Abstract CT233: Serum thymidine kinase 1 levels as a predictive marker for castration resistance in metastatic prostate cancer
Authors: Siltari Aino , Raittinen Paavo , Riikonen Jarno , Koskimäki Juha , Pakarainen Tomi , Anttinen Mikael , Ettala Otto , Tammela Teuvo , Linder Stig , Murtola Teemu J. 
Published: April 2024

Abstract
Introduction
Thymidine kinase 1 (TK1) is an enzyme associated with the cell division cycle; its levels increase during the S-phase of the cell cycle and decrease during the M-phase. In cancer TK1 release is increased both by increased cell proliferation and cell death. Thus, serum concentration of TK1 reflects the tumor volume, proliferation rate, and extent of cell death. Serum Thymidine Kinase 1 (sTK1) enzyme has been used as a prognostic, predictive, and monitoring biomarker in leukemia, Hodgkin's and non-Hodgkin's lymphomas. Additionally, sTK1 enzyme levels have been elevated in breast cancer and prostate cancer. This study evaluated whether sTK1 predicts the risk of castration resistance in patients with metastatic prostate cancer undergoing androgen deprivation therapy (ADT).
Materials and Methods
sTK1 levels was measured in men diagnosed with metastatic prostate cancer at 3 time points; before the initiation of ADT, 6 months after the start of treatment, and within 6 months before the development of castration resistance. Samples at study visits of ESTO2 trial, which compares atorvastatin to placebo in combination with ADT. The trial intervention was blinded at analysis. Cox regression modeling was used to assess whether sTK1 values predict the risk of castration resistance. sTK1 was analyzed both as continuous and categorical variable, stratified by median value 0.12 ng/ml. The time from the initiation of ADT to the formation of castration resistance, death, or a general index date (May 31, 2023) was used as the time-metric.
Results
The study included serum samples from 70 patients, of which 23 had developed castration resistance by May 31,2023. The median follow-up time was 13 months. The sTK1 value measured before ADT initiation did not predict the risk of castration resistance (HR 0.91, 95% CI 0.62-1.35). However, sTK1 at 6 months after the initiation of ADT or up to a maximum of 6 months before formation of castration resistance did predict the risk (HR 1.5, 95% CI 1.07-2.1 and HR 1.79, 95% CI 1.16-2.76, respectively). sTK1 predicted the risk at both measurement points also when analyzed as categorical variable. sTK1 was predictive only among men with metastatic disease; sTK1 no longer predicted the risk of castration resistance who started ADT for biochemically recurrent, non-metastatic disease.
Conclusions
Our findings suggest that sTK1 value can be used to predict the risk of progression to castration resistance during ADT in metastatic prostate cancer. However, these findings need validation from larger datasets.
Citation Format: Aino Siltari, Paavo Raittinen, Jarno Riikonen, Juha Koskimäki, Tomi Pakarainen, Mikael Anttinen, Otto Ettala, Teuvo Tammela, Stig Linder, Teemu J. Murtola. Serum thymidine kinase 1 levels as a predictive marker for castration resistance in metastatic prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT233.