Abstract 1282: Overcoming resistance to inhibitors of CDK4/6 and BET in estrogen receptor-positive breast cancer
Authors: Cosín Marc , Tian Tian , Oliveros Winona , Kharenko Olesya A. , van der Horst Edward H. , Patel Reena G. , Calosing Cyrus , Campeau Eric , Jahagirdar Ravi , Lakhotia Sanjay , Melé Marta , Palafox Marta , Arribas Joaquín , Oliveira Mafalda , Saura Cristina , Serra Violeta , Peiro Sandra 
Published: September 2020

Abstract
Three inhibitors of CDK4/6 kinases have been recently approved for use in combination with endocrine therapy. These inhibitors considerably increase the progression-free survival of patients with advanced Estrogen receptor (ER)-positive breast cancer in first-line treatment. As the resistance to CDK4/6 inhibitors is currently a major problem faced in pre-clinical and clinical drug development, the identification of novel treatment combinations and predictive markers is of high importance. BET proteins, specifically BRD2/4, have important roles in cell cycle regulation and proliferation. Thus, BET inhibitors (BETi)have the potential to overcome some pathways of resistance, especially for those mediated by upregulation of specific transcription factors.
We have investigated the response to BET inhibition in patient-derived xenographs (PDXs) from patients with advanced ER+ breast cancer ex vivo and in vivo. Specifically, 14 breast cancer PDXs (10 ER+/HER2- and 4 ER+/HER2+) that were resistant to CDK4/6i in vivo were treated with ZEN-3694, a selective BET inhibitor currently in clinical development. Nine of the PDXs were sensitive to BET inhibition in monotherapy (65%). The non-responder PDXs showed an improved response when they were treated with ZEN-3694 in combination with fulvestrant (a selective ER degrader) or with a CDK4/6 inhibitor (ribociclib or abemaciclib). RNA-seq and computational analyses comparing responder versus non-responder PDXs revealed 493 differential expressed genes for responders, and 252 for non-responders. From those, 142 genes were differentially expressed in both conditions (untreated and treated). Furthermore, phosphorylation of BRD4 was observed in the ZEN-3694 non-responder PDX group, a mechanism of resistance previously observed in triple-negative breast cancer cell lines. This phosphorylation was abrogated by adding a CDK4/6 inhibitor. In the PDXs, resistance to single agent CDK4/6 and BET inhibitors might be overcome using a combination of both inhibitors. Finally, the molecular mechanism involved in phosphorylation of BRD4 was characterized in a MCF-7 cell line resistant to CDK4/6 inhibitors. Our results show that CDK2 could be one of the kinases involved in the phosphorylation of BRD4.
Conclusion: BET inhibition has an impact in advanced ER+ breast cancer cells, either used alone or in combination with endocrine therapy and CDK4/6 inhibitors. Detection of phosphorylated BRD4 (as a mechanism of resistance to BETi) has high potential for use as a biomarker for selecting patients who could benefit from a combined treatment of BET and CDK4/6 inhibitors.
Citation Format: Marc Cosín, Tian Tian, Winona Oliveros, Olesya A. Kharenko, Edward H. van der Horst, Reena G. Patel, Cyrus Calosing, Eric Campeau, Ravi Jahagirdar, Sanjay Lakhotia, Marta Melé, Marta Palafox, Joaquín Arribas, Mafalda Oliveira, Cristina Saura, Violeta Serra, Sandra Peiro. Overcoming resistance to inhibitors of CDK4/6 and BET in estrogen receptor-positive breast cancer [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 1282.