Adaptive Therapy With Capecitabine for Treatment of Metastatic ER Positive, HER2 Negative Breast Cancer

PRIMARY OBJECTIVE: I. Evaluate the feasibility of adaptive therapy (AT) in hormone receptor positive metastatic breast cancer, defined as the number of patients who can achieve AT modification for 2 or more cycles. SECONDARY OBJECTIVES: I. To evaluate time to progression in patients receiving capecitabine AT defined as the interval between treatment start and tumor progression, or death in patients with no evidence of disease progression. II. Assess overall survival in patients receiving capecitabine as adaptive therapy. III. Evaluate patient related outcomes by measuring quality of life and global health status of patients on AT using the European Organization for Research and Treatment of Cancer (EORTC) Quality of Life Core 30 questionnaire EORTC QLQ C-30. IV. Evaluate adverse events secondary to capecitabine using Common Terminology Criteria for Adverse Events (CTCAE) grading system version 5.0. V. Assess feasibility and accuracy of radiologic 3 dimensional (3D) volumetric approach in measuring target lesions. EXPLORATORY OBJECTIVES: I. Assess circulating tumor deoxyribonucleic acid (DNA) (ctDNA) as a low-cost alternative to imaging for measuring tumor burden. II. Identify gene signatures that could predict response and identify mechanisms of resistance to capecitabine using next generation sequencing technology, including: IIa. Whole exome DNA sequencing from ctDNA at baseline (day 0) and end of treatment; IIb. Whole transcriptome ribonucleic acid (RNA) sequencing from ctDNA at and at baseline (day 0) and end of treatment. III. ctDNA quantification is optional at day 1 (D-1). OUTLINE: INITIAL STANDARD PHASE: Patients receive standard dose of capecitabine orally (PO) twice daily (BID) on days 1-14 of each cycle. Cycles repeat every 21 days in the absence of disease progression or unacceptable toxicity. Patients whose disease has responded or remains stable after 2 cycles continue to the Adaptive Phase. ADAPTIVE PHASE: Patients receive 50% reduced dose of capecitabine PO BID on days 1-14 of each cycle. Patients undergo blood sample collection every cycle and computed tomography (CT) every other cycle for disease response assessment. Patients whose disease burden decreases \< 10% on CT or blood begins receiving an additional 50% reduced dose of capecitabine PO BID on days 1-14 of each cycle. Patients whose disease burden is stable on CT or blood continue receiving initial Adaptive Phase dose of capecitabine PO BID on days 1-14 of each cycle. Patients whose disease burden increases \> 10% on CT or blood begin receiving a 50% dose increase in capecitabine PO BID on days 1-14 of each cycle. Cycles repeat every 21 days in the absence of disease progression or unacceptable toxicity. Additionally, patients undergo blood sample collection, CT or magnetic resonance imaging (MRI), and bone scan if indicated on study. After completion of study treatment, patients are followed up every 3 months for up to 3 years from time of registration.