P-Glycoprotein (ABCB1) Transports the Primary Active Tamoxifen Metabolites Endoxifen and 4-Hydroxytamoxifen and Restricts Their Brain Penetration

P-glycoprotein (P-gp, ABCB1) is a highly efficient drug efflux pump expressed in brain, liver, and small intestine, but also in tumor cells, that affects pharmacokinetics and confers therapy resistance for many anticancer drugs. The aim of this study was to investigate the impact of P-gp on tamoxifen and its primary active metabolites, 4-hydroxytamoxifen, N-desmethyltamoxifen, and endoxifen. We used in vitro transport assays and Abcb1a/1b(-/-) mice to investigate the impact of P-gp on the oral availability and brain penetration of tamoxifen and its metabolites. Systemic exposure of tamoxifen and its metabolites after oral administration of tamoxifen (50 mg/kg) was not changed in the absence of P-gp. However, brain accumulation of tamoxifen, 4-hydroxytamoxifen, and N-desmethyltamoxifen were modestly, but significantly (1.5- to 2-fold), increased. Endoxifen, however, displayed a 9-fold higher brain penetration at 4 h after administration. Endoxifen was transported by P-gp in vitro. Upon direct oral administration of endoxifen (20 mg/kg), systemic exposure was slightly decreased in Abcb1a/1b(-/-) mice, but brain accumulation of endoxifen was dramatically increased (up to 23-fold at 4 h after administration). Shortly after high-dose intravenous administration (5 or 20 mg/kg), endoxifen brain accumulation was increased only 2-fold in Abcb1a/1b(-/-) mice compared with wild-type mice, suggesting a partial saturation of P-gp at the blood-brain barrier. Endoxifen, the clinically most relevant metabolite of tamoxifen, is a P-gp substrate in vitro and in vivo, where P-gp limits its brain penetration. P-gp might thus be relevant for tamoxifen/endoxifen resistance of P-gp-positive breast cancer and tumors positioned behind a functional blood-brain barrier.