Impairment of endoxifen formation in tamoxifen-treated premenopausal breast cancer patients carrying reduced-function CYP2D6 alleles.

Significantly reduced endoxifen levels were observed not only in all homozygous carriers of CYP2D6 null-alleles, but also in carriers of 2 reduced-function alleles. This finding may be highly relevant for future, genotype-based dose considerations.