Abstract 1624: A fragment-based approach to the development of small-molecule STAT3 transcription factor inhibitors
Authors: Nahar Kazi Sharmin , Chamberlain Christopher , Grover Aanchal , Jackson Paul , Rahman Khondaker M. , Thurston David E. 
Published: May 2015

Oncogenic transcription factors are an increasingly important target for anticancer therapies, as inhibition could allow the “reprogramming” of tumor cells leading to apoptosis or differentiation from the malignant phenotype. The STAT (Signal Transducer and Activator of Transcription) proteins are transcription factors that modulate a number of cellular functions. These proteins can exist as monomers, dimerise to form homodimers, or form heterodimers with other family members. STAT3 induces the transcription of genes that control differentiation, inflammation, proliferation, and tumor cell invasion, and its over-expression has been implicated in many tumor types. The protein-protein interaction between two STAT3 monomers (i.e., the dimerisation event in the signalling cascade) has been identified as a valid target to inhibit DNA-binding and the resultant transcriptional activation.

An arylsulphonamidyl thiophene amide library has been synthesized based on a combination of pharmacophores from STAT3 dimerisation inhibitors reported in the literature and a previously reported IL-6 inhibitor (RH-06) that showed significant STAT3 dimerisation inhibition properties in cell free FP assay. The independent fragments of the designed molecules and the mature ligands were tested for their STAT3 dimerization inhibition potential using an in-house fluorescent polarisation assay and a time-resolved fluorescent resonance energy transfer assay (TR-FRET), and their cytotoxicity in tumor cells was evaluated using a MTT assay. Interestingly, two fragments, KSN-57-7 and KSN-57-9, showed significantly greater activity in both the biophysical (45% and 55% inhibition) and biological (IC50 = 6.5 and 7.4 µM) assays, respectively, compared to the mature ligands. The activities were significantly higher than the known STAT3 SH2 domain targeting compound STA-21 (35% inhibition), and comparable in activity to the natural hexapeptide pYLKTKF (42% inhibition).

Based on this, two focused libraries of analogues of KSN-57-7 and KSN-57-9 were synthesized, and members were examined in the TR-FRET and MTT assays. A number of ligands from both library types emerged as early lead compounds with low micromolar IC50 values against the STAT3-dependent MDA-MB-231 cell line (IC50 range 0.4 - 5.2 µM), and these are currently being studied in a number of molecular biology based assays to confirm their mechanism of action. Work on converting the lead fragments into a mature ligand is underway, and examples synthesized to date have either retained or enhanced activity, with the lead structure KSN-57-39 being the most potent PPI inhibitor based on both the TR-FRET and MTT assays (IC50 0.4 µM). The lead optimisation process is currently being guided by detailed Molecular Dynamics (MD) simulations.

Citation Format: Kazi Sharmin Nahar, Christopher Chamberlain, Aanchal Grover, Paul Jackson, Khondaker M. Rahman, David E. Thurston. A fragment-based approach to the development of small-molecule STAT3 transcription factor inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1624. doi:10.1158/1538-7445.AM2014-1624