Evaluating serum thymidine kinase 1 in patients with hormone receptor-positive metastatic breast cancer receiving first-line endocrine therapy in the SWOG S0226 Trial

Abstract

Purpose: Serum thymidine kinase 1 (sTK1) activity is associated with poor prognosis in metastatic breast cancer (MBC).Weassessed the prognostic effect of sTK1 in patients with hormone receptor- positive MBC treated on a prospective randomized trial of anastrozole (A) versus A plus fulvestrant (A + F). Patients and Methods: sTK1 was assessed in 1, 726 serums [baseline (BL), cycles 2, 3, 4, and 7] using the DiviTum assay. A prespecified cutoff of ≥200 Du/L was considered high. Progressionfree survival (PFS) and overall survival (OS) were analyzed by Kaplan-Meier, log-rank tests, and Cox regression. Results: BL sTK1 was elevated in 171 (40%) of 432 patients. Patients with high versus low BL sTK1 had significantly worse PFS [median 11.2 vs. 17.3 months, HR = 1.76; 95% confidence interval (CI; 1.43-2.16); P < 0.0001] and OS [median 30 vs. 58 months, HR = 2.38; 95% CI (1.91-2.98); P < 0.0001]. OS was significantly better for patients with high sTK1 who did not have prior adjuvant tamoxifen and who received A + F versus A alone [median 46 vs. 21 months, HR = 0.58; 95% CI (0.38-0.87); P = 0.0087]. Patients with low sTK1 had no difference in outcomes by therapy (P=0.44). At serial timepoints, high versus low sTK1 had significantly worse subsequent PFS and OS [at cycle 2: PFS HR = 1.70, 95% CI (1.34-2.17); P < 0.0001, OS HR = 2.51, 95% CI (1.93-3.26); P < 0.0001]. Conclusions: High sTK1 at BL and subsequent timepoints is associated with worse prognosis in patients with MBC starting firstline endocrine therapy (ET). Patients with low sTK1 at BL have comparable outcomes on single-agent or combination ET.