Molecular Mechanisms of Resistance and Sensitivity to Palbociclib Re-challenge in ER+ mBC

Eligible patients will receive palbociclib capsules orally for 21 days every four weeks in combination with endocrine therapy (physician's choice based on prior administered agent including tamoxifen, exemestane, fulvestrant, anastrozole, or letrozole). Patients will receive treatment until disease progression (with the exception of patients who develop isolated progression in the brain), unacceptable toxicity, death, or discontinuation from the study treatment for any other reason. Patients discontinuing the study treatment period will enter a post-treatment follow-up period during which survival and new anti-cancer therapy information will be collected every six months (± 14 days) from the last dose of investigational product. The treatment follow-up period will conclude at six months after the last patient has received first treatment dose in the study.