Cytotoxic phenylethanoid glycosides from Digitalis davisiana Heywood: Evaluation of structure activity relationships and chemotaxonomical significance of isolated compounds

The phytochemical studies on the aerial parts of Digitalis davisiana Heywood led to the isolation of three undescribed phenylethanoid glycosides named as digidavisoside A (5), digidavisoside B (7), and davisoside (8), along with 9 known compounds, ferruginoside B (1), isolugrandoside (2), lugrandoside (3), maxoside (4), 3 ''''-O-methylmaxoside (6), trans-lamiuside E (9), digiciliside B (10), p-hydroxyacetophenone (11), and chrysoeriol (12). For the first time compound 11 was reported for Digitalis genus. The chemotaxonomical significance of these compounds in Plantaginaceae family was evaluated and 3'-O-glucosyl substituted phenylethanoid glycosides 4-8 and 10 were found to be chemotaxonomically important for the family. Cytotoxic activity of the aqueous fraction of the methanolic extract was also tested against HEp-2 (human larynx epidermoid carcinoma) and HepG2 (human hepatocellular carcinoma) cancer cell lines. The aqueous fraction showed stronger cytotoxicity on HEp-2 cells than on HepG2. Therefore, the cytotoxic activity of 1-4, 6, 7 and 9 were tested against HEp-2 and L929 (mouse fibroblast cell) cell lines. Other isolated compounds could not be tested due to their insufficient amount. The results were evaluated in the point of structure-activity relationships. IC50 values against HEp-2 cells were established in a range of 71.9-220 mu M. Maxoside (4), isolugrandoside (2) and lugrandoside (3) showed higher cytotoxicity against HEp.2 cell line than other isolated compounds.