Increasing tamoxifen dose in breast cancer patients based on CYP2D6 genotypes and endoxifen levels: effect on active metabolite isomers and the antiestrogenic activity score.

Abstract excerpt

Tamoxifen (Tam), the major drug for estrogen receptor (ER)-positive breast cancer, is converted to its active metabolites, Z- and Z'-endoxifen and 4-OH-Tam isomers, primarily by cytochrome P450 CYP2D6. In 117 patients taking 20 mg/day of Tam, we determined CYP2D6 genotypes and measured the plasma levels of Tam metabolites. The Z-endoxifen levels increased while Z'-endoxifen levels decreased with increasing...

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