Palbociclib With Fulvestrant for Metastatic Breast Cancer After Treatment With Palbociclib and an Aromatase Inhibitor

In this phase II trial, the primary objective is to determine the progression-free survival (PFS) of the cyclin dependent kinase 4/6 inhibitor (CDK4/6i) palbociclib with fulvestrant in women and men with estrogen or progesterone receptor (ER/PR) positive, HER2-negative metastatic breast cancer (MBC) who progressed on treatment with palbociclib and an aromatase inhibitor (AI). The study will also determine the prevalence rate of estrogen receptor α (ESR1) and phosphatidylinositol-3-kinase (PI3K) mutations in the study population. The secondary objectives include evaluating the progression-free survival (PFS) in participants with and without ESR1 mutations, and PI3K mutations through analysis of tumor biopsies and circulating plasma tumor DNA (ptDNA) samples. The study will also describe other alterations in genes and gene products relevant to the cell cycle, drug targets, tumor sensitivity and resistance, and identify novel protein kinases activated in biopsies from participants with hormone refractory MBC who progressed on prior palbociclib and AI. The data will be correlated with tumor subtypes, expression profiles, and candidate phosphoprotein expression with PFS in the study population.