Single arm for Breast Cancer

This is an open-label, multicenter study in patients with metastatic breast cancer who are candidates for standard first line treatment with palbociclib or ribociclib plus endocrine therapy. To be eligible, patients must have received no prior chemotherapeutic or hormonal regimen for metastatic disease. However, patients may still be considered eligible if they have already started treatment with endocrine therapy (an aromatase inhibitor or fulvestrant) plus palbociclib or ribociclib for no longer than 4 weeks prior to study enrollment, as long as they meet all other eligibility criteria. Eligible patients must have had a diagnostic biopsy of the metastatic lesion no more than 4 months prior to study enrollment and with sufficient tissue to complete the proposed biomarker analysis. Patients who develop disease progression within the first 12 months of starting palbociclib or ribociclib plus endocrine therapy will be eligible for an optional additional tissue biopsy at time of disease progression to repeat the analysis at time of disease progression and obtain real-time (10-14-day turn-around) multi-omic data produced under College of American Pathologist (CAP)/Clinical Laboratory Improvement Amendments (CLIA) development and/or compliant practices.