Abstract 5181: Dual inhibition of RNA Pol I transcription and PIM kinase as a new therapy to treat prostate cancer

Abstract Background: The MYC oncogene is commonly over-expressed in prostate cancer (PC). Upregulation of ribosome biogenesis and function is a characteristic feature of MYC-driven tumors. Accordingly, inhibition of ribosomal RNA (rRNA) synthesis with CX-5461, a potent, selective and orally bioavailable inhibitor of RNA polymerase I (Pol I) transcription has been successfully exploited therapeutically in models of hematological malignancy characterized by elevated MYC expression. Additionally, PIM kinases activate MYC signaling and mRNA translation in PC and cooperate with MYC to accelerate tumorigenesis. Here we investigate the efficacy of a dual approach targeting ribosome biogenesis and function to treat PC by combining CX-5461 with the pan-PIM kinase inhibitor CX-6258 in murine and human models Methods: The efficacy of CX-5461 and CX-6258, alone and in combination, was tested in PC cell lines, in the Hi-MYC mouse model of PC (n = 8-11 per group) and in PC metastatic tissues. Primary cell lines derived from Hi-MYC mice were used to analyze signaling events underpinning therapeutic efficacy. Triplicate experiments were analyzed with ANOVA followed by Dunnett's post hoc test. All statistical tests were two-sided. Results: CX-5461 reduced anchorage independent growth and induced cell cycle arrest in human PC cell lines and in primary prostatic epithelial cells from Hi-MYC mice (P<0.001). CX-5461 treatment of Hi-MYC mice induced p53 expression and activity and significantly reduced prostate epithelial cell proliferation (P = 0.02) and invasion. While CX-6258 showed little effect alone, its combination with CX-5461 further suppressed proliferation (P = 0.01), dramatically reduced the incidence of large invasive lesions from 64% to 9% and preserved prostate ductal architecture. This promising combination strategy prevented the growth of PDX tissue characterized by elevated MYC and resistance to conventional therapy (P = 0.04). Conclusions: Our results demonstrate preclinical efficacy of combination therapies targeting the ribosome at multiple levels and provide a new approach for treatment of PC with high MYC activity. Citation Format: Richard J. Rebello, Eric Kusnadi, Don Cameron, Analia Lesmana, Denis Drygin, Ashlee K. Clark, Laura Porter, Shahneen Sandhu, Gail P. Risbridger, Richard B. Pearson, Ross D. Hannan, Luc Furic. Dual inhibition of RNA Pol I transcription and PIM kinase as a new therapy to treat prostate 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 5181.