Key clinical point: Serum thymidine kinase 1 activity (sTKa) proved to be an excellent biomarker of progression risk in patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2–) advanced breast cancer (BC) receiving first-line ribociclib+letrozole.
Major finding: Disease progression risk was significantly higher in patients with high vs low sTKa levels at baseline (hazard ratio 2.21; P = .0002). Patients with high sTKa levels on day 1 of cycle 2 after initial decrease on day 15 of cycle 1 (C1D15; hazard ratio 2.89; P = .0006) or on C1D15 (hazard ratio 5.65; P < .0001) had worse prognosis than those with low sTKa levels at all time points.
Study details: This phase 3 BioItaLEE study included 287 postmenopausal women with HR+/HER2– advanced BC who received ribociclib+letrozole as first-line therapy.
Disclosures: This study was supported by Novartis Farma SpA, Italy. Some authors declared participating on advisory boards and receiving grants, fees, honoraria, or travel support from several sources, including Novartis.
Source: Malorni L et al. Serum thymidine kinase activity in patients with HR-positive/HER2-negative advanced breast cancer treated with ribociclib plus letrozole: Results from the prospective BioItaLEE trial. Eur J Cancer. 2023;186:1-11 (Mar 7). Doi: 10.1016/j.ejca.2023.03.001