Endoxifen levels and metabolic phenotype-associated factors in Mexican Mestizo patients under tamoxifen treatment.

Aim: To evaluate plasma endoxifen levels and metabolic phenotype-associated factors in Mexican Mestizo patients under tamoxifen treatment. Patients & methods: A total of 138 breast cancer patients under tamoxifen treatment were cross-sectionally evaluated and side effects (SE) were recorded. CYP2D6 genetic phenotypes (GP) and metabolic phenotypes (MP) were assessed (metabolic poor [mPM], intermediate [mIM], normal [mNM], and ultrarapid [mUM] metabolizer). Associations were tested in uni-multivariate models for endoxifen >5.9 ng/ml and for mNM + mUM MP. Results: The main SE was hot flashes (62%). Distribution of the CYP2D6 MP was 4.3% mPM; 14.5% mIM; 75.4% mNM; and 5.8% mUM. Endoxifen >5.9 ng/ml was partially associated with SE (p = 0.06); the mNM + mUM MP was associated with treatment time (p = 0.03). Conclusion: The endoxifen-associated factors in Mexican Mestizo patients remain inconclusive, although treatment time was associated with MP.