DiviTum® TKa assay

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.

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.