Abstract 4413: CDK6 overexpression promotes resistance to CDK4/6 inhibitors in breast cancer

Abstract

Abstract CDK4/6 inhibitors (CDK4/6i) are highly active therapies in estrogen receptor-positive (ER+) breast cancer. Unfortunately, tumor relapse on these agents is frequently encountered in clinical practice with limited data on the biologic mechanisms that can mediate resistance. We utilized cell line models, patient derived xenografts and patient samples to interrogate mechanisms of resistance to CDK4/6 inhibitors. A survey of genomic alterations associated with limited benefit of CDK4/6iidentified several alterations involving the Hippo pathway which could potentially impact CDK6 kinase expression. Immunohistochemistry from these patients revealed increased YAP nuclear translocation and high levels of CDK6 protein specifically in the group of patients with short progression-free survival (PFS) on CDK4/6i. A PDX model of acquired resistance to CDK4/6i also showed that prolonged treatment with CDK4/6i led to a subset of tumors that developed resistance, all of which showed Hippo pathway suppression and CDK6 upregulation. Finally, cell lines models exposed to CDK4/6i showed multiple clones that featured upregulation of CDK6 through suppression of the Hippo pathway or amplification of the CDK6 gene. Identifying CDK6 overexpression as a recurrent feature of CDK4/6i resistant tumors, we next investigated the function(s) of CDK6 in our models. We performed knockdown of CDK6 by shRNA and found its overexpression to be necessary for drug resistance in these models while reinforced CDK6 could restore resistance or confer resistance when overexpressed in naïve cells. Given the close homology of CDK4 and CDK6, we asked whether any differences might exist between CDK4 and CDK6 complexes, performing immunoprecipitation and mass spectrometry (IP-MS) and found CDK6 to bind a distinct partition of proteins from CDK4 including INK4 proteins. In vitro kinase assays revealed that INK4 proteins impaired the ability of CDK4/6i to block CDK6 activity. INK4 proteins were found to cooperate with CDK6 in mediating drug resistance as their knockdown restored drug sensitivity to resistant cells, while INK4 overexpression promoted resistance in CDK6-high cells. Taken together, the data revealed that high levels of a CDK6-INK4 complex to be a recurrent mechanism of resistance to CDK4/6i and suggest novel CDK6 inhibitors as a strategy to overcome resistance. Citation Format: Qing Li, Pedram Razavi, Zhiqiang Li, Arnaud F. Da Cruz Paula, Edi Brogi, Maurizio Scaltriti, Jorge S. Reis-Filho, Sarat Chandarlapaty. CDK6 overexpression promotes resistance to CDK4/6 inhibitors in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4413.