Study in Women With Advanced Breast Cancer Receiving Palbociclib With AI or Fulvestrant

Inclusion Criteria: 1. Personally signed written informed consent prior to beginning protocol specific procedures, including expected cooperation of the patient for the treatment and follow-up, must be obtained and documented according to the local regulatory requirements 2. Women with proven diagnosis of advanced, defined as locally advanced inoperable or metastatic, adenocarcinoma of the breast 3. Hormone-receptor-positive (HR+) disease, defined as estrogen-receptor-positive (ER+) and/or progesterone-receptor-positive (PgR+) 4. Human epidermal growth factor receptor 2-negative (HER2-) disease (HER2 neg/+ or HER2++ with CISH/FISH neg.) 5. Pre-/perimenopausal women receiving concomitant therapy with an luteinizing hormone-releasing hormone (LHRH) agonist / ovarian ablation or postmenopausal status 6. Age ≥18 years 7. Measurable disease as per Response Evaluation Criteria in Solid Tumors \[RECIST\] or bone-only disease 8. Patients scheduled for palliative treatment with an combination partner for first- or later-line 9. Eastern Cooperative Oncology Group (ECOG) performance status 0-2 10. Adequate organ and marrow function 11. Resolution of all acute toxic effects of prior therapy, including radiotherapy Grade \<1 (except toxicities not considered a safety risk for the patient) and recovery from surgical procedures 12. Fluent in spoken and written German Exclusion Criteria: 1. Prior treatment with any CDK4/6 inhibitor 2. Prior adjuvant therapy with the respective endocrine combination partner if last intake \<12 months prior to entering the study 3. Prior palliative therapy with the respective endocrine combination partner 4. More than one prior palliative chemotherapy 5. 5\. Known hypersensitivity to letrozole, anastrozole, exemestane, fulvestrant or any of their excipients 6. Current use of food or drugs known to be potent inhibitors or inducers of CYP3A4 (refer to Appendix 15.4) 7. Current use of preparations containing St. John's Wort 8. Participation in other studies involving investigational drug(s) (Phases I-IV) within 2 weeks before the current study treatment begins 9. QTc \> 480 msec on the screening ECG (using the QTcF formula and/or the QTcB (Bazett) formula); history of QT syndrome, Brugada syndrome or known history of QTc prolongation, or Torsade de Pointes 10. High cardiovascular risk, including, but not limited to recent myocardial infarction, severe/unstable angina and severe cardiac dysrhythmias in the past 6 months prior to enrollment 11. Patients with advanced symptomatic, visceral spread, that were at risk of life-threatening complications in the short term (including patients with massive uncontrolled effusions \[pleural, pericardial, peritoneal\], pulmonary lymphangitis, and over 50% liver involvement) 12. Diagnosis of any second malignancy within the last 3 years prior to enrollment, except for adequately treated basal cell or squamous cell skin cancer, or carcinoma in situ of the cervix 13. Known, not-irradiated CNS metastases