The amyloid-β rise and γ-secretase inhibitor potency depend on the level of substrate expression

The amyloid-beta(A beta) peptide, which likely plays a key role in Alzheimer disease, is derived from the amyloid-beta precursor protein (APP) through consecutive proteolytic cleavages by beta-site APP-cleaving enzyme and gamma-secretase. Unexpectedly gamma-secretase inhibitors can increase the secretion of A beta peptides under some circumstances. This A beta rise phenomenon, the same inhibitor causing an increase in A beta at low concentrations but inhibition at higher concentrations, has been widely observed. Here we show that the A beta rise depends on the beta-secretase-derived C-terminal fragment of APP (beta CTF) or C99 levels with low levels causing rises. In contrast, the N-terminally truncated form of A beta, known as p3, formed by alpha-secretase cleavage, did not exhibit a rise. In addition to the A beta rise, low beta CTF or C99 expression decreased gamma-secretase inhibitor potency. This potency shift may be explained by the relatively high enzyme to substrate ratio under conditions of low substrate because increased concentrations of inhibitor would be necessary to affect substrate turnover. Consistent with this hypothesis, gamma-secretase inhibitor radioligand occupancy studies showed that a high level of occupancy was correlated with inhibition of A beta under conditions of low substrate expression. The A beta rise was also observed in rat brain after dosing with the gamma-secretase inhibitor BMS-299897. The A beta rise and potency shift are therefore relevant factors in the development of gamma-secretase inhibitors and can be evaluated using appropriate choices of animal and cell culture models. Hypothetical mechanisms for the A beta rise, including the incomplete processing and endocytic models, are discussed.