SMALL MOLECULE INDUCES TIME‐DEPENDENT INHIBITION OF STAT3 DIMERIZATION AND DNA‐BINDING ACTIVITY AND REGRESSES HUMAN BREAST TUMOR XENOGRAFTS

Aberrantly‐active signal transducer and activator of transcription (Stat)3 has a causal role in many human cancers and represents a validated anticancer drug target, though it has posed significant challenge to drug development. A new small molecule, JKB887, was identified through virtual library screening and is predicted to interact with Lys591, Arg609 and Pro63 in the phospho‐tyrosine (pTyr)‐binding pocket of the Stat3 SH2 domain. JKB887 inhibited Stat3 DNA‐binding activity in vitro in a time‐dependent manner, with IC50 of 2.2‐4.5 µM at 30‐60‐min incubation. It directly disrupted both the Stat3 binding to the cognate, high‐affinity pTyr (pY) peptide, GpYLPQTV‐NH2 in fluorescent polarization assay with IC50 of 3.5‐5.5 µM at 60‐90‐min incubation, and to the IL‐6 receptor/gp130 or Src in treated malignant cells. Treatment with JKB887 selectively blocked constitutive Stat3 phosphorylation, nuclear translocation and transcriptional activity, Stat3‐regulated gene expression, and decreased viable cell numbers, cell growth, colony formation, migration, and survival in human or mouse tumor cells. By contrast, JKB887 had minimal effects on Stat1 activity, pErk1/2MAPK, pShc, pJAK2, pSrc induction, or cells that do not harbor aberrantly‐active Stat3. Additionally, JKB887 inhibited growth of human breast cancer xenografts in mice. JKB887 is a Stat3‐selective inhibitor with demonstrable antitumor effects against Stat3‐dependent human cancers.