Antiviral saponins from Tieghemella heckelii

Arganine C (1) and a new saponin, tieghemelin (2), were isolated from Tieghemella heckelii fruits. Arganine C (1) strongly inhibited HIV entry into cells in a cell fusion assay. The less potent tieghemelin (2) was converted into arganine C (1) by reduction of its ethyl ester with sodium borohydride. The removal of the four-sugar chains from arganine C (1) and tieghemelin (2) to give 16alpha-hydroxyprotobassic acid 3-O-beta-D-glucopyranoside (3) and 16alpha-hydroxyprotobassic acid 3-O-beta-D-glucuronopyranoside (4), respectively, caused total loss of activity in both cases. Arganine C (1) was not significantly cytotoxic to HeLa-CD4(+) cells at the level required to reduce the syncytium count to zero, suggesting it to be a promising candidate for further study as an antiviral drug.