γ-Secretase modulation inhibits amyloid plaque formation and growth and stimulates plaque regression in amyloid precursor protein/presenilin-1 mice

γ-Secretase modulators (GSMs) represent an emerging oral therapy for preventing and targeting Aβ-amyloidosis in Alzheimer disease. Aβ is a family of peptides of varying lengths where both the total and relative amounts of the individual Aβ peptides affect the process of amyloidosis. In contrast to inhibitors of Aβ synthesis, GSMs do not affect the total amount of Aβ peptides generated but decrease longer more amyloidogenic Aβ species while increasing the production of shorter less amyloidogenic Aβ peptides. In this study, we investigated how this modulation of Aβ production affects Aβ plaque dynamics in the brains of APP/PS1dE9 transgenic mice. Similar to studies with different inhibitors of Aβ synthesis, we found that 28 days of once-daily oral treatment with the GSM AZ4126 (100 μmol/kg) resulted in a strong reduction in plaque formation and plaque growth. In addition, and in contrast to Aβ production inhibitors, the GSM AZ4126 caused a significant reduction in the size of established Aβ plaques. Moreover, the antiamyloidogenic activity was accompanied by a marked reduction in brain interstitial fluid Aβ40 and Aβ42 and an increase in Aβ37. Treatment of induced pluripotent stem cell-derived cortical neurons with the GSM AZ4126 reduced secreted Aβ40 and Aβ42 dose-dependently and with a complementary increase in Aβ37 and Aβ38. These studies unravel a previously unknown antiamyloidogenic effect of GSMs, suggesting that they promote the clearance of already established Aβ pathology in addition to their inhibition of Aβ amyloid formation. SIGNIFICANCE STATEMENT: Immunotherapies promoting Aβ-amyloid clearance have shown efficacy in early Alzheimer disease, but complementary Aβ targeting therapeutic approaches are needed. γ-Secretase modulators (GSMs) target Aβ production with an effective and tolerable mechanism. This study demonstrates that a GSM not only inhibits Aβ-amyloid formation but also promotes Aβ-plaque clearance in experiments conducted in an Aβ-amyloidosis mouse model and supports further development of GSMs as an effective oral treatment for Alzheimer disease.