Endoxifen plasma levels

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. 

Tamoxifen (CYP2D6) Paroxetine Greater reduction in plasma levels of endoxifen (active metabolite of tamoxifen formed via CYP2D6) in homozygous EM as compared to patients with at least one variant allele 77... 

Tamoxifen given orally reaches peak plasma levels after 4—7 hours it has two elimination phases (half-lives of 7—14 hours and 4—11 days thus, 3—4 weeks of treatment are required to reach steady-state plasma levels). Tamoxifen is metabolized by hepatic CYPs, some of which it also induces. In humans, the potent antiestrogens 4-hydroxytamoxifen and endoxifen are produced in the liver. The major route of elimination involves N-demethylation and deamination. The drug undergoes entero-hepatic circulation, and excretion is primarily in the feces as conjugates of the deaminated metabolite. 

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