Parthenolide and costunolide decrease microtentacle (McTN) frequency and attachment. (A) Detached Bt-549 and MDA-MB-157 were pretreated for six hours and suspended in drug-containing media for blind McTN scoring. Parthenolide (Parth; 10 μM), costunolide (Cost; 25 μM), and colchicine (Col; 50 μM) show a significant decrease compared to vehicle treated (*P <0.05; ** P <0.001, t-test). Resveratrol (ResV; 50 μg/ml) and Taxol (Tax; 0.5 μg/ml) did not have a significant effect on McTN frequency. Columns, mean %McTNs for six independent experiments in which at least 100 cells were scored blindly; bars, SD. (B) Live population images of suspended Bt-549 and MDA-MB-157. McTNs are observed in vehicle, resveratrol, and Taxol (black arrows).