Synthetically-derived and previously inaccessible modifications of 20′-hydroxy-vinca derivative compounds such as vinblastine, vincristine or vindesine are disclosed that are a stunning 100-fold more active than the natural product (IC50's 50-75 pM vs 7 nM, HCT116), and are now accessible as a result of advances in the total synthesis of the natural product. Illustrative new ultra-potent vinblastines bind tubulin with much higher affinity and likely further disrupt the tubulin head-to-tail α/β dimer-dimer interaction by virtue of the strategic placement of an added conformationally well-defined, rigid and extended C20′-urea along the adjacent protein-protein interface. Added molecular complexity was used to markedly enhance target binding and functional biological activity, and represents a general approach to improving the properties of other natural products targeting a protein-protein interaction. A pharmaceutical composition containing an ultra-potent 20′-hydroxy-vinca derivative compound and a method of treating cancerous cells with such a compound are also disclosed.