MW: 471.52, Purity ≥98% by HPLC. C188-9 is a potent STAT3 inhibitor that binds to STAT3 with high affinity (Kd of 4.7 nM). C188-9 has the ability to inhibit STAT3 binding to its pY-peptide ligand, to inhibit cytokine-stimulated pSTAT3, to reduce constitutive pSTAT3 activity in multiple HNSCC cell lines, and to inhibit anchorage dependent and independent growth of these cells.