Abstract P4-04-12: Therapeutic targeting of CDK4/6 inhibitor resistant breast cancer
Authors: Lim E , Portman N , Alexandrou S , Haupt S , Haupt Y , Caldon E 
Published: February 2018

Abstract
Background:
Selective inhibitors of CDK4/6 kinases (CDK4/6i) were recently FDA approved for use in combination with endocrine therapy (ET), and represent the new standard of care. There are however patients who do not respond or develop resistance to these treatments, and therapies are required in this setting. While there is emerging data on the mechanisms of intrinsic insensitivity to CDK4/6i as monotherapies, which include cyclin E1 amplification, CDK6 amplification and Rb deletion, there is little data on mechanisms of resistance to combined ET and CDK4/6i.
Methods:
We established MCF7 cell line and patient-derived xenograft (PDX) models that are resistant to combined ET and Palbociclib (CDK4/6i) through long-term culture, allowing us to better understand mechanisms underlying CDK4/6i resistance and to model therapeutic strategies in this setting. We also evaluated our therapeutic strategy in vitro and in vivo using MCF cell lines that are resistant to ET, and in an ER+ PDX model derived from a patient who progressed on ET.
Results:
Cells resistant to CDK4/6i alone and in combination with ET show disrupted senescent pathways, and insensitivity to the induction of senescence. MDM2 inhibitors induce cells to enter into senescence, and consequently we are investigating the use of a new generation MDM2 inhibitor (CGM097, Novartis) either in combination with CDK4/6i treatment, or following acquisition of CDK4/6i resistance to prevent exit from senescence. We evaluated a CGM097 either in combination with CDK4/6i treatment, or in combination with fulvestrant following acquisition of CDK4/6i resistance to prevent exit from senescence. CGM097 was effective alone or in combination with fulvestrant in CDK4/6i resistant cells in vitro and in vivo, and resulted in a loss of G1 cells, and a reduction in B galactosidase, a senescence marker.
Another mechanisms of CDK4/6i resistance that has been identified is CDK2 activation, which can occur through Cyclin E amplification. As a second therapeutic strategy, we screened a panel of pan-CDK inhibitors with CDK2 activity in our resistant lines, and identified that CYC065 (Cyclacel), a highly selective CDK2/9 inhibitor, had the most durable response and highest synergy with ET in long-term culture. The combined resistant models were sensitive to CYC065 in vitro and in vivo. CYC065 was mechanistically distinct to CDK4/6i's as it caused arrest in a different phase of the cell cycle and affected expression of different cell cycle proteins.
Conclusion:
An underlying mechanism of combined ET and CDK4/6i resistance is senescent escape, which allows for normal proliferation upon removal of the drug. Using our in vitro and in vivo models of combined ET and CDK4/6i resistance, we have identified two novel therapeutic strategies for this disease, which represents the next clinical challenge in ER+ breast cancer as the natural history of disease is changed with the increasing use of CDK4/6i.
Citation Format: Lim E, Portman N, Alexandrou S, Haupt S, Haupt Y, Caldon E. Therapeutic targeting of CDK4/6 inhibitor resistant breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-04-12.