Eight sesquiterpene lactones were isolated from the roots of Inula helenium and flowers of I. japonica. Among them, isoalantolactone (3) and santamarine (6) exhibited significant
growth inhibitory activities against gynecologic cancer cell lines, while others weakly inhibited the growth of the cell lines (IC50 ≤ 100 μM). In addition, 3 significantly inhibited the
tumour growth of S180 tumour-bearing mice. Compounds 3 and 6 were not toxic to human embryonic lung fibroblast cells in vitro. These results demonstrated that the antitumour
activities are closely related to the structures of the compounds, that is, an α-exomethylene-γ-lactone ring is necessary for these activities.