Abstract 1774: Circulating IL-6, an early biomarker in HR-positive, HER2-negative metastatic breast cancer patients progressing on CDK4/6 inhibitors

Background: CDK4/6 inhibitors (CDK4/6i, e.g. palbociclib) in combination with endocrine therapy (ET) have proven successful in delaying progression in hormone receptor-positive (HR+), HER2-negative (HER2-) metastatic breast cancer patients; however, 60-70% will progress within 1-2 years of treatment. Thus, understanding the mechanisms of resistance and identifying novel treatment strategies are needed to improve survival. Our previously published data shows that palbociclib resistance is marked by significant upregulation of the IL6-STAT3 signaling pathway in HR+, HER2- breast cancer cells and treatment with TTI-101, a small-molecule STAT3 inhibitor completing Phase I studies in solid tumor patients, significantly decreased cell viability. Additionally, matched biopsies from advanced HR+, HER2- breast cancer patients who progressed on palbociclib plus ET had an upregulation in activated STAT3 (STAT3 phosphorylated on Y705, pY-STAT3) as compared to their pre-treatment biopsy. Collectively, these data suggest that targeting the IL-6/STAT3 pathway is a viable therapeutic strategy. Our ongoing studies are directed at delineating the role of IL-6 and pY-STAT3 as predictive biomarkers to provide preclinical rationale for targeting STAT3 in patients who develop CDK4/6i resistance. Methods: We downregulated IL-6 using shRNA in MCF7 and T47D palbociclib sensitive and resistant cells. We tested the efficacy of TTI-101 in patient-derived xenograft (PDX) models from patients with metastatic HR+, HER2- breast cancer who progressed on treatment with palbociclib + ET. We also collected plasma samples from patients with metastatic HR+, HER2- breast cancer just prior to CDK4/6i treatment and at disease progression. Results: Downregulation of IL-6 in palbociclib resistant cells reduced pY-STAT3 levels and re-sensitized cells to ET and palbociclib. Induction of IL-6 is an early event by treating sensitive cells with 1uM palbociclib for 21 days during which the cells adapted and eventually developed resistance. Suppression of IL-6 in sensitive cells revealed that it is needed for cells to overcome growth inhibition by palbociclib. Targeting STAT3 in palbociclib resistant PDX mouse models using TTI-101 (50mg/kg twice daily by oral gavage), resulted in significantly reduced tumor volume; a synergistic effect was observed when TTI-101 was combined with palbociclib. Lastly, IL-6 levels in plasma samples from metastatic HR+, HER2- breast cancer patients were significantly higher a