CCT251545 is an orally bioavailable inhibitor of Wnt signaling (IC50 = 5 nM). It is a potent and selective inhibitor of the human mediator complex-associated protein kinases Cdk8 and Cdk19. Through its effects on Cdk8 and Cdk19, CCT251545 alters Wnt pathway-regulated gene expression. It reduces tumor growth in COLO 205 human colon cancer xenografts after oral dosing (70 mg/kg bid) and in Wnt-dependent tumors.