Novel quinone methides from Salacia kraussii with in vitro antimalarial activity

Three novel quinone methides, i.e., 28-nor-isoiguesterin-17-carbaldehyde (1), 17-(methoxycarbonyl)-28-nor-isoiguesterin (2), and 28-hydroxyisoiguesterin (3), together with the known celastrol (5), pristimerin (6), and isoiguesterol (7), were isolated from the roots of Salacia kraussii (Celastraceae) by bioassay-guided fractionation. The structures of the compounds were determined by DEPT and 2D NMR techniques. The isolates showed antimalarial activity 30-50-fold greater than their cytotoxicity (in HT-29 cells) in vitro, and they showed an additive effect when combined with each other. In vivo, 2 was found to be inactive against blood stages of Plasmodium berghei in mice after oral and parenteral administration, and the compound was toxic with increasing concentrations.