Chem. Pharm. Bull. 53(11) 1451—1454 (2005)

and is an endemic species in Korea. The roots of this plant have been widely used in traditional medicine for the treatment of several diseases, particular malaria and amoebic dysentery. This plant has been investigated extensively, resulting in the isolation of various saponins, ranunculin, anemonin, protoanemonin and triterpenes. Our continuing investigation of the chemical constituents of the P. koreana roots has resulted in the discovery of many saponins (Fig. 1). However, to the best of the author’s knowledge, there is no report on the systematic cytotoxic and antitumor activity of the saponins isolated from the roots of this plant. The variety of their structures motivated us to examine the structure–activity relationship with the in vitro cytotoxic activity as well as the in vivo antitumor activity. 17 saponins, eight lupane-type (1, 3, 5, 7, 9, 11, 13, 15) and nine oleanane-type (2, 4, 6, 8, 10, 12, 14, 16, 17), isolated from P. koreana roots, were evaluated for their cytotoxic activity against four human solid tumor cell lines (A549, SK-OV-3, SK-MEL-2, HCT-15). So far cytotoxic activities against tumor cell lines of these saponins have not been reported. ED50 values obtained were reported in Table 1. As shown in Table 1, it was observed that the saponins 5—17, possessed free carboxylic group at C-28, exhibited moderate to considerable cytotoxic activity (ED50; 1.57—174.34 mM) against tumor lines, whereas their sugar-bonded esters, that is, disdesmoside saponins 1, 2, 3, and 4 were inactive (ED50 300 mM). From this result, we postulate the existence of a free carboxylic group at C-28 is important for the cytotoxic activity. Among the cytotoxic saponins 5—17, oleanolic acid 3-O-a -L-rhamnopyranosyl-(1→2)-[b -D-glucopyranosyl(1→4)]-a-L-arabinopyranoside (10) and oleanolic acid 3-Ob -D-glucopyranosyl-(1→3)-a -L-rhamnopyranosyl-(1→2)-a L-arabinopyranoside (14) exhibited the strongest cytotoxic activity (ED50; 1.57—8.36 mM and 3.47—5.50 mM, respectively), while saponin 6 and 8, possessed hederagenin as an aglycon, exhibited weaker cytotoxic activity. From these results, we postulate the hydroxyl group at C-23 had a negative effect on the cytotoxic activity. This observation bears a striking resemble to another recent study, conducted by Barthomeuf et al. This may be attributed to the electron donating effect of two lone-paired electrons of hydroxyl group toward C-3 of the aglycon (Fig. 2). In a study to determine the relationship between the structure of the sugar moiety at C-3 and the cytotoxic activity, saponins 5 and 6 possessing a trisaccharide of 3-O-a-Lrhamnopyranosyl-(1→2)-O-[b -D-glucopyranosyl-(1→4)]-a L-arabinopyranosyl group, were shown to have a better cytotoxic activity than saponins 15 and 16 which had a disaccharide of the 3-O-b-D-glucopyranosyl-(1→4)-a-L-arabinopyranosyl group. This means that the presence of rhamnopyranose linked at C-2 of the arabinopyranosyl group enhances the cytotoxic activity of monodesmosides. In addition, it was found that there are differences in cytotoxicity between oleanane and lupane saponins. Generally, the cytotoxicities (ED50; 38.14—176.34 mM) of the lupine-type saponins were much weaker than those (ED50; 1.57—13.58 mM) of the oleanane-type saponins, and this result was in agreement with those reported before for similar compounds. Finally, the cell specific action of the saponins was observed. Although the saponins showed no great difference in the cytotoxicity, saponins 6 and 10 exhibited a stronger cytotoxicity with ED50 of 1.57 and 3.04 mM against the SK-MEL-2 cell line, respectively. In order to investigate their antitumor activity, among the 17 saponins isolated from P. koreana roots, 15 saponins which could be isolated in sufficient quantities were applied for the animal test. During the period, there was no remarkNovember 2005 Notes Chem. Pharm. Bull. 53(11) 1451—1454 (2005) 1451