A comparative optical aggregometry study of antiplatelet activity of taxanes from Taxus cuspidata.

Platelets are highly reactive components of the circulatory system. The cytoskeleton of a platelet is an important structure for platelet aggregation as stimulated by several agonists. An anticancer agent, taxol, has been suggested to exert platelet anti-aggregating activity by stabilizing microtubules during the aggregation process. An activity-guided fractionation was performed with a methanol extract of the leaves and twigs of Taxus cuspidata to isolate taxanes with platelet anti-aggregating effects. Compounds 1 to 7 - taxinine (1), taxinine A (2), taxinine B (3), 2-deacetoxytaxinine B (4), taxacin (5), taxchinin B (6), and taxol (7) - were obtained as the antiplatelet components of this plant. These taxane compounds present the possibility of securing new antiplatelet compounds which differ from currently available antiplatelet agents in chemical structure and possibly in mechanisms of action. All compounds showed stronger inhibitory effects than acetylsalicylic acid (ASA) on platelet aggregation induced by arachidonic acid (AA) (IC(50): 14.4, 64.5, 35.5, 16.0, 21.9, 28.6 and 48.2 versus 63.0microM) or U46619 (IC(50): 34.8, 24.9, 36.2, 35.0, 46.9, 71.9 and 68.7 versus 340microM). Compounds 1, 3, 4 and 5, with a cinnamoyl group at the C(5) position, showed strong inhibitory effects against AA-induced aggregation compared to compound 2 (with an -OH group at C(5)) or compounds with an oxetane ring at C(4),(5), such as compounds 6 and 7. All of the seven compounds were 5-13-fold more strongly inhibitory than ASA against U46619-induced aggregation.