This is a prospective study to assess the impact of biomarker driven, early therapeutic switching and delayed imaging with the incorporation of DiviTum® serum TK1 activity ("DiviTum® TKa") in patients with HR positive, HER-2 negative metastatic or unresectable breast cancer. Patients will receive first-line treatment with a CDK4/6 inhibitor (CDK4/6i) and endocrine therapy. All patients will have blood drawn for thymidine kinase activity (TKa) testing at baseline and at C1D15. Patients who are found to have a lack of TKa suppression at C1D15 will be recommended to switch to an alternative therapy. Patients with suppressed C1D15 TKa levels will continue on CDK4/6i and endocrine therapy until clinical progression. Patients with TKa which remains suppressed will be recommended to delay restaging scans from 24 weeks to 36 weeks.

The investigators hypothesize that a patient's TKa level at C1D15 is prognostic for progression-free survival (PFS) on a CDK4/6 inhibitor and early therapeutic switching in patients with a lack of C1D15 TKa suppression will be associated with prolonged PFS.

Inclusion Criteria - Patients

• Diagnosis of metastatic or advanced unresectable invasive breast cancer that is hormone receptor-positive (HR+) and HER2-negative.

• Planned to initiate standard of care first-line therapy with FDA-approved endocrine therapy plus CDK4/6 inhibitor for the stated diagnosis at the time of study enrollment. Ribociclib is the preferred CDK4/6 inhibitor. In the event this drug cannot be obtained due to insurance authorization or if there are specific side effect profile concerns from the treating physician, an alternative CDK4/6 inhibitor is allowed.

• Any prior therapy for early stage breast cancer is allowed, including endocrine therapy and chemotherapy.

• Prior receipt of adjuvant CDK 4/6 inhibitor therapy is permitted provided therapy completion occurred > 12 months prior to study enrollment.

• Presence of RECIST-evaluable disease. Patients with bone-only disease are eligible.

• At least 18 years of age.

• ECOG performance status ≤ 2

• Post-menopausal status, defined as one of the following:

  • Age ≥ 60 years
  • Age < 60 with intact uterus and amenorrhea for 12 consecutive months or more
  • Status post bilateral oophorectomy, total hysterectomy
  • Pre- or peri-menopausal with suppressed ovarian function by use of GnRH agonist/antagonist or surgical bilateral oophorectomy

• Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable).

Exclusion Criteria - Patients

• Receipt of any prior cytotoxic chemotherapy line for metastatic disease. There will be no limit to chemotherapy use in the neoadjuvant or adjuvant setting.
• Patients with a prior or concurrent malignancy are excluded unless that malignancy's natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial.
• Concurrent participation in any investigational therapeutic trial for treatment of metastatic breast cancer.

Eligibility Criteria - Physicians

• Medical Oncologist at Siteman Cancer Center.
• Treating patients with metastatic or advanced unresectable breast cancer.
• Willing to complete Physician Surveys during participation.