Sesquiterpene lactones from the aerial parts of Xanthium sibiricum and their cytotoxic effects on human cancer cell lines

The EtOH extract of the aerial parts of Xanthium sibiricum afforded six sesquiterpene lactones (STLs), including two new eremophilanolides. The structures of the new compounds were established on the basis of spectroscopic methods and the modified Mosher's method to be the C-11 epimers of 2S,4S,5R,7R,8R-2-hydroxy-eremophil-1(10)-en-12,8-olide 11R: 1 (sibiriolide C); 11S: 2 (sibiriolide D)]. The isolated compounds were evaluated for their in vitro cytotoxicities against five human cancer cell lines (Huh-7 hepatocarcinoma, KB nasopharynx carcinoma, Jurkat T cell lymphoblast, BGC-823 and KE-97 gastric carcinoma) using the CellTiter-Glo? luminescent cell viability assay method. Compounds 4–6, each possessing an α-methylene-γ-lactone moiety, were found to have noteworthy cytotoxic effects with IC₅₀ values ranging from 1.1 to 18.0 μM.