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Introduction: Mutations in the ESR1 gene (ESR1m) are important mechanisms of resistance to endocrine therapy (ET) in estrogen receptor-positive (ER+) advanced breast cancer and have been recognized as a prognostic and predictive biomarker as well as a potential therapeutic target. ESR1m prevalence has been described as 9-45% in cohorts of ET-resistant metastatic tumors in a variety of publications. We recently reported a prevalence of 25% in a cohort of ER+ breast cancer patients with visceral metastasis. However, the role of ESR1m as a mechanism of resistance to ET used in early-stage disease is not well studied. Neoadjuvant endocrine therapy (NET) is being increasingly explored, not only to allow less extensive surgery but also as a scientific tool, exploring biomarkers to predict outcomes. The preoperative endocrine prognostic index (PEPI) combining Ki67 score, ER Allred score, tumor size, and nodal status after NET is a surrogate of endocrine sensitivity and can identify a subgroup of patients with primary resistance to ET. In this study, we evaluated the prevalence of ESR1m in tumor samples from patients with ER+ breast cancer that were primarily resistant to neoadjuvant aromatase inhibitor (AI) therapy. Methods: We followed a prospective cohort of postmenopausal patients with ER+ HER2- stages II-III breast cancer treated with neoadjuvant endocrine therapy (NET). Patients were treated with anastrozole for a recommended period of at least three months. Tumor samples from patients with a pattern of primary endocrine-resistant tumors (defined as a PEPI Score higher than 3) were selected and analyzed for the presence of ESR1m. ESR1m were evaluated in formalin-fixed paraffin-embedded (FFPE) breast cancer tissue using real-time quantitative polymerase chain reaction (RT-qPCR). Mutations in codons 380, 537, and 538 of the ESR1 gene were analyzed. Results: 127 patients were included in the cohort, of which 100 (79%) had completed NET and had surgery. Among these patients, the PEPI Score ranged from 0 to 3 in 70% (70/100), and 30% (30/100) had a PEPI Score of 4 or more and were selected. 23 patients were included in the analysis (6 did not consent or were lost to follow-up, and one was HER2-positive). Patients characteristics are summarized in Table 1. The median duration of NET was 22 weeks. All samples of tumor tissue from the surgical specimens after NET were evaluable. Quantification of DNA extraction and reference gene cycle threshold values confirmed that the material was adequate for the analysis. We compared these findings with a study from our group using the same methodology in patients with advanced disease where ESR1 mutations were detected in 25% (n=32) of patients with visceral metastasis of ER+ breast cancer resistant to endocrine therapy. ESR1 mutations were not identified in any of the 23 patients with early-stage ER+ breast cancer resistant to NET (p 0.01, Fisher´s exact test) Discussion: Growing evidence supports the notion that there are different mechanisms of 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, ESR1m 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. Citation Format: Tomas Reinert, Susana Ramalho, Vivian CA Vasconcelos, Leonardo R Silva, Ana Elisa R Silva, Camila A Andrade, Maria Beatriz PL Kraft, Guilherme P Coelho, Jovana Mandelli, Monique Binotto, Cesar Cabello, Geisilene RP Silva, Jose Bines, Carlos H Barrios, Matthew J Ellis, Marcia S Graudenz. ESR1 mutations are not a mechanism of primary resistance to aromatase inhibitors in ER-positive breast cancer treated with neoadjuvant endocrine therapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-11-08.