Oral Presentation NSW State Cancer Conference 2023

Putting the brakes on CDK4/6 inhibitor resistance in estrogen receptor positive breast cancers by harnessing the potential of CDK1 inhibition (#35)

Sarah Alexandrou 1 2 , Max Nobis 1 2 , Kristine J Fernandez 1 , Christine S Lee 1 , Janett Stoehr 1 , Victoria Lee 1 , Heloisa Helena Milioli 1 2 , Elgene Lim 1 2 , Ewan K. A. Millar 3 , Sandra O'Toole 1 2 4 , Paul Timpson 1 2 , Liz Caldon 1 2
  1. Garvan Institute of Medical Research, Sydney, NSW, Australia
  2. St. Vincent's Clinical School, Faculty of Medicine, UNSW Sydney, Sydney, NSW, Australia
  3. Faculty of Medicine, UNSW Sydney, Sydney, NSW, Australia
  4. Tissue Pathology and Diagnostic Oncology, Royal Prince Alfred Hospital, Sydney, NSW, Australia

Rationale: The biggest recent advance in care for patients with metastatic estrogen receptor positive (ER+) breast cancer has been the addition of CDK4/6 inhibitors (CDK4/6i) such as palbociclib/ribociclib to standard-of-care endocrine therapy. CDK4/6i doubles progression-free survival of patients, but unfortunately resistance to CDK4/6i occurs after ~20 months. Almost all ER+ patients with advanced breast cancer will receive CDK4/6i therapy in the future, but there are no treatment strategies for patients who relapse. 

Methods: To identify novel drivers of drug insensitivity we generated and characterised differentially expressed genes in CDK4/6i resistant ER+ breast cancer cells. Lead hits were validated using patient cohorts and by generation and drug treatment of CDK1 biosensor MMTV-PYMT mice.

Results: Palbociclib resistant (PalbR) cells, generated over a 10-month period, were analysed to reveal that CDK1, its binding partner CCNB1 (cyclin B1), along with associated activators were upregulated with palbociclib resistance. This manifested as sustained CDK1 levels and activity during palbociclib treatment, with a prolonged window of CDK1/cyclin B1 activity compared to control cells. Using multiple ER+ breast cancer patient cohorts, we show high expression of cyclin B1/CDK1 correlates with poorer patient survival and recurrent disease. Importantly, CDK1, CCNB1 and CCNB2 expression is high across two cohorts of ER+ breast cancer patients who do not respond to CDK4/6i.

We used CDK1-FRET biosensor MMTV-PYMT mice to visualise the spatiotemporal activity of CDK1 activity in live tumours following treatment with the CDK4/6i ribociclib. Ribociclib treatment led to an increase in CDK1 FRET activity after 72h. In parallel, we demonstrated that this occurs mechanistically in MCF-7 cells that are CDK4/6i sensitive via re-complexing of cyclin B1/CDK1 after short-term palbociclib. Due to the enhanced reliance on CDK1 activity, we treated PalbR cells and an ER+ patient-derived xenograft model resistant to CDK4/6i with dinaciclib, a pan-CDK1 inhibitor. We demonstrate that PalbR cells have enhanced sensitivity to dinaciclib, and dinaciclib reduced tumour growth of resistant disease in vivo.

Conclusions: We have identified a unique escape mechanism for CDK4/6i resistant cancers where they rely upon CDK1 to sustain proliferation. Most importantly, we are developing a way to target this escape mechanism via CDK1 inhibition.