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Background  Detection of circulating  ESR1  mutations is associated with acquired resistance to aromatase inhibitor (AI) in metastatic breast cancer. Until now, the presence of circulating  ESR1  mutations at the end of adjuvant treatment by AI in early breast cancer had never been clearly established. In this context, the aim of the present study was to evaluate the circulating  ESR1  mutation frequency at the end of adjuvant treatment and after relapse.    Methods  This monocentric retrospective study was based on available stored plasmas and included all early breast cancer patients who completed at least 2 years of AI adjuvant treatment and experienced a documented relapse after the end of their treatment. Circulating  ESR1  mutations (D538G, Y537S/N/C) were assessed by droplet digital PCR in plasma samples taken at the end of adjuvant treatment, at time of relapse and at time of progression under first line metastatic treatment.    Results  A total of 42 patients were included, with a median adjuvant AI exposure of 60 months (range 41–85). No circulating  ESR1  mutation was detectable at the end of AI adjuvant therapy. At first relapse, 5.3% of the patients (2/38) had a detectable circulating  ESR1  mutation. At time of progression on first-line metastatic treatment, 33% of the patients (7/21) under AI had a detectable circulating  ESR1  mutation compared to none of the patients under chemotherapy (0/10). The two patients with a detectable  ESR1  mutation at relapse were treated by AI and had an increase of their variant allele fraction at time of progression on first-line metastatic treatment.    Conclusions  Circulating  ESR1  mutation detection at the end of AI-based adjuvant treatment is not clinically useful. Circulating  ESR1  mutation could be assessed as soon as first relapse to guide interventional studies.

Circulating ESR1 mutations at the end of aromatase inhibitor adjuvant treatment and after relapse in breast cancer patients