Poster Presentation 30th Lorne Cancer Conference 2018

Molecular mechanisms underpinning resistance to endocrine therapy and CDK4/6 inhibitors in breast cancer (#116)

Sarah Alexandrou 1 2 , Neil Portman 1 , Christine Lee 1 , Kristine Fernandez 1 , David Blake 3 , Elgene Lim 1 2 , Elizabeth Caldon 1 2
  1. Cancer Division, Garvan Institute of Medical Research, Sydney, NSW, Australia
  2. St. Vincent's Clinical School, Faculty of Medicine, UNSW, Sydney, NSW, Australia
  3. Cyclacel Ltd., Dundee, UK

Endocrine resistant estrogen receptor positive (ER+) breast cancers are particularly dependent upon cyclin-dependent kinase (Cdk) 4/6 kinases for proliferation. Cdk4/6 inhibitors (Cdk4/6i) are now being integrated into standard clinical practice to treat advanced endocrine resistant disease. Unfortunately, clinical resistance to Cdk4/6i is already emerging and there is an urgent need to understand and identify mechanisms of resistance to Cdk4/6i in breast cancer. We have generated a MCF-7 breast cancer cell line that is resistant to Palbociclib (PalbR). We performed cell cycle analysis by flow cytometry, western blotting of cell cycle proteins and metabolic assays to identify mechanisms of resistance.  We identified two potential mechanisms: increased proliferation induced by reduction of the Cdk inhibitor protein p27, and reduced induction of senescence as determined by B-galactosidase staining. Loss of p27 de-represses Cdk2 activity, and we find that PalbR cells have enhanced sensitivity to Cdk2/9 inhibitor CYC-065.

To complement this model, we are developing a panel of in vitro models that mimic the treatment of patients, as Cdk4/6i is always combined with an endocrine therapy in current clinical practice. MCF-7 cells are being chronically treated with Palbociclib in combination with anti-estrogens Tamoxifen or Fulvestrant, or with long-term estrogen deprivation (to mimic aromatase inhibitor treatment). These models demonstrate differences in cellular morphology and growth trajectories compared to the PalbR cell line. In parallel we have generated an in vivo ER+ PDX model which is resistant to chronic Fulvestrant-Palbociclib treatment, and have further in vivo models under development. We will perform comprehensive analyses (RNAseq, flow cytometry, western blotting) to identify mechanisms of resistance that are common to the treatment combinations, or are unique to each treatment regime. Our new panel of models will provide a framework for the identification of mechanisms of resistance, and a vehicle for testing therapies that could counteract this resistance.