Prognostic role of thymidine kinase 1 activity in hormone receptor positive metastatic breast cancer. A systematic review and meta-analysis.
INTRODUCTION: Circulating thymidine kinase 1 activity (TKa) is a potential prognostic biomarker in patients with hormone receptor-positive (HR+) metastatic breast cancer (MBC); however, results are heterogeneous. In this study we aimed to summarize the current evidence on the prognostic role of circulating TKa in women with HR+ MBC. METHODS: We conducted a systematic review of PubMed, Embase, and Cochrane CENTRAL databases and abstracts from main international oncology meetings. Phase II-IV clinical trials and prospective observational studies in patients with MBC assessing circulating TK1 levels or TKa and reporting hazard ratios (HRs) for progression-free survival (PFS) and/or overall survival (OS) were included. HRs were pooled using random-effects models (restricted maximum likelihood with Hartung-Knapp adjustment), with heterogeneity quantified by I2 and prediction intervals. The primary study outcome was the association of baseline and on-treatment TKa with PFS and OS. Secondary analyses aimed at exploring the source of heterogeneity. RESULTS: Eighteen studies, reporting data from nearly 3000 women, were included in the systematic review and 15 studies were meta-analyzed. Patients with HR+ MBC and high baseline TKa showed a significantly higher risk of progression (PFS: HR 1.90; 95% CI 1.57-2.30; p < 0.001) and death (OS: HR 2.48; 95% CI 1.94-3.17; p < 0.001) than those with low TKa. TKa at 2 and 4 weeks on-treatment was also prognostic (2 weeks, PFS: HR 2.76; 95% CI 2.34-3.26; p < 0.001; 4 weeks, HR 2.26; 95% CI 1.93-2.66; p < 0.001). Similar pooled effects were obtained when accounting for different cut-offs, TKa assessment, and sample-type. CONCLUSION: Pre-treatment high TKa is an adverse prognostic factor in women with HR+ MBC. High on-treatment TKa is also associated with worse outcome, potentially serving as an early signal of treatment resistance. We provide a comprehensive summary of the currently available evidence on the prognostic value of circulating TKa.