Anti-complementary activity of protostane-type triterpenes from Alismatis rhizoma

Four protostane-type triterpenes, alisol B 23-acetate (1a), alisol C 23-acetate (2a), alisol B (3a), and alisol A 24-acetate (4a), were isolated from the rhizome of Alismatis plantago-aquatice L. var. orientale Samuelson (Alismataceae) and eleven protostane derivatives (compounds 1-11) were obtained by selective modification from alisol B 23-acetate (1a). These compounds were investigated for their anti-complement activity against the classical pathway of the complement system. Alisol B (3a) and alisol A 24-acetate (4a) exhibited anti-complement activity with IC50 values of 150 and 130 microM. Among the synthetic derivatives, the tetrahydroxylated protostane triterpene (9) showed moderate inhibitory activity with IC50 value of 97.1 microM. Introduction of an aldehyde group at C-23 (10; IC50 value, 47.7 microM) showed the most potent inhibitory effect on the complement system in vitro.