Two Different Schedules of Palbociclib + Second Line Endocrine Therapy in Estrogen Receptor Positive, HER2 Neg Advanced/Metastatic Breast Cancer

Inclusion Criteria: * Premenopausal and postmenopausal women 18 years of age or older. * Histologically confirmed adenocarcinoma of the breast, with ER positive and HER2 negative status based on local testing on most recent pathological tumour specimen. * Patients must satisfy the following criteria for prior therapy: * Progressed during treatment or within 12 months of completion of adjuvant endocrine therapy or * Progressed during prior endocrine therapy for advanced/metastatic disease. Note: 'Progressed during endocrine therapy' means that the patient progressed while on or within 1 month after discontinuation of endocrine therapy. * One line of chemotherapy for advanced/metastatic disease (regardless of prior adjuvant chemotherapy use) is allowed in addition to endocrine therapy. * Patients must have evidence of disease to be eligible for the study, but measurable disease is not mandatory. * For those patient with measureable disease who will be included in the response assessment, the following criteria must apply: * X-ray ≥ 20 mm * Spiral CT scan or physical exam ≥ 10 mm (lymph nodes must be ≥ 15 mm in the short axis) * Conventional CT scan, MRI ≥ 20 mm * Measurable lesions must be outside a previous radiotherapy field if they are the sole site of disease, unless disease progression has been documented. Tumor lesions previously irradiated or subjected to other loco regional therapy will only be deemed measurable if progression at the treated site after c…