Abstract 3101: In-vitro characterization of Abemaciclib pharmacology in ER+ breast cancer cell lines

Dysregulation of the cell-cycle is a hallmark of cancer and genetic alterations in its regulatory machinery (or checkpoints) occur in most human tumors. The majority these defects are found in genes encoding for proteins regulating G1 phase progression, such as Rb, E2F1, CyclinD1, CDK4 and CDK6. Aberrant regulation of the G1 kinases CDK4 and CDK6, as well as overexpression or gene amplification of CyclinD, lead to inhibition of tumor suppressors such as Rb resulting in an accelerated cell cycle progression. Alterations in the CyclinD-CDK4/6-Rb pathway are common in breast cancer. Amplification of CCND1 gene encoding CyclinD1, occurs in 15% to 20% of breast cancers, and CyclinD1 overexpression is even more common (up to 50% of breast cancers). Abemaciclib is a reversible, ATP competitive, kinase inhibitor selective for CDK4 and CDK6 that has been shown to prevent growth of malignant cells in-vitro and in-vivo. This antitumor activity is mediated by inhibiting the phosphorylation of Rb and subsequent blockade of tumor cell cycle progression through G1/S. CDK4/6 inhibitors in general have shown significant potential for the treatment of metastatic breast cancer and Abemaciclib, in particular, is currently being evaluated in advanced clinical trials (Phase II as single agent and Phase III in combination with anti-hormone therapy) in hormone receptor positive metastatic breast cancer patients. The goal of this study was to investigate the mechanism of action of Abemaciclib in ER+ luminal breast cancer. We have evaluated the response of the drug in a diversity of breast cancer cell lines. Phenotypic characterization of sensitive cell lines was carried out by monitoring proliferation, cell cycle progression and phosphorylation of Rb using High Content Imaging. Senescence markers were included in the study to monitor the final outcome of the cells upon sustained exposure to the drug. Luminal ER+ breast cancer cells showed a marked sensitivity to treatment with Abemaciclib with IC50 values ranging from 5nM to 2uM. Simultaneous decrease in Rb phosphorylation with sustained accumulation of the 2N subpopulation was observed. Associated to the G1S arrest phenotype, Abemaciclib treatment resulted in a decrease of cell proliferation markers (Ki67 and BrdU). Additionally, a marked hyper-methylation profile (Histone H3K9met3) and a decrease of FOXM1 expression were observed, as well as an accumulation of endogenous beta-galactosidase and p21. Taken together this profile suggests that Abemaciclib acts through promotion of senescence in breast cancer cells. Abemaciclib prevents proliferation of breast cancer cell lines expressing D-types cyclins by promoting cell cycle arrest mediated by inhibition of Rb phosphorylation. Abemaciclib is a CDK4/6 inhibitor with potential to treat breast cancer by blocking cell proliferation leading to induction of senescence. Citation Format: Maria Jose Lallena, Karsten Boehnke, Raquel Torres, Ana Hermoso, Joaquin Amat, Bruna Calsina, Alfonso De Dios, Sean Buchanan, Jian Du, Richard Paul Beckmann, Xueqian Gong, Ann Mcnulty. In-vitro characterization of Abemaciclib pharmacology in ER+ breast cancer cell lines. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3101. doi:10.1158/1538-7445.AM2015-3101