Palbociclib in Combination With Tamoxifen as First Line Therapy for Metastatic Hormone Receptor Positive Breast Cancer

Inclusion Criteria: Subjects must meet all of the following applicable inclusion criteria to participate in this study: * Male or female ≥ 18 years of age at time of consent. NOTE: Both pre- and post-menopausal women are eligible. Pre-menopausal status is defined as either: * Last menstrual period within the last 12 months. * In case of therapy-induced amenorrhea, plasma estradiol and /or FSH is in the premenopausal range per local normal range. * Locally advanced, locoregionally recurrent, or metastatic disease, not amenable to curative therapy. NOTE: Although not required as a protocol procedure, a patient with a new metastatic lesion should be considered for biopsy whenever possible to reassess ER/PR/HER2 status if clinically indicated. If a biopsy is prospectively done as part of standard of care, the study would like to store samples for correlative research. * Histologically and/or cytologically confirmed diagnosis of ER positive and/or PR positive (ER \>1%, PR \>1%), HER2 negative breast cancer. NOTE: Subject has HER2-negative breast cancer (based on most recently analyzed biopsy) is defined as a negative in situ hybridization test or an IHC status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (e.g. FISH, CISH, SISH, DISH, etc.) test is required by local laboratory testing. * Metastatic disease evaluable on imaging studies. Subjects may have measurable disease as per RECIST 1.1 or bone-only disease. NOTE: Bone-only subjects are eligible if their…