C188 is a STAT3 inhibitor that inhibits IL-6-stimulated STAT3 phosphorylation and nuclear translocation in HepG2 cells by targeting STAT3 SH2 domain peptide-binding pocket. C188, in particular, was highly active in inducing apoptosis of the breast cancer cell line MB-MDA-468 in vitro ( EC 50 = 0.7 μM).