentially masking the compound effect on A beta(1-42) levels.