IL-6 predicts CDK4/6 inhibitor resistance, identifying STAT3 as a target in HR + /HER2-negative metastatic breast cancer.

Resistance to CDK4/6 inhibitors (CDK4/6i) is a major challenge in treating hormone receptor-positive, HER2-negative metastatic breast cancer. This study aimed to identify a biomarker predictive of resistance that could also serve as a therapeutic target. Circulating IL-6 levels in 166 patients significantly increased at progression, making IL-6 a non-invasive biomarker to predict CDK4/6i resistance. Knockdown of IL-6 re-sensitized resistant cells to palbociclib and endocrine therapy, underscoring IL-6's critical role in maintaining resistance. Patient-derived xenograft models from patients who progressed within 3 months versus ≥6 months of palbociclib therapy revealed distinct transcriptomic profiles, with later progressors exhibiting IL-6/STAT3 activation, epithelial-to-mesenchymal transition, and immune evasion signatures. Treatment with TTI-101, a STAT3 inhibitor, significantly reduced tumor growth and improved survival in these models, providing preclinical validation for targeting the IL-6/STAT3 axis. These findings support using IL-6 to guide personalized treatment and combining STAT3 inhibitors with CDK4/6i as a transformative strategy to overcome resistance.