Phase III Palbociclib With Endocrine Therapy vs. Capecitabine in HR+/HER2- MBC With Resistance to Aromatase Inhibitors

Inclusion Criteria

• The patient has signed the informed consent document.
• a) Patients in cohort 1: Females with histologically confirmed MBC whose disease is resistant to previous non-steroidal aromatase inhibitors (letrozole or anastrozole) b) Patients in cohort 2: Females with histologically confirmed MBC whose disease was resistant to previous aromatase inhibitors (exemestane, letrozole or anastrozole).

Resistance is defined as: Recurrence while on or within 12 months after the end of adjuvant treatment with NSAI/AI or Progression while on or within 1 month after the end of treatment with NSAI/AI for advanced disease.

• Previous chemotherapy is permitted either in the (neo) adjuvant setting and/or first line therapy for MBC (chemotherapy administered as "second adjuvant therapy" for locoregional recurrence should be considered as first line chemotherapy for MBC).
• It is not mandatory to have exemestane, letrozole or anastrozole as the most recent treatment before randomization but recurrence or progression of breast cancer while receiving (or immediately after the enf of) the most recent systemic therapy has to be documented before randomization.
• Hormonal receptor positive (HR+) breast cancer based on local laboratory determination. HR+ defined as major or equal to 1 percent positive cells by Immunohistochemistry (IHC) for ER and/or Progesterone Receptor (PgR).
• Documented HER2 negative breast cancer based on local laboratory determination on most recent tumor biopsy. HER2 negative tumor is determined as IHC score 0 or 1+ or negative by ISH (FISH/Chromogenic In Situ Hybridization (CISH)/SISH) defined as a HER2/CEP17 ratio minor to 2 or for single probe assessment a HER2 copy number minor to 4.
• Measurable disease or at least one bone lesion, lytic or mixed (lytic+blastic), which has not been previously irradiated and is assessable by CT/MRI in the absence of measurable disease according to RECIST 1.1 criteria.
• Patient is at least 18 years of age.
• Eastern Cooperative Oncology Group (ECOG) Performance Status minor or equal to 1.
• Life expectancy major or equal to 12 weeks.
• Adequate organ and bone marrow function.
• Postmenopausal women defined as women with:

Prior bilateral surgical oophorectomy, or Age > 60 years, or Age < 60 years and medically confirmed post-menopausal status defined as spontaneous cessation of regular menses for at least 12 consecutive months with no alternative pathological or physiological cause or follicle-stimulating hormone (FSH) and estradiol blood levels in their respective postmenopausal ranges

• Resolution of all acute toxic effects of prior anti-cancer therapy or surgical procedures to NCI CTCAE version 4.0 Grade minor or equal to 1 (except alopecia or other toxicities not considered a safety risk for the patient at investigator´s discretion).
• Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests and other study procedures.

Exclusion Criteria

• Have received more than 1 prior chemotherapy regimen for MBC. (NOTE: Chemotherapy administered as "second adjuvant therapy" for locoregional recurrence should be considered one prior chemotherapy for MBC).Other previous anticancer endocrine treatments for advanced disease are allowed.
• Patients with advanced, symptomatic, visceral spread that are 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).
• Known active uncontrolled or symptomatic central nervous system (CNS) metastases, carcinomatous meningitis or leptomeningeal disease as indicated by clinical symptoms, cerebral edema, and/or progressive growth. Patients with a history of CNS metastases or cord compression are eligible if they have been definitively treated with local therapy (eg, radiotherapy,) and are clinically stable off anticonvulsants and steroids for at