CK1
0.3 μM (IC 50 )
D4476 is a potent and rather selective inhibitor of CK1 in vitro and in cells. In H4IIE hepatoma cells, D4476 specifically inhibits the phosphorylation of endogenous forkhead box transcription factor O1a (FOXO1a) on Ser322 and Ser325 within its MPD, without affecting the phosphorylation of other sites. CK1δ assayed at 0.1 mM ATP using a phosphorylated peptide TFRPRTSpSNASTIS corresponding to residues 312–325 of FOXO1a is inhibited with an IC 50 value of 0.3 μM. The IC 50 value for CK1δ decreases progressively as the concentration of ATP is lowered, indicating that D4476 is an ATP-competitive inhibitor of CK1. CK1.