Single-Cell RNA Sequencing Identifies Molecular Biomarkers Predicting Late Progression to CDK4/6 Inhibition in Patients with HR+/HER2- Metastatic Breast Cancer [tissue]

This study addresses the critical need for predictive biomarkers in patients with hormone receptor–positive, HER2-negative (HR+/HER2-) metastatic breast cancer (mBC) who have been treated with cyclin-dependent kinase 4/6 inhibitors (CDK4/6is). Using single-cell RNA sequencing, we reveal differential mechanisms of early versus late progression, enhancing our understanding of resistance to CDK4/6is. We identify and validate tumor-specific molecular biomarkers and demonstrate that tumor-infiltrating cytotoxic natural killer and CD8+ T cells are predictive of therapeutic response. Additionally, increased immune activity suppression in late progressing tumors suggests that combining CDK4/6is with checkpoint inhibitors could be beneficial against drug resistance. Validation across 2 independent cohorts confirms the robustness and clinical relevance of our findings. These insights pave the way for optimizing therapeutic strategies based on microenvironment-specific changes, offering a personalized and effective approach to managing HR+/HER2- mBC and improving patient outcomes. Metastatic tumor biopsies were obtained from HR+/HER2- BC patients pre-CDK4/6i treatment at baseline (BL) and/or at disease progression. BL samples were from CDK4/6i responders (n = 8, median progression-free survival [mPFS] = 17 months), while progressors were categorized as early-progressors (EP, n = 3, mPFS = 3 months) and late-progressors (LP, n = 8, mPFS = 10 months). Metastatic sites included liver (10), pleural effusions (6), ascites (2), and bone (1). InferCNV distinguished tumor cells, and functional analysis utilized the Molecular Signatures Database. Signature panel genes were validated in two independent cohorts (MDACC n = 89, Korean n = 61) with RNAseq data from baseline biopsies from HR+/HER2- mBC patients pre-CDK4/6i treatment.