Factors affecting inter-individual variability in endoxifen concentrations in patients with breast cancer: results from the prospective TOTAM trial.
C Louwrens BraalJustin D WestenbergSanne M BuijsSteven AbramsTessa A M MulderRon H N van SchaikStijn L W KoolenAgnes JagerRon H J MathijssenPublished in: Breast cancer research and treatment (2022)
The inter-individual variability of endoxifen concentration could largely be explained by CYP2D6 genotype and for a small proportion by age and weight. The model showed a sensitivity and specificity of 66 and 98%, respectively, indicating a high probability of (misclassification) error for the patients with subtherapeutic endoxifen concentrations (< 16 nmol/L). The remaining unexplained inter-individual variability is still high and therefore model-informed tamoxifen dosing should be accompanied by therapeutic drug monitoring.