Structural Characterization of Cholestane Rhamnosides from Ornithogalum saundersiae Bulbs and Their Cytotoxic Activity against Cultured Tumor Cells

Previous phytochemical studies of the bulbs of Ornithogalum saundersiae, an ornamental perennial plant native to South Africa, resulted in the isolation of 29 new cholestane glycosides, some of which were structurally unique and showed potent cytotoxic activity against cultured tumor cell lines. Therefore, we aimed to perform further phytochemical examinations of methanolic extracts obtained from Ornithogalum saundersiae bulbs, isolating 12 new cholestane rhamnosides (1–12) and seven known compounds (13–19). The structures of the new compounds (1–12) were identified via NMR-based structural characterization methods, and through a sequence of chemical transformations followed by spectroscopic and chromatographic analysis. The cytotoxic activity of the isolated compounds (1–19) and the derivatives (1a and 6a) against HL-60 human promyelocytic leukemia cells and A549 human lung adenocarcinoma cells was evaluated. Compounds 10–12, 16, and 17 showed cytotoxicity against both HL-60 and A549 cells. Compound 11 showed potent cytotoxicity with an IC(50) value of 0.16 µM against HL-60 cells and induced apoptotic cell death via a mitochondrion-independent pathway.