DNA Topoisomerase Inhibitory Activity of Constituents from the Flowers of Inula japonica

Fourteen compounds were isolated from the flowers of Inula japonica THUNB. (Asteraceae), including two new compounds, (1S,2S,4S,5S,8S,10R)-2-acetoxy-4,3-dihydroxy-pseudoguai-7(11)-en-12,8-olide (1) and (1S,2S,4S,5S,8S,10R)-2,4,13-trihydroxy-pseudoguai-7(11)-en-12,8-olide (2), and twelve known compounds, budlein B (3), 6β-hydroxytomentosin (4), 6-deacetoxybritanin (5), 4-epipulchellin (6), britanin (7), tomentosin (8), (+)-dihydroquercetin (9), (-)-syringaresinol (10), quercetagetin 3,4'-dimethyl ether (11), luteolin (12), britanin G (13) and inuchinenolide C (14). Structures of 1 and 2 were determined based on one and two dimensional (1D)- and (2D)-NMR data and Mosher's esterification method. Compounds 9 and 12 showed inhibitory activities toward DNA Topoisomerase I with IC50 values of 55.7 and 37.0 µM, respectively, compared to camptothecin (CPT) with an IC50 of 24.5 µM. Compounds 7-9 and 11-14 exhibited more potent inhibitory activity against topoisomerases II with IC50 values of 6.9, 3.8, 3.0, 6.9, 10.0, 14.7 and 13.8 µM, respectively, than that of etoposide (VP-16) with an IC50 of 26.9 µM. Compounds 4-7 and 10-14 exhibited weak cytotoxicities to the selected Cancer cell lines.