Dimeric sesquiterpene thioalkaloids with potent immunosuppressive activity from the rhizome of Nuphar pumilum: structural requirements of nuphar alkaloids for immunosuppressive activity

Potent immunosuppressive dimeric sesquiterpene thioalkaloids, 6-hydroxythiobinupharidine, 6,6'-dihydroxythiobinupharidine, 6-hydroxythionuphlutine B and 6'-hydroxythionuphlutine B, were isolated from the rhizome of Nuphar pumilum together with five inactive quinolizidine alkaloids, neothiobinupharidine, nupharidine, deoxynupharidine, 7-epideoxynupharidine and nupharolutine. These dimeric sesquiterpene thioalkaloids were found to significantly inhibit anti-sheep erythrocyte plaque forming cell formation in mouse splenocytes at 1 microM. At this concentration. 6-hydroxythiobinupharidine, 6-hydroxythionuphlutine B and 6'-hydroxythionuphlutine B did not show cytotoxic effects to mouse splenocytes, and 6,6'-dihydroxythiobinupharidine also showed only minor or minimal cytotoxicity. By comparison of the inhibitory activity of several Nuphar alkaloids on anti-sheep erythrocyte plaque forming cell formation, some structural requirements of Nuphar alkaloids for immunosuppressive activity were obtained. Namely, the 6- or 6'-hydroxyl group at the quinolizidine ring of dimeric sesquiterpene thioalkaloids is essential for the immunosuppressive effect. The number of hydroxyl groups appears to be related to the cytotoxicity, and the influence on splenocytes is greater with increasing numbers of hydroxyl groups.