Carboxylesterase-IN-2 (compound 4u) is a potent inhibitor of Carboxylesterase Notum with an IC 50 less than or equal to 10 nM. Notum is a negative regulator of Wnt signaling acting through the hydrolysis of a palmitoleoylate ester, which is required for Wnt activity. Carboxylesterase-IN-2 has the potential for the research of cancer disease.