Treatment Monitoring of Patients Receiving CDK 4/6 Inhibitors for Hormone Receptor (HR) Positive, HER2 Negative Metastatic Breast Cancer (MBC) With or Without the Addition of DiviTum® Serum Thymidine Kinase 1 (TK1) Activity Testing

Historically, serial testing of patients with metastatic breast cancer has included a combination of physical exam, symptom evaluation, laboratory testing, and imaging. Circulating tumor biomarkers are sometimes also incorporated. Frequent testing with numerous diagnostics at each time point is a significant burden to patients and to healthcare systems. The DiviTum® TKa assay measures TK1 activity. Numerous studies have illustrated the prognostic nature of plasma or serum TK1 activity level in metastatic cancer. The investigators hypothesize that the incorporation of data from DiviTum® TKa measurement into the treatment monitoring schema will be associated with physician desire to change the near-term usage and/or timing of other routine restaging tests, including either standard tumor imaging or tumor marker testing. Given the relatively low rate of disease progression in this first-line population, it is expected that most of this change will be an intended reduction in scheduling of routine treatment surveillance testing with increase in intervals of subsequent tumor restaging imaging by at least 4 weeks. Secondarily, the consequences of rescheduling of routine surveillance testing may ultimately result in an absolute reduction in the number of some tests used during the time period examined.