PURPOSE: Breast cancers with ESR1 mutations are resistant to antiestrogen therapy. In this study, we aimed to investigate the association of ESR1 mutations with resistance to CDK4/6 inhibitors (CDK4/6i) using real-world data analysis and experimental validation. PATIENTS AND METHODS: A total of 3,958 patients with estrogen receptor-positive (ER+) metastatic breast cancer with DNA sequencing data were analyzed. Breast tumor DNA and circulating tumor (ct) DNA were sequenced using the Tempus xT tumor assay and Tempus xF liquid biopsy, respectively. Patients were stratified into either treated with CDK4/6i (tumor tissue: 1,070
ctDNA: 1,885) or CDK4/6i naïve (tumor tissue: 750
ctDNA: 253). Engineered MCF7 cells carrying ESR1 Y537S or D538G knock-in mutations were used to study antitumor efficacy of the CDK4/6i palbociclib in vitro and in vivo. RESULTS: In both xF and xT assays, ESR1 mutations were the only somatic alterations significantly more frequent in patients who received CDK4/6i compared to those who did not. Knock-in of ESR1 Y537S or ESR1 D538G in MCF7 cells resulted in upregulation of cell cycle-related gene signatures upon treatment with CDK4/6i ± antiestrogen compared to cells with non-mutant ESR1. MCF7 xenografts harboring ESR1 Y537S and D538G mutations established in nude mice were resistant to palbociclib. CONCLUSIONS: We report herein real-world and preclinical evidence that ESR1 mutations, particularly Y537S and D538G, can drive resistance to CDK4/6 inhibitors.