Selective mitotic microtubule antagonist that exhibits > 20 fold selectivity over axonal microtubules. Inhibits proliferation of multiple human tumor cell lines (IC50 = 1.25 nM in HeLa cells) and blocks metaphase/anaphase transition by suppression of microtubule dynamics (IC50 = 3.8 nM). Reduces spindle length by 29% and inhibits microtubule polymerization at micromolar concentrations.