Phenylethanoid glycosides from Digitalis purpurea and Penstemon linarioides with PKC alpha-inhibitory activity

In a continuation of our search for potential tumor inhibitors from plants, it was found that the CH2-Cl-2-MeOH (1:1) extracts from Digitalis purpurea and Penstemon linarioides both showed PKC alpha-inhibitory bioactivity. Bi oassay-directed fractionation of the extract from D. purpurea yielded the n ew, weakly active phenylethanoid glycoside 2-(3-hydroxy-4-methoxy-phenyl)-e thyl-O-(alpha-L-rhamnosyl)-(1-->3)-O-(alpha-L-rhamnosyl)-(1-->6)-4-O-E-feru loyl-beta-D-glucopyranoside (1) together with the four known compounds calc eolarioside A (2), calceolarioside B (3), forsythiaside (4), and plantainos ide D (5), The extract from P. linarioides yielded the three known glycosid es leucosceptoside A (6), acteoside (7), and poliumoside (8), together with the iridoid plantarenaloside (9). All of the isolated compounds, except co mpound 9, showed inhibitory activity against PKC alpha with IC50 values (in mu M) of 125 (1), 0.6 (2), 4.6 (3), 1.9 (4), 14.8 (5), 19.0 (6), 9.3 (7), and 24.4 (8).