Abstract B21: Protein phosphatase 2A (PP2A) activation functions synergistically with kinase inhibition in pancreatic cancer

The 5-year survival rate for pancreatic cancer patients is only 6%, the lowest of all major cancers, indicating a critical need for increased understanding of pancreatic cancer development and identification of new therapeutic targets. Several genetic mutations are associated with the progression of human pancreatic ductal adenocarcinoma, including KRAS, which is mutated in ~95% of all pancreatic ductal adenocarcinomas (PDAC). Targeted therapies, such as kinase inhibitors, provide a means to reduce oncogenic signaling pathways that drive pancreatic cancer; however, the presence of resistance mechanisms, through extensive feedback loops, reduces the clinical efficacy of single agent treatments. Protein Phosphatase 2A (PP2A) is a critical tumor suppressor that negatively regulates several key oncogenic pathways implicated in mediating therapeutic resistance, including the PI3K/Akt, RAS/ERK, and Myc pathways. The goal of this research is to identify the therapeutic benefit of combined phosphatase activation and kinase inhibition, as a means to attenuate resistance mechanisms, reduce pancreatic tumor growth, and improve therapeutic efficacy in pancreatic cancer patients. We have determined that SET, an endogenous inhibitor of PP2A, is overexpressed in pancreatic cancer cell lines and primary patient samples, suggesting that suppression of PP2A contributes to pancreatic oncogenic signaling. As a result, we have started investigating the therapeutic efficacy of two novel therapeutic agents that activate PP2A: OP449, a SET antagonist, and DT1154, a small molecule direct PP2A activator. Treatment of pancreatic cancer cell lines with these compounds results in increased PP2A activity, reduced MYC levels, and reduced tumorigenic potential both in vitro and in vivo. Since these compounds reduce oncogenic signals known to contribute to pancreatic cancer, we hypothesize that phosphatase activation will function synergistically with select kinase inhibitors, resulting in sustained attenuation of oncogenic signaling in pancreatic cancer cells. In order to identify pathways that function synergistically with PP2A activation, a panel of pancreatic cancer cell lines was plated with and without OP449 into a 384-well kinase inhibitor (KI) screen, which evaluates the efficacy of over 130 kinase inhibitors at seven serial dilutions spanning the predicted IC50. Several signaling nodes were found to be synergistic with OP449, with the PI3K/AKT/mTOR pathways being particularly susceptible. Specifically, the combination of OP449 with Ink128, an mTOR1/2 inhibitor, reduced pancreatic cancer cell survival, oncogenic signaling, and transformed phenotypes, over either drug alone. These results provide new insight into our understanding of the regulation of pancreatic oncogenic signaling by phosphatases and identify new combination therapies that reduce resistance mechanisms. Citation Format: Brittany L. Allen-Petersen, Amy S. Farrell, Zina P. Jenny, Colin J. Daniel, Zhiping Wang, Charles D. Lopez, Dale J. Christensen, Goutham Narla, Brett C. Sheppard, Rosalie C. Sears. Protein phosphatase 2A (PP2A) activation functions synergistically with kinase inhibition in pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B21.