Abstract 5907: Fulvestrant resistance in estrogen receptor positive breast cancer models is driven by heterogeneous ER independent transcriptional programs

Abstract

Abstract Endocrine therapy is the mainstay of treatment for metastatic estrogen receptor (ER) positive breast cancer. The selective ER degrader (SERD) fulvestrant is increasingly relevant due to its high therapeutic efficacy and often used in combination with other targeted treatments in the metastatic setting. However, most metastatic patients inevitably progress during therapy and develop resistance, constituting a major clinical problem. Our aim is to identify mechanisms conferring resistance to fulvestrant and evaluate potential therapeutic interventions. We have generated several fulvestrant-resistant (FR) in vitro models through chronic exposure of established breast cancer cell lines (i.e. MCF7, Cama1, T47D and HCC1428) to increasing concentrations of fulvestrant. The models have been characterized by assessing gene expression alterations using microarrays, protein expression by western blots, cell cycle profiling by flow cytometry and drug sensitivity by proliferation assays. These established FR models demonstrate a marked decrease in sensitivity to fulvestrant and ability to progress through the cell cycle in the presence of fulvestrant despite downregulation of ERα and decrease in ER signaling. Gene Ontology analysis revealed involvement of mitotic cell cycle, cell proliferation and DNA replication and repair genes in development of resistance. Investigation of cell cycle regulation showed cell line specific alterations in the FR models. Striking molecular alterations included upregulation of cyclin D3, as well as cyclin E2 and its binding partner CDK2 in Cama1 FR cells, and upregulation of CDK6 in MCF7 FR cells. These changes indicate importance of cyclin D-CDK4/6 and cyclin E-CDK2 nodes in supporting cell cycle progression of cells that have developed resistance to fulvestrant. Subsequently, we evaluated response of the FR cells to pharmacological inhibition of CDK4/6 by palbociclib and CDK2 by dinaciclib. MCF7 parental and FR cells were equally responsive to palbociclib treatment, while Cama1 FR cells displayed less sensitivity to palbociclib than their parental counterpart. In contrast, dinaciclib induced comparable growth inhibition in Cama1 parental and FR cells, while demonstrating lower efficacy in MCF7 FR model comparing to parental cells. Over all, our findings show heterogeneous mechanisms of endocrine resistance and demonstrate that our in vitro models can represent diverse mechanisms of fulvestrant resistance. This heterogeneity likely occurs also in breast cancer patients whose tumors develop resistance to endocrine therapy. The FR models display ER-independent growth, for which the underlying mechanisms include alterations in cell cycle regulation. Importantly, some of the changes could predispose FR cells to respond better to specific therapeutic interventions, such as treatment with CDK4/6 or CDK2 inhibitors. Citation Format: Kamila K. Kaminska, Nina Akrap, Johan Staaf, Åke Borg, Ana Bosch, Gabriella Honeth. Fulvestrant resistance in estrogen receptor positive breast cancer models is driven by heterogeneous ER independent transcriptional programs [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 5907.