In view of future pharmacokinetic studies, a highly sensitive ultra performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) method has been developed for the simultaneous quantification of tamoxifen and three of its main phase I metabolites in human lithium heparinized plasma. The analytical method has been thoroughly validated in agreement with FDA recommendations. Plasma samples of 200μl were purified by liquid-liquid extraction with 1ml n-hexane/isopropanol, after deproteination through addition of 50μl acetone and 50μl deuterated internal standards in acetonitrile. Tamoxifen, N-desmethyl-tamoxifen, 4-hydroxy-tamoxifen and endoxifen were chromatographically separated on an Acquity UPLC®BEH C18 1.7μm 2.1mm×100mm column eluted at a flow-rate of 0.300ml/min on a gradient of 0.2mM ammonium formate and acetonitrile, both acidified with 0.1% formic acid. The overall run time of the method was 10min, with elution times of 2.9, 3.0, 4.1 and 4.2min for endoxifen, 4-hydroxy-tamoxifen, N-desmethyl-tamoxifen and tamoxifen, respectively. Tamoxifen and its metabolites were quantified by triple-quadrupole mass spectrometry in the positive ion electrospray ionization mode. The multiple reaction monitoring transitions were set at 372>72 (m/z) for tamoxifen, 358>58 (m/z) for N-desmethyl-tamoxifen, 388>72 (m/z) for 4-hydroxy-tamoxifen and 374>58 (m/z) for endoxifen. The analytical method was highly sensitive with the lower limit of quantification validated at 5.00nM for tamoxifen and N-desmethyl-tamoxifen and 0.500nM for 4-hydroxy-tamoxifen and endoxifen, which is equivalent to 1.86, 1.78, 0.194 and 0.187ng/ml for tamoxifen, N-desmethyl-tamoxifen, 4-hydroxy-tamoxifen and endoxifen, respectively. The method was also precise and accurate, with within-run and between-run precisions within 12.0% and accuracy ranging from 89.5 to 105.3%. The method has been applied to samples from a clinical study and cross-validated with a validated LC-MS/MS method in serum.

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Keywords 4-Hydroxy-tamoxifen, Endoxifen, LC-MS/MS, N-desmethyl-tamoxifen, Tamoxifen
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Binkhorst, L, Mathijssen, A.H.J, Ghobadi Moghadam-Helmantel, I.M, de Bruijn, P, van Gelder, T, Wiemer, E.A.C, & Loos, W.J. (2011). Quantification of tamoxifen and three of its phase-I metabolites in human plasma by liquid chromatography/triple-quadrupole mass spectrometry. Journal of Pharmaceutical and Biomedical Analysis, 56(5), 1016–1023. doi:10.1016/j.jpba.2011.08.002