Functional Imaging in Prediction of Response to Abemaciclib for Advanced Hormone Receptor-Positive, HER2-Negative Breast Cancer

OUTLINE: Patients receive FFNP intravenously (IV) and undergo PET/CT imaging at baseline. Patients then receive estradiol orally every 8 hours (Q8H) over a 24-hour period, followed again by FFNP IV and PET/CT imaging. Patients then receive abemaciclib orally (PO) twice daily (BID) on days 1-28 of each cycle. Cycles repeat every 28 days in the absence of disease progression or unacceptable toxicity. Patients also receive endocrine therapy (ET) of the treating physician choice. Patients also receive FDG IV and undergo PET/CT imaging at baseline, with additional diagnostic imaging for tumor assessment every 3 cycles, and undergo blood sample collection throughout the study. After study completion of study, patients are followed every 3 months.