Endoxifen plasma concentrations

Of these active metabolites, endoxifen is suggested to be the primary active metabolite responsible for the majority of tamoxifen clinical efficacy, as endoxifen plasma concentrations are about five to tenfold higher than those of 4-hydroxy-tamoxifen [11, 165], Endoxifen may have additional mechanisms of action than 4-hydroxy-tamoxifen by targeting Era for degradation by proteasome [166] and through the promotion of ERa/ERf) heterodimerization, blocking ERa transcriptional activity [167],... 

However, whether the monitoring of endoxifen plasma concentrations in breast cancer patients would constitute a valid approach to optimize individual dosage and improve treatment effectiveness remains to be demonstrated. So far, only one study... 

Whether the monitoring of endoxifen plasma concentrations in breast cancer patients would constitute a valid approach to optimize individual dosage and improve treatment efficacy is under scrutiny and remains to be demonstrated. In that purpose large prospective studies relating endoxifen plasma levels to clinical outcomes are as yet needed. In this perspective, it is critical to settle analytical and selectivity discrepancies between methods and laboratories and to ensure reproducible quantification results between laboratories. These concerted harmonization efforts can be carried out within the frame of an international external quality control program, which as yet, remains to be organized. 

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