Abstract A061: Targeting PRMT5 enhances the response to CDK4/6 inhibitors in multiple cancer types
Authors: AbuHammad Shatha , McArthur Grant A , Sheppard Karen 
Published: March 2020

Abstract
CDK4 is a central regulator of cell cycle progression and is frequently hyper-activated by various genomic changes in numerous cancer types. CDK4/6 inhibitors such as palbociclib are approved for treatment of HR-positive breast cancer and has shown efficacy across many cancers. However, CDK4/6 inhibitors are best used in combination therapy rather than as a monotherapy. Finding novel combinations that can enhance the efficacy of CDK4/6 inhibitors and overcome or delay the emergence of resistance may provide new treatment strategies for patients.PRMT5 is an emerging target for the treatment of cancer, with the newly developed inhibitor GSK3326595 currently in phase 1/2 clinical trials.PRMT5 is a type II protein methyl-transferase that primarily catalyses the formation of symmetric di-methylarginine modifications. PRMT5 methylates the arginine residue on Histones 2A, 3 and 4 and this is considered an important epigenetic regulatory mechanism. PRMT5 can also methylate a number of non-histone proteins that govern important cellular processes including, cell signalling, RNA nucleo-cytoplasmic transport, pre-RNA processing, and ribosome biogenesis. The combined inhibition of CDK4 and PRMT5 can be seen as a promising strategy for the treatment of many cancers. A high throughput drug screen with a boutique library of targeted therapeutics was performed in order to identify potential targets whose inhibition enhances palbociclib efficacy. The effectiveness of combination treatment was evaluated by measuring cell proliferation clonogenicity, and cell death. To understand the mechanism/s of response to CDK4 and PRMT5 inhibitors, we utilized RPPA and RNA sequencing assays. In addition to this, western blotting, PCR and real-time PCR were also used to study both gene and protein expression levels as well as to validate specific targets. Furthermore, gene depletion via shRNA was performed to validate the response to these treatments. Dual inhibition of CDK4/6 and PRMT5 resulted in a robust and more durable response than the single agents in breast cancer, oesophageal, pancreatic and melanoma cell lines. The response to this combination was observed in both p53-wild type and mutant cell lines. High throughput proteomic and genomic analysis revealed that Inhibition of CDK4/6 and/or PRMT5 acts in p53-dependent and independent mechanisms. In p53 wild type, the combination of palbociclib and GSKGSK3326595 resulted in a permenant state of cell cycle arrest, whereas in p53-mutant cells the combinaiton caused mitotic catastrophe and induced cell death. In conclusion, Inhibition of PRMT5 enhances the response to CDK4/6 inhibitors and the combination of palbociclib and GSK3326595 is a potentially promising treatment strategy for various cancers.
Citation Format: Shatha AbuHammad, Grant A McArthur, Karen Sheppard. Targeting PRMT5 enhances the response to CDK4/6 inhibitors in multiple cancer types [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A061. doi:10.1158/1535-7163.TARG-19-A061