A plant alkaloid that arrests the cell cycle in G2/M phase by blocking mitotic spindle formation. Inhibits normal microtubule assembly and causes aberrant tubulin polymerization. Blocks binding of tubulin to microtubule proteins. Induces apoptosis. Triggers Raf-1 activation, phosphorylation of Bcl-2 family proteins and induction of p53 expression. Cancer chemotherapeutic agent.