Abstract Tamoxifen’s pharmacokinetics is strongly influenced by the highly polymorphic CYP2D6 while the influence of other genetic variants has been inconclusive. To further delineate this genotypic-phenotypic impact, we conducted a multi-ancestry genome wide association study in 636 hormone-receptor positive (HR+) breast cancer (BC) patients treated with 20mg tamoxifen daily for 8 weeks and validated these genetic determinants in another 869 patients. Association with clinical outcomes was examined in 1326 non-metastatic HR+ patients receiving adjuvant tamoxifen. A genome-wide significant association with Z-endoxifen levels was observed at CYP2D6 locus on chromosome 22 and its downstream region of TCF20 rs932376 A>G. Both CYP2D6 metabolizer status and TCF20 rs932376 A>G were independent predictors of endoxifen levels in multivariable analysis. CYP2D6 metabolizer status accounted for greater variability of mean endoxifen levels compared to TCF20 rs932376 A>G (91.2% vs 48.8%). These findings were replicated in validation cohorts. Neither TCF20 rs932376 nor CYP2D6 metabolizer status was significantly associated with BC outcomes after adjustment for known prognostic factors. Our study confirmed that CYP2D6 metabolizer status remains as the prime predictor of steady-state Z endoxifen levels, while TCF20 rs932376 A>G has a smaller, independent effect. Both genetic factors were not associated with breast cancer clinical outcomes.
Khor et al. (2026) studied this question.