HR+/HER2- aBC Undergoing 1L Treatment With CDK4/6i (Palbociclib, Abemaciclib or Ribociclib) and AI (Anastrozole or Letrozole)

Many patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) advanced breast cancer in the first therapy line is endocrine sensitive. For these patients the established 1st line standard therapy is a combination of a CDK4/6 inhibitor and an aromatase inhibitor. This combination therapy is usually given until clinical progression. Recent studies suggest that a detection of a somatic ESR1 mutation in the ctDNA can be clinically used to prompt an earlier change from the aromatase inhibitor to a selective estrogen receptor degrader. For implementation in the clinical routine it is important to gain knowledge about the dynamics of ESR1 mutation in HR+/HER2- advanced breast cancer patients during their first line therapy. At the beginning of the first line therapy about 5-8% of patients show an ESR1 mutation. An identification of patients with resistance to aromatase inhibitors at that timepoint might not be feasible and cost-efficient. At the timepoint of clinical progression up to 30-40% of patients will have acquired an ESR1 mutation. Knowledge about the mutation frequencies at different timepoints will be very helpful to direct testing strategies for implementing testing broadly in a population. The aim of this study is to test ESR1 mutation frequencies in groups of patients (single time test per patient) with no clinical progression and different durations under first-line treatment with aromatase inhibitor and CDK4/4 inhibitor.