The use of the 13C-dextromethorphan breath test for phenotyping CYP2D6 in breast cancer patients using tamoxifen: association with CYP2D6 genotype and serum endoxifen levels

Purpose Adjuvant therapy with tamoxifen significantly reduces breast cancer recurrence and mortality in estrogen receptor positive disease. CYP2D6 is the main enzyme involved in the activation of the prodrug tamoxifen into the anti-estrogen endoxifen. Endoxifen is thought to be a main determinant for clinical efficacy in breast cancer patients using tamoxifen. As the large interindividual variation in endoxifen levels is only partly explained by CYP2D6 genotype, we explored the use of the 13 C-dextromethorphan breath test (DM-BT) for phenotyping CYP2D6 and to predict serum steady-state endoxifen levels as a marker for clinical outcome in breast cancer patients using tamoxifen. Methods In 65 patients with early breast cancer using tamoxifen, CYP2D6 phenotype was assessed by DM-BT. CYP2D6 genotype using Amplichip and serum steady-state levels of endoxifen were determined. Genotype was translated into the gene activity score and into ultrarapid, extensive, heterozygous extensive, intermediate or poor metabolizer CYP2D6 predicted phenotype. Results CYP2D6 phenotype determined by the DM-BT explained variation in serum steady-state endoxifen levels for 47.5 % ( R 2 = 0.475, p