The goal of this study is to evaluate the extended Overall Survival (OS) from PARSIFAL trial - NCT02491983 focused on the efficacy and safety of palbociclib in combination with fulvestrant or letrozole in patients with Human Epidermal growth factor Receptor 2 (HER2)-negative, Endocrine Receptor (ER)-positive metastatic breast cancer. It was designed to test the superiority of fulvestrant plus palbociclib compared with letrozole plus palbociclib first and then the non-inferiority of fulvestrant plus palbociclib compared with letrozole plus palbociclib if the superiority objective was not achieved.

This is an observational, international multi-center study with the objective to evaluate the extended overall survival from PARSIFAL trial (NCT02491983) analyzing the efficacy and safety of palbociclib in combination with fulvestrant or letrozole in patients with HER2-negative, ER-positive locally advanced or metastatic breast cancer. The patients included in this trial were previously randomized in PARSIFAL trial and did not withdraw consent to participate in the PARSIFAL clinical trial. The primary objective is to compare the efficacy (in terms of OS) of palbociclib in combination with fulvestrant (interventional arm) versus palbociclib plus letrozole (control arm) during extended follow-up of PARSIFAL trial and the secondary objectives are to assess the extended efficacy, of palbociclib combined with fulvestrant or letrozole in terms of progression-free survival (PFS), to estimate the extended efficacy, of palbociclib combined with endocrine therapy (fulvestrant or letrozole) in terms of OS and PFS and to assess the subsequent antineoplastic therapies to palbociclib combined with fulvestrant or letrozole in this population. The PARSIFAL-LONG study is non-interventional. There are no protocol-mandated visits or procedures associated with the study. In this study the data required to document the defined study endpoints will be collected using the medical histories of patients accrued in the PARSIFAL study as data source. This additional data will be analyzed along with the database of PARSIFAL trial. The estimated study duration is 24 months and the expected period for data validation, analysis, and reporting is around 4 more months.