Sensitive HPLC-PDA determination of tamoxifen and its metabolites N-desmethyltamoxifen, 4-hydroxytamoxifen and endoxifen in human plasma

► Tamoxifen is the most widely used drug in breast cancer treatment. ► Most of the biological activity of tamoxifen is due to the metabolite endoxifen, formed mainly by CYP2D6 hydroxilation of N-demethyltamoxifen. ► We developed the most sensitive HPLC-UV method for the determination of plasma concentrations of tamoxifen, endoxifen and other revelevant metabolites. ► The method was tested in 110 plasma samples from breast cancer patients under tamoxifen therapy and endoxifen was always above the limit of detection. ► Endoxifen can be measured with adequate accuracy and precision in clinical samples by the developed method.