Abstract P6-03-09: Role of IL-6 in promoting endocrine therapy and palbociclib resistance estrogen receptor positive breast cancer cells

Background: CDK4/6 inhibitors combined with endocrine therapy (ET) are mainstay to treat metastatic estrogen receptor (ER) positive patients. Yet, almost 60% develop resistance to CDK4/6 inhibition within 2 years of initial treatment. An ongoing clinical challenge has thus been identifying biomarkers of response to predict patients that will either respond or not respond to palbociclib. Further, there is an unmet need to identify actionable targets for patients that have progressed on CDK4/6 blockade regimens. Currently, the only biomarker being used to identify patients for anti-CDK4/6 therapy is estrogen receptor (ER) by IHC. The goal of our study was thus to identify the therapeutic vulnerabilities of CDK4/6 inhibitor resistance cells and identify key markers that can longitudinally correlate with development of resistance. Methods: We have developed MCF7 and T47D resistant cells by treating them with increasing doses (1.2, 2.4, 3.6, and 4.8μΜ) of palbociclib over a 6-month period. At each dose, resistant cells were harvested to observe dose dependent changes that occur as the cells acquire resistance to palbociclib. At endpoint i.e. 4.8μM, multi-omics approach was used to compare sensitive vs. resistant cells. Results: Our multi-omics data indicates that palbociclib resistant cells show i) Upregulation of the STAT3-IL6 pathway ii) Loss of ERα and iii) Loss RB protein levels. Primarily, resistant cells adapt to palbociclib treatment by a dose dependent upregulation of IL6. Further, we observe that induction of IL-6 is an early event which also correlates with senescence-associated secretory phenotype (SASP) signature in resistant cells, predicted to be induced with CDK4/6 inhibition. Importantly, time-course studies using 1μM palbociclib indicates that IL-6 is needed in overcome SASP and promote proliferation in cells adapting and eventually developing resistance to CDK4/6 inhibition. Lastly, we subjected MCF7 and T47D parental cells, which are sensitive to endocrine therapy and palbociclib, with exogenous IL-6 and IL-6 receptor (IL-6R). Treatment with IL-6/IL-6R lead to the downregulation of ER and RB protein levels. IL-6 treated parental cells are 5-fold more resistant to tamoxifen and 7-fold more resistant to fulvestrant compared to parental cells not treated with IL-6. Additionally, dose response studies with palbociclib showed parental cells treated with IL-6 are 5-fold more resistant to palbociclib treatment compared to the cells cultured in the absence of IL-6. Conclusions: Collectively, our data suggests that resistance to palbociclib results in a cascade of events initiating with induction of senescence, leading to sustained upregulation of IL-6 which leads to the promotion of an aggressive tumor growth, accompanied by resistance to endocrine therapy. Our ongoing studies are geared towards delineating the role of each step of CDK4/6 resistance and determining if IL-6 is required to maintain CDK4/6 inhibition resistance. Our long-term goal is to correlate provide the pre-clinical rationale for using IL-6 as a predictive biomarker in patients receiving CDK4/6 inhibitor based therapies. Citation Format: Nicole M Kettner, Tuyen Bui, Xian Chen, Kelly K Hunt, Debu Tripathy, Khandan Keyomarsi. Role of IL-6 in promoting endocrine therapy and palbociclib resistance estrogen receptor positive breast cancer cells [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-03-09.