PALbociclib CoLlaborative Adjuvant Study

Inclusion Criteria: * Signed informed consent prior to study specific procedures. * Age ≥18 years (or per national guidelines). * Pre- and postmenopausal women or men with Stage II (Stage IIA limited to max. 1000 patients) or Stage III early invasive breast cancer * Patients with multicentric and/or multifocal and/or bilateral early invasive breast cancer are eligible if all histopathologically examined tumors meet pathologic criteria for ER+ and/or PR+ and HER2-. * Patients must have histologically confirmed ER+ and/or PR+, HER2-, early invasive breast cancer. * Patients must have undergone adequate (definitive) breast surgery for the current malignancy. FFPE tumor tissue block must be confirmed to be received at the central sample repository prior to randomization. * ECOG performance status 0-1. * Patients must be able and willing to swallow and retain oral medication. * Serum or urine pregnancy test must be negative in premenopausal women within 14 days of randomization, or in women with amenorrhea of less than 12 months at time of randomization. * Patients who received neo/adjuvant therapy must be after last dose of chemotherapy and/or biologic therapy and must have sufficient resolution of side effects. * Patients who received breast/axilla/post-mastectomy chest wall radiotherapy must be after last dose of radiotherapy and must have sufficient resolution of side effects. * Patients must have sufficient resolution of any surgical side effects (no active wound healing complications). -Patients must either be initiating or have already started adjuvant hormonal treatment. - * Patients who already received neo/adjuvant endocrine therapy are eligible as long as they are enrolled within 12 months of initial histological diagnosis and after completing no more than 6 months of adjuvant endocrine therapy. * Absolute neutrophil count ≥ 1,500/µL * Platelets ≥ 100,000/ mm3 * Hemoglobin ≥ 10g/dL * Total serum bilirubin ≤ ULN; or total bilirubin ≤ 3.0 × ULN with direct bilirubin within normal range in patients with documented Gilbert's Syndrome. * Aspartate amino transferase (AST or SGOT) and alanine amino transferase (ALT or SGPT) ≤ 1.5 × institutional ULN. * Serum creatinine below the upper limit of the institutional normal range (ULN) or creatinine clearance ≥ 60 mL/min/1.73 m2 for patients with serum creatinine levels above institutional ULN. Exclusion Criteria: * Concurrent therapy with other Investigational Products. * Prior therapy with any CDK inhibitor. * Patients with Stage I or IV breast cancer are not eligible. * History of allergic reactions attributed to compounds of chemical or biologic composition similar to palbociclib. * Patients receiving any medications or substances that are potent inhibitors or inducers of * CYP3A isoenzymes within 7 days of randomization. * Uncontrolled intercurrent illness that would limit compliance with study requirements. * Pregnant women, or women of childbearing potential without a negative pregnancy test within 14 days prior to randomization. * Patients with a history of any malignancy are ineligible * Patients who previously received endocrine therapy within 5 years prior to diagnosis of the current malignancy. * Patients on antiretroviral therapy. * Patients with clinically significant history of any chronic liver disease. * Patients receiving concurrent exogenous hormone therapy (topical vaginal estrogen therapy is allowable).