Abstract B23: Inhibition of ribosomal RNA synthesis as a new therapeutic approach to treat advanced prostate cancer
Authors: Rebello Richard J. , Kusnadi Eric , Cameron Don P. , Pearson Helen B. , Lesmana Analia , Devlin Jennifer R. , Drygin Denis , Clark Ashlee K. , Porter Laura , Pedersen John , Sandhu Shahneen , Risbridger Gail P. , Pearson Richard B. , Hannan Ross D. , Furic Luc 
Published: May 2017

Abstract
Background: Prostate epithelium is exquisitely sensitive to the overexpression of the proto-oncogene MYC which causes neoplastic transformation. Indeed, MYC protein is almost universally overexpressed in metastatic castration-resistant prostate cancer (CRPC) making targeting MYC an attractive option for treating advanced stage disease. Unfortunately, the development of therapeutic agents directly targeting MYC has been largely unsuccessful, thus emphasizing the need to indirectly target MYC activity through inhibition of downstream cellular processes it regulates. One of the main effects of MYC in cancer cells is to accelerate proliferative growth via stimulation of high levels of ribosome biogenesis. Accordingly, the control of protein synthesis rate has emerged as the “Achilles' heel” of a wide array of tumors. MYC also regulates and cooperates with PIM kinases to increase the activity of the eIF4F translation initiation complex and MYC-driven tumors are addicted to eIF4E. Here, we investigate the efficacy of a single and dual approach targeting ribosome biogenesis and function to treat prostate cancer (PC).
Experimental design: We employed numerous models of PC, including a novel CRPC patient derived xenograft system, which showed the pre-clinical efficacy of therapies that combine to target MYC directed signaling to the ribosome. The 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, but only in models of hematological malignancy. CX-5461 and CX-6258, a pan-PIM kinase inhibitor, were tested alone and in combination in PC cell lines, in Hi-MYC and PTEN-deficient mouse models and in patient derived xenografts (PDX) of metastatic tissue obtained from a castration-resistant PC patient.
Results: CX-5461 inhibited anchorage-independent growth and induced cell cycle arrest in PC cell lines at nanomolar concentrations. Oral administration of 50 mg/kg CX-5461 induced p53 expression and activity and reduced proliferation (Ki-67) and invasion (loss of ductal actin) in Hi-MYC tumors, but not in PTEN null (low MYC) tumors. While 100 mg/kg CX-6258 showed limited effect alone, its combination with CX-5461 further suppressed proliferation and dramatically reduced large invasive lesions in both models. This rational combination strategy significantly inhibited proliferation and induced cell death in PDX of PC.
Conclusion: Our results demonstrate preclinical efficacy of targeting the ribosome at multiple levels and provide a new approach for the treatment of PC. In addition, a key conclusion of our study is that the androgen receptor (AR) presence or activity has no significant impact on the therapeutic activity of our novel combination therapy. Therefore, we believe our new exciting combination therapy could be used in the clinic in combination with current anti-androgens or as salvage therapy in multi-drug resistant CRPC.
Citation Format: Richard J. Rebello, Eric Kusnadi, Don P. Cameron, Helen B. Pearson, Analia Lesmana, Jennifer R. Devlin, Denis Drygin, Ashlee K. Clark, Laura Porter, John Pedersen, Shahneen Sandhu, Gail P. Risbridger, Richard B. Pearson, Ross D. Hannan, Luc Furic. Inhibition of ribosomal RNA synthesis as a new therapeutic approach to treat advanced prostate cancer. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B23.