Antiproliferative Carvotacetones from Sphaeranthus africanus.

Five carvotacetone derivatives, including two known ones, 3,5-diangeloyloxy-7-hydroxycarvotacetone (1) and 3-angeloyloxy-5-[2″,3″-epoxy-2″-methylbutanoyloxy]-7-hydroxycarvotacetone (2), along with three new compounds, 3-angeloyloxy-5-[3″-chloro-2″-hydroxy-2″-methylbutanoyloxy]-7-hydroxycarvotacetone (3), 5-angeloyloxy-7-hydroxy-3-tigloyloxycarvotacetone (4), and 3-angeloyloxy-5,7-dihydroxycarvotacetone (5), were isolated from the aerial parts of Sphaeranthus africanus collected in Vietnam. Bioassay-guided fractionation was monitored by the antiproliferative activity on CCRF-CEM human cancer cells. The structures of compounds 1-5 were determined on the basis of NMR spectroscopic and mass spectrometric data. Activities of compounds 1-5 were evaluated in vitro against the human cancer cell lines CCRF-CEM, MDA-MB-231, U-251, and HCT-116. All compounds exhibited significant antiproliferative activity against all four cancer cell lines. CCRF-CEM was most sensitive to the compounds, with IC50 values ranging from 0.6 to 1.5 μM. Compounds 3 and 4 possessed the highest activity, with IC50 values in the four cell lines ranging from 0.6 to 2.9 μM and 1.3 to 2.5 μM, respectively. These compounds also showed inhibitory activity toward the HEK-293 human embryonic kidney cells with IC50 values ranging from 2.5 to 5.5 μM. This is the first time that antiproliferative activity of S. africanus has been reported, and 1-5 are the most cytotoxic carvotacetone derivatives reported so far.