Oncology Abstracts

Endocrine resistant estrogen receptor positive (ER+) breast cancers are dependent upon cyclin-dependent kinases (CDK) 4/6 for proliferation, making them highly suitable for CDK4/6 inhibitor treatment. Despite initial efficacy, acquired resistance to CDK4/6 inhibitors is emerging and is now a major consideration in pre-clinical and clinical drug development. Current models of CDK4/6 inhibitor resistance do not mimic the clinical scenario where CDK4/6 inhibition will occur in the context of endocrine therapy and/or resistance. We aimed to characterise the mechanisms of resistance, identify clinically targetable pathways and evaluate novel therapeutic strategies for endocrine therapy and CDK4/6 inhibitor resistant ER+ breast cancer. To identify mechanisms of resistance we generated a palbociclib resistant (PalbR) MCF-7 breast cancer cell line. We show that PalbR cells have disrupted cell dynamics that result in an elongation of the S phase of the cell cycle. Prolonging S phase leads to a reduction in the CDK inhibitor proteins p21^Cip1 and p27^Kip1, and an activation of CDK2. CDK2 is therapeutically targetable using the CDK2/9 inhibitor CYC065 (Cyclacel, Phase I). CDK2 inhibition in combination with palbociclib enhanced growth inhibition and promoted the induction of senescence in PalbR cells. To complement this model, we have developed a panel of in vitro models that mimic the clinical treatment of patients. Here we combine palbociclib with an endocrine therapy; tamoxifen or fulvestrant, and show that palbociclib synergises with endocrine therapy to inhibit proliferation. In parallel we have generated an in vivo ER+ patient-derived xenograft model resistant to chronic fulvestrant+palbociclib treatment that has downregulation of cell cycle associated transcripts. Resistance to palbociclib occurs via cell cycle dysregulation of S phase, supressing CDK inhibitors. Our novel panel of resistant models provides a framework to identify mechanisms of acquired resistance, and a vehicle for testing clinically relevant therapies that could counteract this resistance.