Plasma thymidine kinase 1 activity and outcome of ER+ HER2- metastatic breast cancer patients treated with palbociclib and endocrine therapy.

Previous cohort studies have reported plasma TK1 activity (pTKa) as a potential prognostic biomarker in estrogen receptor-positive (ER+) HER2-negative (HER2-) metastatic breast cancer (MBC). In this prospective study, we report here the prognostic impact of pTKa in ER+/HER2- MBC patients treated with endocrine therapy and CDK4/6 inhibitor. Patients were included into the prospective, ethics committee-approved ALCINA study (NCT02866149). Eligibility criteria were patients with ER+/HER2- MBC treated at Institut Curie with endocrine therapy and palbociclib. Plasma samples were obtained at baseline and after 4 weeks of treatment. pTKa was quantified by the DiviTum® assay (Biovica, Sweden). From May 2016 to August 2018, 103 patients treated with endocrine therapy and palbociclib were included. Patients had received a median of two prior systemic therapies for MBC (range 0-14). Median follow-up was 13.8 months (range 6-31), with median PFS and OS of 9.6 months (95%CI [7.0-11.3]) and 28 months (95%CI [23-not reached]), respectively. Median baseline pTKa was 292 Du/L (range 20-27,312 Du/L, IQR [89-853]). After adjusting for other prognostic factors, baseline pTKa remained an independent prognostic factor for both PFS (HR = 1.3 95%CI [1.1-1.4], p = 0.0005) and OS (HR = 1.3 95%CI [1.2-1.6], p < 0.0001), and 4-week pTKa was associated with OS (HR = 1.6 95%CI [1.3-2], p < 0.0001). That survival prediction was significantly improved by the addition of baseline pTKa to clinicopathological characteristics. Adding pTKa changes at 4 weeks to baseline pTKa did not further increase survival prediction. This study demonstrates the clinical validity of pTKa as a new circulating prognostic marker in ER+/HER2- MBC patients treated with endocrine therapy and palbociclib.