Abstract 5811: Combined inhibition of CDK2 and CDK4/6 overcomes acquired resistance to CDK4/6 inhibitors in breast cancer via RB-independent pathway

Abstract

Abstract Objective: FDA-approved CDK4/6 inhibitors (palbociclib, ribociclib and abemaciclib) are widely used to treat metastatic hormone receptor-positive (HR+) breast cancer. Despite the excellent disease control with CDK4/6 inhibitors, acquired resistance emerges in almost all cases. The strategies to combat the resistance are not yet defined. We investigated therapeutic strategies as well as mechanisms of resistance to CDK4/6 inhibitors in breast cancer. Methods: Palbociclib resistant cells (MCF7-PR and T47D-PR) were generated from the parental MCF and T47D cell lines by treating them with gradually increasing concentration of palbociclib for over 9 months. Cell viability assay, cell cycle, and western blot were performed to confirm the acquired resistance. Microarray analyses were carried out to identify altered gene expression. We validated the overexpressed genes by qRT-PCR and western blot and further confirmed the significant inhibition by CDK2 siRNA in combination with palbociclib both in vitro and in vivo. Results: Palbociclib-resistant MCF7 and T47D cells had about 10 fold higher half maximal inhibitory concentration (IC50), compared with parental cells. Morphological changes and alteration in EMT markers were observed in resistant cells. Loss of RB and overexpression of cyclin E1 confirmed that the cells gained resistance. Overexpression of C-MYC was observed in resistant cells compared to sensitive counterparts. Furthermore, from analysis of pleural effusion samples from HR+ breast cancer patients, we confirmed again that loss of RB and overexpression of cyclin E are significantly correlated with resistance to CDK4/6 inhibitors. Mechanistically, we found that, cyclin E-CDK2 mediated phosphorylation of C-MYC was responsible for suppressing C-MYC induced senescence. To overcome CDK4/6 inhibitor resistance, combined inhibition of CDK2 and CDK4/6 showed a synergistic effect in palbociclib-resistant in vitro and in vivo model. Further, combination-treated group showed significantly higher β-galactosidase expression compared to no- treatment control or palbociclib alone treated group. Conclusion: The synergistic anti-proliferative effect of the combined inhibition of CDK2 and CDK4/6 overcame acquired resistance to CDK4/6 inhibitors via RB-independent pathway, that is, suppressing C-MYC. These findings could pave the way for the development of next generation CDK2 inhibitor in CDK4/6 inhibitor-resistant breast cancer, which is characterized by overexpressed cyclin E and C-MYC. Keywords: CDK4/6, hormone receptor-positive breast cancer, drug resistance Citation Format: Kamal Pandey. Combined inhibition of CDK2 and CDK4/6 overcomes acquired resistance to CDK4/6 inhibitors in breast cancer via RB-independent pathway [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5811.