Fast and Adequate Liquid Chromatography-Tandem Mass Spectrometric Determination of Z-endoxifen Serum Levels for Therapeutic Drug Monitoring

摘要

Background: Z-endoxifen (further referred to as endoxifen, unless stated otherwise) is proposed as the most important metabolite of tamoxifen. Patients receiving adjuvant tamoxifen treatment with endoxifen levels below the threshold of 5.9 ng/mL may have an increased risk of breast cancer recurrence. Several factors, such as genetic polymorphisms, drug interactions, and (non) adherence, lead to large interpatient variability in endoxifen exposure, resulting in a substantial number of patients showing subtherapeutic levels. As genotyping and phenotyping are not able to adequately predict endoxifen exposure, therapeutic drug monitoring (TDM) seems to be the best approach for tailored tamoxifen therapy.