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.