A Randomized, Open-label, Multi-center Phase IV Study Evaluating Palbociclib Plus Endocrine Treatment Versus a Chemotherapy-based Treatment Strategy in Patients With Hormone Receptor Positive / HER2 Negative Breast Cancer in a Real World Setting (GBG 93 - PADMA Study).

Inclusion Criteria: 1. Written informed consent prior to beginning specific protocol procedures, including expected cooperation of the patients for the treatment and follow-up, willingness and ability to complete collection of data via wearable device and study mobile must be obtained and documented according to the local regulatory requirements. 2. Female or male patients. 3. Age ≥ 18 years old. 4. Metastatic invasive hormone receptor positive and HER2 negative breast cancer (histologically confirmed). 5. Patients who in the opinion of the treating physician are candidates suitable for randomization for mono-chemotherapy treatment, that has either an approved label in Europe and/or is supported by guidelines for the treatment of first-line advanced BC, which are based on evidence on safety and efficacy in this setting. 6. Symptomatic or asymptomatic metastatic breast cancer. 7. Resolution of all acute toxic effects of prior anti-cancer therapy or surgical procedures to NCI CTCAE version 4.0 grade ≤ 1 (except alopecia or other toxicities not considered a safety risk for the patient at investigator's discretion). 8. Life-expectancy \> 6 months. 9. For female patients: The patients need to be either A) of non-childbearing potential (documented postmenopausal or post hysterectomy) B) childbearing potential with negative serum or urinary pregnancy test (in this case patients need to use highly effective non-hormonal contraceptive methods). Exclusion Criteria: 1. Indication for poly-chemotherapy or single-agent endocrine therapy only or bevacizumab. 2. Asymptomatic oligometastases of the bone as the only site of metastatic disease. 3. Uncontrolled/untreated central nervous system lesions. 4. Patients who received treatment for metastatic/relapsed breast cancer. 5. Inadequate organ function as per physician's assessment immediate prior to randomization. 6. Treatment with preparations containing St. John´s Wort within the last 7 days prior to randomization and/or concurrent use. 7. Known severe hypersensitivity reactions to compounds or excipients similar to palbociclib, planned chemotherapy or planned endocrine therapy. 8. Existing contraindication against the use of palbociclib, planned chemotherapy or planned endocrine therapy. 9. Patients with hereditary problems of galactose intolerance, the Lapp lactase deficiency, and glucose-galactose malabsorption. 10. Female patients: pregnancy or lactation at the time of randomization or intention to become pregnant during the study and up to six months after treatment. Male patients: Intention to beget a child during the study and up to six months after treatment.