a dosage range of 1 to 20 mg/day in women with breast cancer. Tamoxifen has been found to decrease insulin-like growth factor 1 (IGF-1) levels by 17 to 38% in women and men. Suppression of IGF-1 production in the liver is a well-known action of estrogens and SERMs. A 10 mg/day dosage of tamoxifen is nearly as effective as a 20 mg/day dosage in suppressing IGF-1 levels. Other activities Afimoxifene is an agonist of the G protein-coupled estrogen receptor (GPER) with relatively low affinity. Its affinity for the receptor is in the range of 100 to 1,000 nM, relative to 3 to 6 nM for estradiol. In addition to its activity as a SERM, afimoxifene binds to both the estrogen-related receptor β and estrogen-related receptor γ and is an antagonist of the estrogen-related receptor γ (ERRγ). Norendoxifen (4-hydroxy-N,N-didesmethyltamoxifen), another active metabolite of tamoxifen, has been found to act as a potent competitive aromatase inhibitor (IC50 = 90 nM), and may also be involved in the antiestrogenic activity of tamoxifen. In addition to its activity as a SERM, tamoxifen is a potent and selective protein kinase C inhibitor, and is active in this regard at therapeutic concentrations. This action is thought to underlie the efficacy of tamoxifen in the treatment of bipolar disorder. Tamoxifen is an inhibitor of P-glycoprotein. Pharmacokinetics Absorption Tamoxifen is rapidly and extensively absorbed from the intestines with oral administration. The oral bioavailability of tamoxifen is approximately 100%, which is suggestive of minimal first-pass metabolism in the intestines and liver. Following intake, peak levels of tamoxifen occur after three to seven hours. Steady state levels of tamoxifen are reached typically after 3 to 4 weeks but possibly up to 16 weeks of daily administration. Steady state levels of afimoxifene are achieved after 8 weeks of daily tamoxifen administration. Peak levels of tamoxifen after a single 40 mg oral dose were 65 ng/mL and steady state levels at 20 mg/day were 310 ng/mL. Levels of tamoxifen show clear dose dependency across a dosage range of 1 to 20 mg/day. Endoxifen levels are approximately 5 to 10 times higher than afimoxifene levels, with large interindividual variability. Endoxifen levels have been reported as 10.8 to 15.9 ng/mL at steady state in CYP2D6 normal metabolizers during therapy with 20 mg/day tamoxifen. The most abundant metabolites of tamoxifen in terms of circulating concentrations are N-desmethyltamoxifen, N,N-didesmethyltamoxifen, (Z)-endoxifen, and tamoxifen N-oxide. Distribution The volume