Potent anti-metastatic activity of dimeric sesquiterpene thioalkaloids from the rhizome of Nuphar pumilum

The methanolic extract and its alkaloid fraction from the rhizomes of Nuphar pumilum inhibited invasion of B16 melanoma cells across collagen-coated filters in vitro. Dimeric sesquiterpene thioalkaloids with the 6-hydroxyl group, 6-hydroxythiobinupharidine, 6,6′-dihydroxythiobinupharidine, and 6-hydroxythionuphlutine B, showed potent activity with IC50 values of 0.029, 0.087, and 0.36 μM, respectively, but dimeric sesquiterpene thioalkaloids lacking the 6-hydroxyl group (thiobinupharidine, neothiobinupharidine, syn-thiobinupharidine sulfoxide, thionuphultine B β-sulfoxide, and neothiobinupharidine β-sulfoxide) and monomeric sesquiterpene alkaloids (nupharidine, deoxynupharidine, 7-epideoxynupharidine, and nupharolutine) showed weak activity. The alkaloid fraction (20 mg/kg/d, po) and the principal dimeric sesquiterpene thioalkaloid 6-hydroxythiobinupharidine (5 mg/kg/d, po) significantly inhibited lung tumor formation by more than 90% 10 days after injection of B16 melanoma cells in mice.