Cytotoxic epimeric withaphysalins from leaves of Acnistus arborescens

Phytochemical analysis of the leaves of Acnistus arborescens (Solanaceae) resulted in the isolation of two new epimeric witha-physalins (17S,20R,22R)-5beta,6beta:18,20-diepoxy-4beta,18-dihydroxyl-oxowitha-24-enotide (2, 18R and 18S), together with the known withaphysalin F (1, 18R and 18S). Their structures were established by spectroscopic methods, including 2D NMR data and comparison with literature data. Compounds 1 and 2 displayed potent cytotoxic activities against several cancer cell lines with IC50 values in the range of 0.20 to 1.46 mug/mL for 1 and 0.89 to 8.08 mug/mL for 2. The strong cytotoxicity presented by 1 suggests that in this series of compounds, the 2,3-unsaturated ketone moiety is an important pharmacophoric unit. The cytotoxic activity seemed to be related to DNA synthesis inhibition, as revealed by the reduction of 5-bromo-2'-deoxyuridine incorporation after 24 hours of incubation on leukemic cells.