Top/HDAC-IN-1 (Compound 29b) is a topoisomerase / HDAC dual inhibitor with IC 50 s of 18, 230, 790, 87, and 5250 nM for HDAC1 , HDAC2 , HDAC3 , HDAC6 , and HDAC8 , respectively. Top/HDAC-IN-1 exhibits potent antitumor activities against the HCT116 cell line with the IC 50 of 180 nM. Top/HDAC-IN-1 efficiently induces apoptosis with G2 cell cycle arrest in HCT116 cells.