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Mutations in the  ESR1  gene ( ESR1 m) are important mechanisms of resistance to endocrine therapy in estrogen receptor–positive (ER+) metastatic breast cancer and have been studied as a potential therapeutic target, as well as a predictive and prognostic biomarker. Nonetheless, the role of  ESR1 m as a possible mechanism of primary endocrine resistance, as well as whether it also occurs in tumors that are resistant to ET administered in early-stage disease as (neo)adjuvant, has not been adequately studied. In this study, we evaluated the prevalence of  ESR1 m in tumor samples from patients with ER+ breast cancer resistant to neoadjuvant aromatase inhibitor therapy.   Methods:  We followed a prospective cohort of patients with ER+ HER2– stages II and III breast cancer treated with neoadjuvant endocrine therapy (NET). Tumor samples from patients with a pattern of primary endocrine resistance [defined as a Preoperative Endocrine Prognostic Index (PEPI) score of ≥4] were identified and analyzed for the presence of  ESR1 m.   Results:  One hundred twenty-seven patients were included in the cohort, of which 100 (79%) had completed NET and underwent surgery. Among these patients, the PEPI score ranged from 0 to 3 in 70% (70/100), whereas 30% (30/100) had a PEPI score of 4 or more. Twenty-three of these patients were included in the analysis.  ESR1  mutations were not identified in any of the 23 patients with early-stage ER+ breast cancer resistant to NET.   Discussion:  Growing evidence supports the notion that there are different mechanisms for primary and secondary endocrine resistance. Our study suggests that  ESR1  mutations do not evolve rapidly and do not represent a common mechanism of primary endocrine resistance in the neoadjuvant setting. Therefore,  ESR1 m should be considered a mechanism of acquired endocrine resistance in the context of advanced disease. Further research should be conducted to identify factors associated with intrinsic resistance to ET.

frontiers in oncology

ESR1 Mutations Are Not a Common Mechanism of Endocrine Resistance in Patients With Estrogen Receptor–Positive Breast Cancer Treated With Neoadjuvant Aromatase Inhibitor Therapy