CDK8-IN-12 is a selective, potent and orally active inhibitor of CDK8 (Ki: 14 nM) and is an anti-cancer agent. CDK8-IN-12 inhibits GSK-3α, GSK-3β and PCK-θ with Ki of 13 nM, 4 nM and 109 nM respectively. CDK8-IN-12 showed anti-proliferative activity against MV4-11 cells.