This phase II trial evaluates the effect of capecitabine on tumor response using imaging and tumor markers to adjust dose (adaptive therapy) in patients with estrogen receptor (ER) positive, HER2 negative breast cancer that has spread from where it first started to other areas in the body (metastatic). Capecitabine is in a class of medications called antimetabolites. It is taken up by tumor cells and breaks down into fluorouracil, a substance that kills tumor cells. Adaptive therapy with capecitabine based on tumor burden response may slow or stop the growth of tumor cells in patients with metastatic ER positive, HER2 negative breast cancer.
PRE-REGISTRATION: Provide written informed consent Note: Pre-registration should occur prior to screening research blood draws being completed
PRE-REGISTRATION: Provider anticipates the patient will begin on capecitabine within 14 days or has started capecitabine within the past 42 days and has had a maximum of two cycles Note: Pre-registration should occur prior to screening research blood draws being completed
Age ≥ 18 years
Histological confirmation of estrogen-receptor positive (ER+), HER2-negative overexpression or amplification negative as per American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines, metastatic breast cancer
Prior chemotherapy or use of antibody drug conjugate in the metastatic setting
Note - Cohort 2 only: Patients can have had up to two cycles of capecitabine before joining the study
Pregnant persons
Nursing persons