Plasma Thymidine Kinase Activity as a Biomarker in Patients with Luminal Metastatic Breast Cancer Treated with Palbociclib within the TREnd Trial

Clin Cancer Res. 2020 May 1;26(9):2131-2139. doi: 10.1158/1078-0432.CCR-19-3271. Epub 2020 Jan 14.


Purpose: Thymidine kinase 1 (TK1) is downstream to the CDK4/6 pathway, and TK activity (TKa) measured in blood is a dynamic marker of outcome in patients with advanced breast cancer (ABC). This study explores TK1 as a biomarker of palbociclib response, both in vitro and in patients with ABC.

Experimental design: Modulation of TK1 levels and activity by palbociclib were studied in seven estrogen receptor-positive breast cancer cell lines: sensitive (PDS) and with palbociclib acquired resistance (PDR). TKa was assayed in plasma obtained at baseline (T0), after one cycle (T1), and at disease progression on palbociclib (T2) in patients enrolled in the "To Reverse ENDocrine Resistance" (TREnd) trial (n = 46).

Results: Among E2F-dependent genes, TK1 was significantly downregulated after short-term palbociclib. Early TKa reduction by palbociclib occurred in PDS but not in PDR cells. In patients, median TKa (mTKa) at T0 was 75 DiviTum units per liter (Du/L), with baseline TKa not proving prognostic. At T1, mTKa decreased to 35 Du/L, with a minority of patients (n = 8) showing an increase-correlating with a worse outcome than those with decreased/stable TKa (n = 33; mPFS 3.0 vs 9.0 months; P = 0.002). At T2, mTKa was 251 Du/L; patients with TKa above the median had worse outcomes on post-study treatment compared with those with lower TKa (2.9 vs 8.7 months; P = 0.05).

Conclusions: TK is a dynamic marker of resistance to palbociclib which may lead to early identification of patients in whom treatment escalation may be feasible. In addition, TKa may stratify prognosis in patients with acquired resistance to palbociclib.

Publication types

  • Clinical Trial, Phase II
  • Multicenter Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Biomarkers, Tumor / blood
  • Breast Neoplasms / blood
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / enzymology*
  • Breast Neoplasms / pathology
  • Cell Line, Tumor
  • Cyclin-Dependent Kinase 4 / antagonists & inhibitors
  • Cyclin-Dependent Kinase 4 / blood
  • Cyclin-Dependent Kinase 6 / antagonists & inhibitors
  • Cyclin-Dependent Kinase 6 / blood
  • Drug Resistance, Neoplasm
  • Female
  • Humans
  • Middle Aged
  • Neoplasm Metastasis
  • Piperazines / therapeutic use*
  • Protein Kinase Inhibitors / therapeutic use
  • Pyridines / therapeutic use*
  • Receptors, Estrogen / metabolism
  • Survival Rate
  • Thymidine Kinase / blood
  • Treatment Switching


  • Biomarkers, Tumor
  • Piperazines
  • Protein Kinase Inhibitors
  • Pyridines
  • Receptors, Estrogen
  • Thymidine Kinase
  • thymidine kinase 1
  • CDK4 protein, human
  • CDK6 protein, human
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinase 6
  • palbociclib