Thymidine kinase activity levels in serum can identify HR+ metastatic breast cancer patients with a low risk of early progression (SWOG S0226)

Abstract

Background Some patients with metastatic breast cancer (MBC) stay on endocrine therapy (ET) for years and others progress quickly. Serum thymidine kinase activity (TKa), an indicator of cell-proliferation, is a potential biomarker for monitoring ET and predicting MBC outcome. We have previously reported TKa as being prognostic in MBC in SWOG S0226. Here, new data on progression within 30/60 days post sampling, with a new, FDA approved version of DiviTum®TKa highlighting differences vs. a Research Use Only version is reported. Methods 1,546 serum samples from 454 patients were assessed, collected at baseline and at 4 subsequent timepoints during treatment. A new predefined cut-off tested the ability to predict disease progression. A new measuring unit, DuA (DiviTum® unit of Activity) is adopted. Results A DiviTum®TKa score <250 DuA provides a much lower risk of progression within 30/60 days after blood draw, the negative predictive value (NPV) was 96.7% and 93.5%, respectively. Patients <250 DuA experienced significantly longer progression-free survival and overall survival, demonstrated at baseline and for all time intervals. Conclusions DiviTum®TKa provides clinically meaningful information for patients with HR+ MBC. Low TKa levels provide such a high NPV for rapid progression that such patients might forego additional therapy added to single agent ET. Trial registration: NCT00075764