Acquired ESR1 mutations are a major mechanism of resistance to
aromatase inhibitors (AIs). We developed ultra high-sensitivity multiplex digital polymerase chain reaction assays for ESR1 mutations in
circulating tumor DNA (ctDNA) and investigated the clinical relevance and origin of ESR1 mutations in 171 women with advanced
breast cancer. ESR1 mutation status in ctDNA showed high concordance with contemporaneous
tumor biopsies and was accurately assessed in samples shipped at room temperature in preservative tubes. ESR1 mutations were found exclusively in
estrogen receptor-positive
breast cancer patients previously exposed to AI. Patients with ESR1 mutations had a substantially shorter progression-free survival on subsequent AI-based
therapy [hazard ratio, 3.1; 95% confidence interval (CI), 1.9 to 23.1; P = 0.0041]. ESR1 mutation prevalence differed markedly between patients who were first exposed to AI during the adjuvant and metastatic settings [5.8% (3 of 52) versus 36.4% (16 of 44), respectively; P = 0.0002]. In an independent cohort, ESR1 mutations were identified in 0% (0 of 32; 95% CI, 0 to 10.9)
tumor biopsies taken after progression on adjuvant AI. In a patient with serial sampling, ESR1 mutation was selected during metastatic AI
therapy to become the dominant clone in the
cancer. ESR1 mutations can be robustly identified with ctDNA analysis and predict for resistance to subsequent AI
therapy. ESR1 mutations are rarely acquired during adjuvant AI but are commonly selected by
therapy for metastatic disease, providing evidence that mechanisms of resistance to targeted
therapy may be substantially different between the treatment of micrometastatic and overt metastatic
cancer.