ASCO, Author Interviews, Biomarkers, Breast Cancer / 30.05.2020 Interview with: Francois-Clement Bidard, MD PhD Head of Breast Cancer Group, Institut Curie Professor of Med. Oncology, UVSQ/Paris What is the background for this study? Response: A timely question is how to optimize the endocrine therapy of ER+ HER2- metastatic breast cancers? Aromatase inhibitors (AI) are currently the standard of care in first line, in combination with CDK4/6 inhibitors. Mutations in the estrogen receptor gene (ESR1) can be detected in up to 40% of AI-resistant metastatic breast cancers, but no data was available in the current first line setting (AI combined to CDK4/6 inhibitor). This exploratory study of the first line PADA-1 study reports on the detection rate of ESR1 mutations in cell-free DNA from an “AI-sensitive” population (with no metastatic relapse during adjuvant AI therapy), before the start of therapy (at inclusion). As expected, we found a low prevalence of 3.2% in the general population (N=1,017 patients included). The prevalence of ESR1 mutations among subgroups appeared primarily driven by the type of adjuvant endocrine therapy: patients with prior exposure to adjuvant therapy with AI displayed the highest prevalence of ESR1 mutations (7.1%), followed by patients with no prior adjuvant endocrine therapy (mostly de novo stage IV; ESR1 mutation prevalence: 2.4%). Patients who received adjuvant endocrine therapy with no AI (e.g. tamoxifen only) had the lowest ESR1 mutation prevalence (1.2%). (more…)