Abstract 5779: CDK4/6 inhibitors and anaplastic thyroid cancer: An investigation into the retinoblastoma cancer pathway and potential therapeutic targets
Authors: Wong Kristen A. , Cristofano Antonio Di 
Published: August 2018

Abstract
Introduction/Background: Recently, Cyclin-dependent kinase (CDK) 4/6 inhibitors have been shown to increase progression-free survival in estrogen receptor-positive, stage IV breast cancer patients. We aim to assess whether Palbociclib, a selective CDK4/6 inhibitor, prevents tumor cell growth in vitro and in vivo in anaplastic thyroid cancer (ATC). ATC carries an automatic stage IV classification and a 6-month survival prognosis; there has been very little advancement in chemotherapeutic options, making it a good disease target for this new class of drug. Retinoblastoma (Rb) is a tumor suppressor protein that is rarely mutated in ATC (&lt;3%). It blocks cell entry into S phase, preventing DNA synthesis and cell proliferation. Hyper-phosphorylation of Rb by CDK4/6 causes the release of transcription factors and induces cell division. By blocking CDK4/6 with the inhibitor, the tumor suppressor function of Rb stays activated. Methods: We characterized our large library of thyroid cancer cell lines by mRNA and proteins involved in the Rb pathway using qPCR and western blot. We then performed 72-hour dose-dependent cell growth assays using viable cell counts with differing concentrations of Palbociclib to determine EC50 values. The same cell counting technique was used to test Palbociclib in combination with other targeted chemotherapy agents and again in resistance/adaptation experiments with long term exposure. Cell cycle analysis was performed using BD FACSCanto II fluidics system. Results: Our human lines show more susceptibility compared to murine lines. Interestingly, the PI3K and RAS pathway-activating mutated lines required the highest relative doses of Palbociclib followed by p53 and BRAF mutated cells. The Rb negative cells showed no growth inhibition when treated. Using flow cytometry, it was confirmed that Palbociclib's cystostatic effect is due to a G1 arrest. The presence of synergistic effects between Palbociclib and Omipalisib, a PI3K inhibitor, occurs in the majority of the cell lines and preliminary experiments show this combination treatment slows the development of resistance to Palbociclib. Conclusions: In vitro testing has shown that Palbociclib causes dose-dependent growth inhibition on Rb positive ATC cells, and genetic mutations may predict response to the drug. Rb protein is necessary for the drug to have effect. Current work is focused on designing a novel mouse model for in vivo study and to further investigate the related parallel pathways for combined targeted therapies, namely those whose over-activation is associated with the least sensitivity to Palbociclib - PI3K/AKT and RAS pathways.
Citation Format: Kristen A. Wong, Antonio Di Cristofano. CDK4/6 inhibitors and anaplastic thyroid cancer: An investigation into the retinoblastoma cancer pathway and potential therapeutic targets [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5779.