Abstract B60: Targeting the polyamine addiction of pancreatic cancers: Combination therapies and biomarkers

Abstract

Abstract This project developed a combination therapy which inhibits both polyamine transport and biosynthesis in an effort to target the polyamine addiction of pancreatic cancers. Several parameters were measured in six human pancreatic cell lines (L3.6pl, Panc-1, BxPC3, AsPC-1, Capan-1 and HPNE) and one murine line (Pan02). These included the kinetics of 3H-spermidine uptake (Vmax and Km values), the 72h IC50 value for difluoromethylornithine (DFMO, an inhibitor of polyamine biosynthesis), and the ability of exogenous spermidine to rescue the viability of DFMO-treated cells. DFMO has a mixed clinical history and cancers often escape DFMO via polyamine import. Three polyamine transport inhibitors (PTIs) were evaluated for their ability to block the uptake of exogenous spermidine into DFMO-treated pancreatic cancer cells (via EC50 values). The combination therapy (DFMO+PTI) was shown to effectively block spermidine import in vitro and improved survival times in an orthotopic mouse model. Basal c-Myc and ATP13A3 protein expression were shown to track well with polyamine transport activity (Vmax) in untreated cell lines. In the presence of DFMO, both polyamine transport activity and ATP13A3 protein expression of L3.6pl cells were increased, whereas c-Myc protein expression decreased. This suggested that ATP13A3 may be involved in the DFMO-stimulated polyamine transport response and was confirmed via siRNA experiments. Using ATP13A3 as a biomarker of polyamine transport, our results could be explained in the context of relative ATP13A3 expression and the caveolin-1 status of each cell line. Cell lines with high ATP13A3 and low Cav-1 had the highest Vmax (e.g., L3.6pl and Panc-1) and could be readily rescued from DFMO by exogenous spermidine (1 µM). In contrast, cells with low ATP13A3 and low Cav-1 expression (i.e., Capan-1 and AsPC-1) gave low Vmax values and could not be rescued from DFMO by exogenous spermidine. BxPC3 cells were found to have high ATP13A3 and high Cav-1 expression and were found to be poorly rescued by exogenous spermidine in the presence of DFMO. In summary, the combination therapy of DFMO+PTI was shown to effectively deplete pancreatic cancers of intracellular polyamines and holds great promise as a potential therapy. The discovery of ATP13A3 as a polyamine transport protein is very significant as there are few proteins identified for this pathway and this marker (along with Cav-1 status) could be used to stratify patients in ongoing DFMO trials. In this regard, the DFMO+PTI approach represents a potential catch-all therapy for tumors addicted to polyamine metabolites. Citation Format: Otto Phanstiel, IV, Meenu Madan, Arjun Patel, Kristen Skruber, Deborah A. Altomare.{Authors}. Targeting the polyamine addiction of pancreatic cancers: Combination therapies and biomarkers. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr B60.