Abstract C180: Novel small molecule inhibitors of signal transducer and activator of transcription (STAT3) for cancer treatment.

Transcription factors (TFs) represent attractive targets for cancer therapy. However, very few direct TF inhibitors are currently in the clinic. STAT3 is overexpressed and activated in many human cancers promoting proliferation, survival and metabolic adaptation of cancer cells. In this study we investigated the mechanism of action of two new anticancer compounds, OPB-31121 and OPB-51602, currently evaluated in phase I clinical trials. Computational docking and molecular dynamic simulation (MDS) showed that the two compounds bound to the SH2 domain of STAT3 with an estimated binding affinity 2-3 orders of magnitude lower than other known STAT3 inhibitors (STAT3i). Furthermore, the compounds shared a common binding pocket that did not overlap with that of the other STAT3i. Binding assays using isothermal titration calorimetry (ITC) confirmed that OPB-31121 and OPB-51602 bound to recombinant STAT3-SH2 with Kd in the nM range. Binding of the two compounds was disrupted by mutations of amino acids in the predicted binding pocket. Interestingly, competition assays demonstrated that OPB-31121 and OPB-51602 did not prevent binding of other STAT3i, in agreement with the MDS predictions. OPB-51602 and OPB-31121 blocked both Tyr705 and Ser727 phosphorylation, which are required for full STAT3 activation, and inhibited the activity of STAT3 dependent luciferase reporter in human cancer cells. Both compounds strongly reduced anchorage-dependent growth and colony formation in soft-agar. Intriguingly, both compounds were more effective in conditions of metabolic stress (e.g., nutrient and glucose depletion), suggesting that they affected STAT3 functions relevant for metabolic adaptation of cancer cells. In vivo OPB-51602 (20-40 mg/kg PO daily for 2 weeks) arrested growth of DU145 prostate tumor xenografts and concomitantly reduced Tyr705 and Ser727 phosphorylation in tumor tissues. Interestingly, growth inhibition persisted after discontinuation of the treatment, suggesting a stable impairment of tumor-initiating capability and tumor regrowth. Altogether, this study identifies STAT3 as a relevant target of these new compounds. We show that both drugs interact directly with STAT3 and interfere with STAT3 functions in cells and tumor xenografts. Notably, the two compounds bind with high affinity to a unique pocket in the STAT3-SH2 domain that is not shared by other known STAT3i. These features might be relevant for their ability to block specific STAT3 functions and protein interactions resulting in distinctive biological activity and pharmacological properties of these novel compounds. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C180. Citation Format: Lara Brambilla, Davide Genini, Erik Laurini, Gianluca Civenni, Sandra Pinton, Manuela Sarti, Laurent Perez, Sabrina Pricl, Giuseppina M. Carbone, Carlo V. Catapano. Novel small molecule inhibitors of signal transducer and activator of transcription (STAT3) for cancer treatment. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C180.