The novel colchicine site binders, cc-3, cc-4, 4c, gc-1, and gc-2 were further tested in vitro for their effects on the dynamics of individual microtubules (Fig. ). All of these compounds effectively decreased microtubule growth rates by approximately 50% and exerted a statistically significant effect on microtubule catastrophes when added to the tubulin polymerization mixture at concentrations of 5-10 μM (Fig. ). Consistently, all tested drugs arrested A549 cells in mitosis, as evidenced by flow cytometry analysis using the PI cell cycle assay (Fig. , Fig. ).