CYP2D6 Metabolizer Phenotype and Tamoxifen Endoxifen Plasma Concentrations in Breast Cancer: Clinical Impact on Recurrence-Free Survival in 2,184 Patients

This study investigates association between CYP2D6 metabolizer phenotype, steady-state endoxifen plasma concentration, and 5-year recurrence-free survival in ER+ breast cancer patients receiving adjuvant tamoxifen within the context of pharmacogenomics and oncology, an area of growing scientific importance given its implications for CYP2D6 pretreatment genotyping guidelines, endoxifen-guided tamoxifen dose adjustment, and extended adjuvant therapy decision support. Using prospective pharmacogenomic cohort study with CYP2D6 star-allele genotyping, mass spectrometry endoxifen quantification, and 5-year recurrence-free survival analysis by Cox regression, we examine CYP2D6 enzymatic activity converting tamoxifen to active metabolite endoxifen, with poor metabolizer phenotype reducing endoxifen concentrations below the 16 nM therapeutic threshold and impairing ER antagonism in 2,184 ER+ breast cancer patients (stage I-III) from 8 oncology centers genotyped for CYP2D6 and followed for 5 years after adjuvant tamoxifen initiation drawn from 8 NCI-designated cancer center outpatient oncology clinics with centralized CYP2D6 genotyping and mass spectrometry endoxifen assay. Results indicate that CYP2D6 poor metabolizers have 52.4% lower steady-state endoxifen (8.2 vs. 17.4 nM) and 28.4% lower 5-year RFS (62.4% vs. 84.2%) versus extensive metabolizers; endoxifen <16 nM threshold independently predicts recurrence (HR 1.84) (p < 0.001), with 28.4% lower RFS in poor metabolizers; HR 1.84 for endoxifen <16 nM as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to pharmacogenomics and oncology and carry actionable implications for the design of programs and policies targeting CYP2D6 pretreatment genotyping guidelines, endoxifen-guided tamoxifen dose adjustment, and extended adjuvant therapy decision support.