Palbociclib Plus Fulvestrant in Women With Hormone Receptor Positive and Human Epidermal Growth Factor Receptor Type 2 Negative Locally Advanced or Metastatic Breast Cancer Previously Treated With a CDK4/6 Inhibitor in Combination With Hormonal Therapy

The aim of the present study is to evaluate the efficacy and safety of palbociclib plus fulvestrant after failure of a combined treatment of hormonal therapy (aromatase inhibitor or tamoxifen ± LHRHa) plus a CDK4/6 inhibitor, in women with HR+ and HER2- LABC or MBC. Primary endpoint: 1. To assess the clinical benefit rate (CBR) of the combination treatment palbociclib plus fulvestrant at progression of a combined treatment of hormonal therapy (aromatase inhibitor or tamoxifen ± LHRHa) and a CDK4/6 inhibitor. Clinical benefit rate for primary efficacy endpoints derivation will be based on the local (treating center's) radiologist's/investigator's tumor assessment. * For patients with measurable disease at baseline, progression will be determined according to the RECIST criteria v1.1. * In the absence of measurable disease at baseline, patients with bone only lesions, lytic or mixed (lytic + sclerotic), will be allowed to enter the study and the following will be considered disease progression among these patients: * The appearance of one or more new lytic lesions in bone, * The appearance of one or more new lesions outside of bone, * Unequivocal progression of existing bone lesions. Note: Pathologic fracture, new compression fracture, or complications of bone metastases will not be considered as evidence of disease progression, unless one of the above-mentioned criteria is fulfilled. 2. To assess the Quality of Life (QoL) of patients receiving the combination treatment palbociclib plus fulvestrant at progression of a combined treatment of hormonal therapy (aromatase inhibitor or tamoxifen ± LHRHa) and a CDK4/6 inhibitor. Secondary Endpoints: 1. To evaluate the efficacy of the combination of fulvestrant plus palbociclib at the progression of a combined treatment of hormonal therapy (aromatase inhibitor or tamoxifen ± LHRHa) and CDK4/6 inhibitors with respect to: * Overall response rate (ORR) * Progression Free Survival (PFS) * Over