Inhibition of HT-1080, SK-MEL-28, T24 and MDA-MB-231 cells invasion in vitro by r-viridistatin 2. Tumor cells (7.5 × 105 cells/mL, 0.2 mL) were treated with 6.3 μM of r-viridistatin 2 for 30 min at 37 °C, and then placed in the upper Boyden chamber containing a matrigel-coated filter membrane. Invasion was induced by adding McCoy’s 5A minimum essential medium, Eagle’s minimum essential medium and RPMI-1640 medium with FBS to the lower chamber for 24 h. After fixation and removal of non-migrated cells, the cells that migrated to the underside of filter membrane were quantified by light microscopy (3 fields/40×). *p < 0.05 (r-disintegrins compared to negative control).