Description_x000D_ Biochem/physiol Actions_x000D_ (Z)-Endoxifen (endoxifen) is an active tamoxifen metabolite generated via actions of cytochrome P450 (CYP) enzymes CYP3A4/5 and CYP2D6. Endoxifen is more potent than tamoxifen as a selective estrogen receptor modulator (SERM) both in vitro and in vivo with good pharmacokinetics and oral availability (∼80% MCF-7 tumor growth inhibition with 4-8 mg/kg/day endoxifen or 20 mg/kg/day tamoxifen in mice via p.o.). Endoxifen also exhibits 4-fold higher PKC inhibitory potency than tamoxifen and can overcome tamoxifen resistance due to cytochrome CYP2D6 polymorphism.