Furostane Series Asterosaponins and Other Unusual Steroid Oligoglycosides from the Tropical Starfish Pentaceraster regulus

Seven new asterosaponins, pentaregulosides A-G (1-7), including two furostane-type steroid oligoglycosides (2, 3), along with four previously known compounds (8-11) were isolated from the ethanolic extract of the starfish Pentaceraster regulus, collected off the coast of Vietnam. The structures of 1-7 were elucidated by extensive NMR and ESIMS techniques as well as chemical transformations. The aglycons of compounds 1 and 3 have not previously been observed in starfish steroid oligoglycosides, while the aglycons of compounds 2 and 4-6 are very rare for this structural group. Compound 1 exhibited cytotoxic activity with an IC50 value of 6.4 +/- 0.3 mu M against RAW 264.7 murine macrophages. In contrast, nontoxic asterosaponins 3, 4, and 5 showed a potential immunomodulatory action at a concentration of 5 tiM, reducing by 40%, 28%, and 55%, respectively, reactive oxygen species formation in the RAW 264.7 cells, co-stimulated with the pro-inflammatory endotoxic lipopolysaccharide from E. oli.