A water-soluble prodrug derived from the African bush willow (Combretum caffrum) with antineoplastic activity. Fosbretabulin is dephosphorylated to its active metabolite, combretastatin A4, which binds to tubulin and inhibits microtubule polymerization, resulting in mitotic arrest and apoptosis in endothelial cells. As apoptotic endothelial cells detach from their substrata, tumor blood vessels collapse; the acute disruption of tumor blood flow may result in tumor necrosis.