• Female patient has given written informed consent

• Patient is ≥ 18 years of age at time of signing the written informed consent

• Patient has been diagnosed with histologically confirmed metastatic adenocarcinoma of the breast

• Patient has documented histological or cytological confirmation of estrogen receptor positive (ER+) and HER2 negative (HER2-) disease

• Patient has no curative treatment option by surgery or radiotherapy

• Patient was treated with CDK4/6 inhibitor plus endocrine therapy for at least 12 months with disease control (complete remission, partial remission or stable disease) as judged by the treating physician before planned study treatment initiation

• Patient has a preserved performance status (ECOG ≤ 2)

• Patient has adequate bone marrow, renal and hepatic function:

∙ Hemoglobin \> 9.0 g/dL

‣ Absolute neutrophil count judged as appropriate for study therapy by the investigator

‣ Platelets ≥ 100 x 109/L

‣ Calculated creatinine clearance judged as appropriate for study therapy by the investigator

‣ AST (SGOT) / ALT (SGPT) and alkaline phosphatase ≤ 2.5x ULN

‣ Serum albumin \> 30 g/L

• Patients considered postmenopausal according to one of the following definition:

∙ Women \<50 years of age who are amenorrheic for 12 months or more following cessation of exogenous hormonal treatments and whose levels of luteinizing hormone and follicle-stimulating hormone are in the post-menopausal institutional range

‣ Women ≥50 years of age who are amenorrheic for 12 months or more following cessation of all exogenous hormonal treatments, had radiation-induced menopause with last menses \>1 year ago or had chemotherapy-induced menopause with last menses \>1 year ago

‣ Artificially induced postmenopausal women (by Gonadotropin-releasing hormone \[GnRH\] analogs)

⁃ WOCBP must have a negative serum pregnancy test within 7 days prior to start of trial