Prediction of individual tamoxifen activation in breast cancer patients.

e13514 Background: Endoxifen (EN), the main active metabolite of tamoxifen (T), reaches steady state (SS) plasma levels after about 4 months of administration. It derives principally by cytochrome CYP2D6 activity, which is highly variable among patients. In order to predict individual SS EN exposure, three different approaches were investigated. Methods: Before starting T administration, 73 breast cancer patients (median age: 59 yrs, range: 30-89) were characterized by means of: 1) a phenotyping test of CYP2D6 activity, based on the urinary metabolic ratio of dextromethorphan/dextrorphan (log transformed, LMR). 2) CYP2D6 genotyping (alleles *1, *3, *4, *5, *6, *9, *10, *41), to classify patients in 3 functional groups: Extensive (EM), Intermediate (IM) and Poor (PM) Metabolizers. After starting T treatment (20mg/day), EN plasma levels were measured after 1 month (1M EN, pre- steady state) and 4 months of therapy (SS EN). ANOVA, paired and unpaired T test and linear regressions were performed to analyze as...