The effect of rs5758550 on CYP2D6☆2 phenotype and formation of endoxifen in breast cancer patients using tamoxifen

Aim: CYP2D6(star)2 is considered fully active, but it has been suggested that it only happens in the presence of rs5758550. This study aims to elucidate the impact of this enhancer. Materials & methods: DNA and blood samples from women enrolled in the CYPTAM study (NTR1509) were analyzed. Fourteen CYP2D6(star)2 carriers without the enhancer were reclassified. The relationship of CYP2D6 phenotypes and drug levels was studied. Results: After correction for the absence of the enhancer, the correlation between CYP2D6 phenotypes and endoxifen did not improve (R-2: 0.290 vs 0.279). No difference was observed in mean concentrations between CYP2D6(star)2 individuals with and without the enhancer. Conclusion: The rs5758550 enhancer does not lead to improved prediction of endoxifen levels in breast cancer patients.