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.
To support TDM services, a rapid and sensitive high-performance liquid chromatography-tandem mass spectrometry assay for the quantification of endoxifen in human serum was developed and validated. Validation was performed according to the latest US FDA and EMA guidelines on bioanalytical method validation.
All validation parameters fulfilled their acceptance criteria, and the developed assay is now successfully being used to support TDM services. Thus far, 32.7% of the more than 500 determined endoxifen serum levels were below the threshold of 5.9 ng/mL.
The successfully validated serum assay quantifies endoxifen with a linear regression calibration model (weighted 1/x) in the concentration range from 1.00 to 25.0 ng/mL. The assay was validated with an inaccuracy of ±7.7% and an imprecision of ≤3.9%, obtained with an IS normalized matrix factor of 0.925 and a signal-to-noise ratio of >66.
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