Abstract 1246: Development of STAT3 dual-targeting strategies for the treatment of pancreatic cancer

Abstract

Abstract Pancreatic cancer remains a largely incurable disease, with patients facing the worst 5-year survival rate of any cancer. The challenge is to identify the molecular effectors that regulate the survival of pancreatic ductal adenocarcinoma (PDAC) cells, to devise molecular-targeted strategies that are effective in the metastatic setting, and overcome the protective role of the tumor-associated fibrosis and stroma. Strategies targeting multiple molecular effectors in PDAC are likely going to make a bigger impact. Thus, we are identifying molecular targets or synthetic lethal pairs that regulate critical pro-survival or pro-invasive pathways in PDAC. Constitutively activated Signal Transducer and Activator of Transcription 3 (STAT3) protein has been found to be a key regulator of pancreatic cancer and a target for molecular therapeutic intervention. To better model the tumor and its microenvironment, we utilized ex vivo 3-Dimensional (3D) cultures of patient-derived pancreatic cancer cells in the absence and presence of cancer-associated fibroblasts (CAFs). We can quantitate the inhibitory effect on both the tumor and CAFs as they are labeled with different fluorescent markers. In this co-culture model, inhibition of tumor growth is maintained following STAT3 inhibition in the presence of CAFs. We screened several STAT3 small molecule inhibitors, derived from the SH-4-54 class of STAT3 inhibitors, and found inhibition of pancreatic cancer cell proliferation in the low μM range. Our inhibitors bind the STAT3 protein potently, as shown by SPR, and demonstrate no effect in a kinome screen. In vitro studies demonstrated potent cell killing as well as inhibition of STAT3 activation in the 3D co-culture model. We have previously reported that Ref-1 (redox factor-1) regulates STAT3 activity through its redox function and blocking STAT3 through phosphorylation and redox inhibition synergizes for PDAC cell killing. In our 3D co-culture system, Ref-1 inhibitor, APX3330 decreases tumor area and intensity in a dose-dependent manner. The addition of APX3330 to STAT3 pathway inhibition via Ruxolitinib (Rux, Jak 2 inhibitor) or direct STAT3 inhibitor potentiated the killing effect in the tumor. However, the combination treatment did not appear to sensitize CAF cells, suggesting that targeting of Ref-1 and the STAT3 pathway is more specifically targeting tumor cells. Utilizing APX3330, Rux, and our lead STAT3 inhibitors, we evaluated the effects of Ref-1/STAT3 inhibition in PDAC low passage patient-derived cell lines. The activity of STAT3 and specificity of lead compounds was assessed by immunoblotting for levels of phosphorylated proteins including STAT3 (Y705) and STAT5 (Y694) in 3D culture. These studies establish the rationale for the development of STAT3 dual-targeting strategies for the treatment of pancreatic cancer and suggest that Ref-1 and STAT3 may be a synthetic lethal pair. Citation Format: Melissa L. Fishel, Michelle L. Grimard, Mark R. Kelley, David A. Rosa, Andrew Shouksmith, Gary Tin, Ji Park, Patrick T. Gunning. Development of STAT3 dual-targeting strategies for the treatment of pancreatic cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1246.