Application of Mice Humanised for Cytochrome P450 CYP2D6 to the Study of Tamoxifen Metabolism and Drug-Drug Interaction with Antidepressants

Tamoxifen is an oestrogen-receptor (ER) antagonist used in the treatment of breast cancer. It is a prodrug, converted by several P450 enzymes to a primary metabolite, N-desmethyltamoxifen (NDT), which is then further modified by CYP2D6 to a pharmacologically potent secondary metabolite, 4-hydroxy-N-desmethyltamoxifen (endoxifen). Anti-depressants (ADs), which are often co-prescribed to patients receiving tamoxifen, are also metabolised by CYP2D6 and evidence suggests that a drugdrug interaction (DDI) between these agents adversely affects the outcome of tamoxifen therapy by inhibiting endoxifen formation. Here, we have evaluated this potentially important DDI in vivo, in mice humanised for CYP2D6 (hCYP2D6). The rate of conversion of NDT to endoxifen by hCYP2D6 mouse liver microsomes (MLM) in vitro was similar to that of the most active members of a panel of 13 individual human liver microsomes (HLM). Co-incubation with the CYP2D6 inhibitor, quinidine, ablated endoxifen generation by hCYP2D6 MLM. The NDT-hydroxylation activity of wild-type MLM was 7.4 times higher than that of hCYP2D6, while MLM from Cyp2d knockout animals were inactive. Hydroxylation of NDT correlated with that of bufuralol, a CYP2D6 probe substrate, in the HLM panel. In vitro, ADs of the selective serotonin reuptake inhibitor (SSRI) class were, by an order of magnitude, more potent inhibitors of NDT hydroxylation by hCYP2D6 MLM than were compounds of the tricyclic class. At a clinically-relevant dose, paroxetine pre-treatment inhibited the generation of endoxifen from NDT in hCYP2D6 mice in vivo. These data demonstrate the potential of ADs to affect endoxifen generation and, thereby, the outcome of tamoxifen therapy.