ECLECTIC: EstroTEP and Circulating Biomarkers for ER-positive HER2-negative Metastatic Breast Cancer Patients

All patients deemed eligible for a second line endocrine therapy will undergo a 18F-FES PET/CT scan and circulating tumor biomarkers assessment (circulating tumor cells (CTC) and, if not available, circulating tumor DNA (ctDNA)). All 18F-FES PET/CT scan will be anonymized and reviewed centrally, and compared to the 18Fluorodeoxyglucose (18F-FDG) PET/CT results before treatment initiation; circulating biomarkers status will be assessed centrally and will remain blinded to investigator and patients.

Endocrine therapy in Arm A and C may consist in single agent endocrine therapy or in combination with targeted therapy. Luteinizing Hormone-Releasing Hormone (LH-RH) agonist will be used in combination with endocrine therapy whenever appropriate and per label. Chemotherapy in Arm B may consist in single agent chemotherapy, poly-chemotherapy, or antibody-drug conjugates. Patients who are eligible (per drug label) may receive Poly-adenosine-5'-diphosphate-ribose Polymerase (PARP) inhibitor if allocated to Arm B.