PYTHIA

Serum thymidine kinase activity in patients with hormone receptor-positive and HER2-negative metastatic breast cancer treated with palbociclib and fulvestrant
European Journal of Cancer, Volume 164, March 2022, Pages 39-51

Study type
Phase II, single-arm, multicenter trial (Italy, Belgium, UK) that enrolled 124 post-menopausal women with endocrine-resistant hormone receptor-positive/HER2-negative metastatic breast cancer.

Background
Biomarkers for cyclin-dependent kinase 4/6 inhibitors, such as palbociclib, for patients with hormone receptor-positive/HER2-negative metastatic breast cancer are lacking. Thymidine kinase is a proliferation marker downstream of the cyclin-dependent kinase 4/6 pathway.

Objective
We prospectively investigated the prognostic role of serum thymidine kinase activity (sTKa), in patients treated with Palbociclib + fulvestrant.

Results
Data from 122 women were analyzed. Pre-treatment sTKa was not associated with clinical characteristics and moderately correlated with tissue Ki-67. Palbociclib + fulvestrant markedly suppressed sTKa levels at D15, with 83% of patients recording levels below limit of detection. At D28, sTKa showed a rebound in 60% of patients. At each timepoint, higher sTKa was associated with shorter progression-free survival (each p < 0.001), with the strongest effect at D15.

Conclusion
TKa is an independent prognostic biomarker in patients treated with palbociclib. High pre-treatment sTKa and its incomplete suppression during treatment may identify patients with poorer prognosis and primary resistance. This warrants validation in prospective comparative trials.