• Hormone receptor-positive, HER2-negative metastatic breast cancer.

• Measurable or non-measurable but evaluable disease by RECIST v1.1.

• Candidate for capecitabine treatment per physician decision. See below phase-specific eligibility criteria for further guidance.

• No more than one prior chemotherapy for metastatic disease.

• Patient must have received prior endocrine therapy with CDK4/6 inhibitor.

• If patient is on denosumab or zoledronic acid prior to enrollment, patient must have been on the regimen for at least 6 months prior to study. However, a washout of 3 weeks is required prior to C1D1.

• At least 18 years of age.

• ECOG performance status 0, 1, or 2

• Life expectancy of at least 12 weeks.

• Adequate bone marrow and organ function as defined below:

‣ Leukocytes ≥ 3 K/cumm

⁃ Absolute neutrophil count (ANC) ≥ 1.5 K/cumm

⁃ Platelets ≥ 100 K/cumm

⁃ Total bilirubin ≤ 1.5 x IULN (or total bilirubin ≤ 3 mg/dL if patient has known Gilbert Syndrome)

⁃ AST(SGOT)/ALT(SGPT) ≤ 3.0 x IULN

⁃ Creatinine clearance \> 60 mL/min by Cockcroft-Gault

⁃ Calcium within normal limits

• Women of childbearing potential and men who are heterosexually active must agree to use adequate contraception as specified in the protocol. Contraception should continue for 6 months (for women) or 3 months (for men) after the end of treatment. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately.

• Ability to understand and willingness to sign an IRB approved written informed consent document.

• Patients must have archival tissue sample available from prior metastatic biopsy. If no tissue is available, patient may still be able to enroll with PI approval.

• Presence of bone metastasis is not required.

• Candidate for, or currently on stable doses of capecitabine, defined as capecitabine: 1000 mg/m\^2 BID, 14 days on and 7 days off. A stable dose of capecitabine is defined as no more than grade 1 AEs related to capecitabine on the 1000 mg/m\^2 BID, 14 days on and 7 days off dose for at least 1 cycle. Capecitabine is not counted as a prior chemotherapy regimen in these patients.

• Progressive bone metastasis per the most recent tumor imaging studies by RECIST 1.1 or clinical progression (such as worsening bone pain, elevation of tumor marker) per treating physician.