Cytotoxic Tirucallane and Apotirucallane Triterpenoids from the Stems of Picrasma quassioides

Phytochemical investigation on the stems of Picrasma quassioides led to the isolation of a novel compound, picraquassin A (1), with an unprecedented 21,24-cycloapotirucallane skeleton, and four new apotirucallane-type triterpenoids (2–5), together with 15 new tirucallane-type triterpenoids (6–20) and 10 known tirucallane-type triterpenoids (21–30). To our knowledge, this is the first report demonstrating the presence of apotirucallane-type triterpenoids in the genus Picrasma. The structures of the new compounds were determined based on spectroscopic data interpretation. Cytotoxicities of the isolated compounds were evaluated using three human cancer cell lines, MKN-28, A-549, and MCF-7. Compound 2 exhibited the most potent activity against MKN-28 cells with an IC₅₀ value of 2.5 μM. Flow cytometry and Western blot analysis revealed that 2 induces the apoptosis of MKN-28 cells via activating caspase-3/-9, while increasing Bax and Bad and decreasing Bcl-2 expression levels.