Cytotoxic oleanane triterpenoid saponins from Albizia julibrissin

Abstract

Bioassay-guided fractionation of the ethanolic extract of the stem bark of Albizia julibrissin led to the isolation of ten new oleanane-type triterpenoid saponins, julibrosides J37–J46 (1–10), along with six known analogues (11–16). In addition, 11 prosapogenins (17–27) were prepared by mild or strong alkaline hydrolysis of the total saponin. The structures of 1−27 were determined by spectroscopic and chemical means, and their cytotoxicities against four human cancer cell lines, BGC-823, A549, HCT-116, and HepG2 were evaluated. Compounds 5–16 exhibited significant inhibitory activity with IC50 values ranging from 2.59 to 9.30μM, and 8 turned out to be the most active compound with all IC50 values <5μM. A preliminary structure-activity relationship of these saponins clearly indicated that the outer monoterpenoid moiety (MT′) is a crucial substituent for cytotoxicity, and the linkage sites of the MT′ unit greatly influenced the activity. It could also be inferred that the existence of 16-OH of the aglycone almost has no effect on cytotoxicity and the N-acetyl-glucosamine moiety at C-3 seems to enhance activity.