According to 2 large breast cancer trials, CYP2D6 genotyping was not predictive of the effectiveness of tamoxifen in postmenopausal women. Thus, the results of these studies are not generalizable to premenopausal women.
CYP2D6 genotyping has been a focus of research interest, but studies have been inconclusive as to the value of testing. In theory, certain CYP2D6 genotypes and phenotypes would be associated with breast cancer outcomes on treatment with tamoxifen; that is, tamoxifen would be less effective inpoor and intermediate metabolizers of endoxifen (a metabolite of tamoxifen) and more effective in extensive metabolizers of endoxifen.
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