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

• 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.
• Female or male patients.
• Age ≥ 18 years old.
• Metastatic invasive hormone receptor positive and HER2 negative breast cancer (histologically confirmed).
• 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.
• Symptomatic or asymptomatic metastatic breast cancer.
• 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).
• Life-expectancy > 6 months.
• 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

• Indication for poly-chemotherapy or single-agent endocrine therapy only or bevacizumab.
• Asymptomatic oligometastases of the bone as the only site of metastatic disease.
• Uncontrolled/untreated central nervous system lesions.
• Patients who received treatment for metastatic/relapsed breast cancer.
• Inadequate organ function as per physician's assessment immediate prior to randomization.
• Treatment with preparations containing St. John´s Wort within the last 7 days prior to randomization and/or concurrent use.
• Known severe hypersensitivity reactions to compounds or excipients similar to palbociclib, planned chemotherapy or planned endocrine therapy.
• Existing contraindication against the use of palbociclib, planned chemotherapy or planned endocrine therapy.
• Patients with hereditary problems of galactose intolerance, the Lapp lactase deficiency, and glucose-galactose malabsorption.
• 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.