The cis/trans isomerization of X-Pro peptide bonds in proteins in some instances acts as a molecular switch in biological pathways. Our prior work suggests that the cis isomer of the phospho-Thr668-Pro669 motif, located in the cytoplasmic domain of the amyloid-beta protein precursor (A beta PP), is correlated with an increase in amyloidogenic processing of A beta PP and production of amyloid-beta (A beta), the neurotoxic peptide fragment in Alzheimer's disease (AD). We designed a 100% cis-locked cyclic dipeptide composed of cyclized phospho-Thr-Pro (pCDP) as a mimic for this putative pathological conformation, and three phosphate-blocked derivatives (pCDP-diBzl, pCDP-Bzl, and pCDP-diPOM). Two H4 neuroglioma cell lines were established as AD cell models for use in testing these compounds: H4-A beta PP695 for stable overexpression of wild-type A beta PP695, and H4-BACE1 for stable overexpression of beta-site A beta PP cleaving enzyme-1 (BACE1). The level of the secreted A beta PP fragment resulting from BACE1 activity, sA beta PP beta, served as a key proxy for amyloidogenic processing, since cleavage of A beta PP by BACE1 is a requisite first step in A beta production. Of the compounds tested, pCDP-diBzl decreased sA beta PP beta levels in both cell lines, while pCDP-diPOM decreased sA beta PP beta levels in only H4-BACE1 cells, all with similar dose-dependences and patterns of proteolytic A beta PP fragments. Enzymatic assays showed that none of the pCDP derivatives directly inhibit BACE1 catalytic activity. These results suggest a model in which pCDP-diBzl and pCDP-diPOM act at a common point to inhibit entry of A beta PP into the amyloidogenic A beta PP processing pathway but through different targets, and provide important insights for the development of novel AD therapeutics.