of distribution of tamoxifen is 50 to 60 L/kg and its clearance has been estimated as 1.2 to 5.1 L/hour. High concentrations of tamoxifen have been found in breast, uterus, liver, kidney, lung, pancreas, and ovary tissue in animals and humans. Levels of tamoxifen in the uterus have been found to be 2- to 3-fold higher than in the circulation and in the breasts 10-fold higher than in the circulation. The plasma protein binding of tamoxifen and afimoxifene is greater than 99%. A majority of tamoxifen is bound to albumin. Albumin alone binds 98.8% of tamoxifen while other plasma proteins are not greatly involved. Metabolism Tamoxifen is a prodrug and is metabolized in the liver by the cytochrome P450 isoforms CYP3A4, CYP2C9, and CYP2D6 into active metabolites such as endoxifen (4-hydroxy-N-desmethyltamoxifen) and afimoxifene (4-hydroxytamoxifen). Conversion of tamoxifen by N-demethylation into N-desmethyltamoxifen, which is catalyzed primarily by CYP3A4 and CYP3A5, is responsible for approximately 92% of tamoxifen metabolism. Conversely, 4-hydroxylation of tamoxifen into afimoxifene is responsible for only about 7% of tamoxifen metabolism. Following its formation, N-desmethyltamoxifen is oxidized into several other metabolites, the most notable of which is endoxifen. Another active metabolite, norendoxifen (4-hydroxy-N,N-didesmethyltamoxifen), is formed via N-demethylation of endoxifen or 4-hydroxylation of N,N-didesmethyltamoxifen. Tamoxifen and its metabolites undergo conjugation, including glucuronidation and sulfation. Tamoxifen may inhibit its own metabolism. Elimination Tamoxifen has a long elimination half-life of typically 5 to 7 days, with a range of 4 to 11 days. Similarly, the half-life of afimoxifene is 14 days. Conversely, the half-life of endoxifen is 50 to 70 hours (2–3 days). The long half-lives of tamoxifen and afimoxifene are attributed to their high plasma protein binding as well as to enterohepatic recirculation. Upon discontinuation of treatment, levels of tamoxifen and its metabolites persist in the circulation for at least 6 weeks. Tamoxifen is excreted in bile and is eliminated in feces, while small amounts are eliminated in urine. Chemistry Tamoxifen is a nonsteroidal SERM of the triphenylethylene family and was structurally derived from diethylstilbestrol-like estrogens and antiestrogens such as chlorotrianisene and ethamoxytriphetol. Initially, clomifene was synthesized, and tamoxifen was developed subsequently. Tamoxifen is closely related structurally to other triphenylethylenes, such as clomifene, nafoxidine, ospemifene, toremifene, and numerous others. Other SERMs, like raloxifene, are structurally distinct from tamoxifen and other triphenylethylenes. History In the late 1950s, pharmaceutical companies were actively researching a newly discovered class of anti-estrogen compounds in the hope