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

Inclusion Criteria:

1. The patient has signed the informed consent document.
2. 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.
3. 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).
4. 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.
5. 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).
6. 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.
7. 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.
8. Patient is at least 18 years of age.
9. Eastern Cooperative Oncology Group (ECOG) Performance Status minor or equal to 1.
10. Life expectancy major or equal to 12 weeks.
11. Adequate organ and bone marrow function.
12. 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
13. 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).
14. Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests and other study procedures.

Exclusion Criteria:

1. 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.
2. 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).
3. 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 least 4 weeks before randomization.
4. Prior treatment with any CDK4/6, mTOR or PI3K inhibitor (any agent whose mechanism of action is to inhibit the PI3 kinase-mTOR pathway) or capecitabine.
5. a) Patients included in cohort 1: Prior treatment with exemestane in the metastatic setting. If the patient has received exemestane in the adjuvant setting and developed MBC, she will be eligible for the study provided:

   * She has received letrozole/anastrozole as first-line MBC and progressed.
   * At least 1 year has elapsed since the end of adjuvant exemestane treatment. b) Patients included in Cohort 2: Prior treatment with fulvestrant in the metastatic setting. If the patient has received fulvestrant in the adjuvant setting and developed MBC, she will be eligible for the study provided:
   * She has received letrozole/anastrozole as first-line MBC and progressed.
   * At least 1 year has elapsed since the end of adjuvant fulvestrant treatment.
6. Patients treated within the last 7 days prior to randomization with:

   * Food or drugs that are known to be CYP3A4 inhibitors
   * Drugs that are known to be CYP3A4 inducers
   * Drugs that are known to prolong the QT interval
7. Patients who received before randomization:

   * Any investigational agent within 4 weeks
   * Chemotherapy within a period of time that is minor than the cycle length used for that treatment (e.g. less 3 weeks for fluorouracil, doxorubicine, epirubicin or less than 1 week for weekly chemotherapy)
   * Previous endocrine therapy is permitted without any window
   * Radiotherapy within 2 weeks (all acute toxic effects must be resolved to NCI CTCAE version 4.0 grade minor 1, except toxicities not considered a safety risk for the patient at investigator´s discretion) but patients who received prior radiotherapy to less than 25 per cent of bone marrow are not eligible independent of when it was received
   * Major surgery or other anti-cancer therapy not previously specified within 4 weeks, (all acute toxic effects must be resolved to NCI CTCAE version 4.0 grade minor 1, except toxicities not considered a safety risk for the patient at investigator´s discretion)
8. Diagnosis of any other malignancy within 3 years prior to randomization, except for adequately treated basal cell or squamous cell skin cancer, or carcinoma in situ of the cervix.
9. QTc major 480msec, family or personal history of long or short QT syndrome, Brugada syndrome or known history of QTc prolongation, or Torsade de Pointes (TdP).
10. Uncontrolled electrolyte disorders that can compound the effects of a QTc-prolonging drug (eg, hypocalcemia, hypokalemia, hypomagnesemia).
11. Any of the following within 6 months of randomization: myocardial infarction, severe/unstable angina, ongoing cardiac dysrhythmias of NCI CTCAE version 4.0 Grade major or equal to 2, atrial fibrillation of any grade, coronary/peripheral artery bypass graft, symptomatic congestive heart failure, cerebrovascular accident including transient ischemic attack, or symptomatic pulmonary embolism.
12. Difficulties to swallow tablets, malabsorption syndrome disease significantly affecting gastrointestinal function, resection of the stomach or small bowel, or active inflammatory bowel disease or chronic diarrhea.
13. Known hypersensitivity to exemestane, palbociclib, capecitabine, fulvestrant or any of their excipients.
14. Any of the following contraindications for chemotherapy with capecitabine:

    * Known deficiency or family history of deficiency of dihydropyrimidine dehydrogenase.
    * Requirement for concurrent use of the antiviral agent sorivudine (antiviral) or chemically related analogues, such as brivudine.
15. Only for patients in Cohort 2 any of the following contraindications for treatment with fulvestrant:

    \- Bleeding diathesis (i.e., disseminated intravascular coagulation, clotting factor deficiency) or long-term anticoagulant therapy (other than antiplatelet therapy and low dose warfarin) provided that the International Normalised Ratio (INR) is less than 1.6.
16. Known human immunodeficiency virus infection.
17. Other severe acute or chronic medical or psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the patient inappropriate for entry into this study.
18. Recent or active suicidal ideation or behavior