New UPLC–MS/MS Assay for the Determination of Tamoxifen and Its Metabolites in Human Plasma, Application to Patients

Aim: A rapid UPLC–MS/MS method for the determination of tamoxifen (TAM), N -desmethyltamoxifen, 4-hydroxytamoxifen and endoxifen in human plasma was validated, after a simple protein precipitation. Materials and methods: The analysis was achieved on a C 18 analytical column, using a gradient elution with a mobile phase of water and acetonitrile for 4.5 min. Results: The validated method demonstrated good linearity between 1 and 500 ng/ml for TAM and N -desmethyltamoxifen; between 0.2 and 100 ng/ml for endoxifen and between 0.1 and 50 ng/ml for 4-hydroxytamoxifen. The method also provided satisfactory results in terms of within day and between day imprecisions and accuracy, and also in terms of time stability and specificity. Conclusion: The method is applied routinely for TAM monitoring from patients undergoing therapy.