A Novel Cardenolide Glycoside Isolated from Xysmalobium undulatum Reduces Levels of the Alzheimer's Disease-Associated β-Amyloid Peptides Aβ42 In Vitro

Elevated levels of the amylo β-proteins (Aβ), particularly Aβ42, are associated with a high risk of Alzheimer's disease (AD). The Aβ proteins are produced from cellular processing of the amyloid precursor proteins (APPs). To identify natural products that block the formation of Aβ-proteins from APPs, we previously screened a library of plant extracts and identified Xysmalobium undulaum (Apocynaceae) as a potential plant for further research. Here, we provide a report on the isolation and identification of the active principles from the plant species using a bioassay-guided fractionation. Fractions and resulting pure compounds from the purification process of the extract of X. undulatum were screened in vitro against APPs transfected HeLa cell lines. Three compounds, acetylated glycosydated crotoxogenin (1), xysmalogenin-3, β-d-glucopyranoside (2), and crotoxigenin 3-O-glucopyranoside (3), were subsequently isolated and their structures elucidated using NMR and mass spectrometry. Compound 1, a novel cardenolide, and 2 significantly decreased the Aβ42 levels in a dose-dependent manner while compound 3 was inactive. In silico investigations identified the AD's β-secretase enzyme, BACE1, as a potential target for these compounds with the glycoside moiety being of significance in binding to the enzyme active site. Our study provides the first report of a novel cardenolide and the potential of cardenolides as chemical scaffolds for developing AD treatment drugs.