CNS amyloid-ß, soluble APP-α and -ß kinetics during BACE inhibition.

BACE, a ß-secretase, is an attractive potential disease-modifying therapeutic strategy for Alzheimer's disease (AD) as it results directly in the decrease of amyloid precursor protein (APP) processing through the ß-secretase pathway and a lowering of CNS amyloid-ß (Aß) levels. The interaction of the ß-secretase and α-secretase pathway-mediated processing of APP in the rhesus monkey (nonhuman primate; NHP) CNS is not understood. We hypothesized that CNS inhibition of BACE would result in decreased newly generated Aß and soluble APPß (sAPPß), with increased newly generated sAPPα. A stable isotope labeling kinetics experiment in NHPs was performed with a (13)C6-leucine infusion protocol to evaluate effects of BACE inhibition on CNS APP processing by measuring the kinetics of sAPPα, sAPPß, and Aß in CSF. Each NHP received a low, medium, or high dose of MBI-5 (BACE inhibitor) or vehicle in a four-way crossover design. CSF sAPPα, sAPPß, and Aß were measured by ELISA and newly incorporated label following immunoprecipitation and liquid chromatography-mass spectrometry. Concentrations, kinetics, and amount of newly generated APP fragments were calculated. sAPPß and sAPPα kinetics were similar, but both significantly slower than Aß. BACE inhibition resulted in decreased labeled sAPPß and Aß in CSF, without observable changes in labeled CSF sAPPα. ELISA concentrations of sAPPß and Aß both decreased and sAPPα increased. sAPPα increased by ELISA, with no difference by labeled sAPPα kinetics indicating increases in product may be due to APP shunting from the ß-secretase to the α-secretase pathway. These results provide a quantitative understanding of pharmacodynamic effects of BACE inhibition on NHP CNS, which can inform about target development.

Diurnal patterns of soluble amyloid precursor protein metabolites in the human central nervous system.

The amyloid-ß (Aß) protein is diurnally regulated in both the cerebrospinal fluid and blood in healthy adults; circadian amplitudes decrease with aging and the presence of cerebral Aß deposits. The cause of the Aß diurnal pattern is poorly understood. One hypothesis is that the Amyloid Precursor Protein (APP) is diurnally regulated, leading to APP product diurnal patterns. APP in the central nervous system is processed either via the ß-pathway (amyloidogenic), generating soluble APP-ß (sAPPß) and Aß, or the α-pathway (non-amyloidogenic), releasing soluble APP-α (sAPPα). To elucidate the potential contributions of APP to the Aß diurnal pattern and the balance of the α- and ß- pathways in APP processing, we measured APP proteolytic products over 36 hours in human cerebrospinal fluid from cognitively normal and Alzheimer's disease participants. We found diurnal patterns in sAPPα, sAPPß, Aß40, and Aß42, which diminish with increased age, that support the hypothesis that APP is diurnally regulated in the human central nervous system and thus results in Aß diurnal patterns. We also found that the four APP metabolites were positively correlated in all participants without cerebral Aß deposits. This positive correlation suggests that the α- and ß- APP pathways are non-competitive under normal physiologic conditions where APP availability may be the limiting factor that determines sAPPα and sAPPß production. However, in participants with cerebral Aß deposits, there was no correlation of Aß to sAPP metabolites, suggesting that normal physiologic regulation of cerebrospinal fluid Aß is impaired in the presence of amyloidosis. Lastly, we found that the ratio of sAPPß to sAPPα was significantly higher in participants with cerebral Aß deposits versus those without deposits. Therefore, the sAPPß to sAPPα ratio may be a useful biomarker for cerebral amyloidosis.