An UPLC–MS/MS method for separation and accurate quantification of tamoxifen and its metabolites isomers

Abstract A selective and accurate analytical method is needed to quantify tamoxifen and its phase I metabolites in a prospective clinical protocol, for evaluation of pharmacokinetic parameters of tamoxifen and its metabolites in adjuvant treatment of breast cancer. The selectivity of the analytical method is a fundamental criteria to allow the quantification of the main active metabolites (Z)-isomers from (Z)′-isomers. An UPLC–MS/MS method was developed and validated for the quantification of (Z)-tamoxifen, (Z)-endoxifen, (E)-endoxifen, Z′-endoxifen, (Z)′-endoxifen, (Z)-4-hydroxytamoxifen, (Z)-4′-hydroxytamoxifen, N-desmethyl tamoxifen, and tamoxifen-N-oxide. The validation range was set between 0.5 ng/mL and 125 ng/mL for 4-hydroxytamoxifen and endoxifen isomers, and between 12.5 ng/mL and 300 ng/mL for tamoxifen, tamoxifen N-desmethyl and tamoxifen-N-oxide. The application to patient plasma samples was performed.