Figure 1.

Endoxifen concentrations at baseline and 4 months by CYP2D6 phenotype group. At baseline, with all patients receiving 20 mg/day of tamoxifen, there were significant differences in endoxifen concentrations in EM patients compared with IM or PM patients (p < .001). After 4 months of dose escalation to 40 mg/day in the IM and PM patients, only the IM patients had endoxifen concentrations similar to EM patients (p = .08), whereas the PM patients continued to have lower endoxifen concentrations (p = .01), recapitulating the findings reported in the analysis of the first 120 patients enrolled on this trial [18].

Abbreviations: EM, extensive metabolizer; IM, intermediate metabolizer; PM, poor metabolizer; UM, ultra-rapid metabolizer.