Therapeutic Drug Monitoring of Endoxifen for Tamoxifen Precision Dosing: Feasible in Patients with Hormone-Sensitive Breast Cancer.

Background: Endoxifen is the most important active metabolite of tamoxifen. Several retrospective studies have suggested a minimal or threshold endoxifen systemic concentration of 14-16 nM is required for a lower recurrence rate. The aim of this study was to investigate the feasibility of reaching a predefined endoxifen level of ≥ 16 nM (5.