Signal transducer and activator of transcription 3 (STAT3) inhibitor, S3I-201, acts as a potent and non-selective alkylating agent

Oncotarget. 2016 Apr 12;7(15):20669-79. doi: 10.18632/oncotarget.7838.


The Signal Transducer and Activator of Transcription 3 (STAT3) oncogene is a master regulator of many human cancers, and a well-recognized target for therapeutic intervention. A well known STAT3 inhibitor, S3I-201 (NSC 74859), is hypothesized to block STAT3 function in cancer cells by binding the STAT3 SH2 domain and disrupt STAT3 protein complexation events. In this study, liquid chromatography tandem mass spectrometry analysis revealed that STAT3, in the presence of S3I-201, showed a minimum of five specific sites of modification, cysteine's 108, 259, 367, 542, and 687. Moreover, a prepared fluorescently labeled chemical probe of S3I-201 (DB-6-055) revealed that S3I-201 non-specifically and globally alkylated intracellular proteins at concentrations consistent with S3I-201's reported IC50. These data are consistent with the hypothesis that S3I-201 is a sub-optimal probe for interrogating STAT3-related cell biology.

Keywords: NSC 74859; S3I-201; STAT3; covalent modification; oncology.

MeSH terms

  • Alkylating Agents / pharmacology*
  • Aminosalicylic Acids / pharmacology
  • Apoptosis / drug effects
  • Benzenesulfonates / pharmacology*
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / metabolism
  • Breast Neoplasms / pathology
  • Cell Proliferation / drug effects
  • Female
  • Humans
  • STAT3 Transcription Factor / antagonists & inhibitors*
  • Tumor Cells, Cultured


  • Alkylating Agents
  • Aminosalicylic Acids
  • Benzenesulfonates
  • NSC 74859
  • STAT3 Transcription Factor