Tamoxifen pharmacokinetics and molecular data

Pharmacokinetics
Clinical pharmacology Absorption and distribution Metabolism Excretion Special populations

Clinical pharmacology
Tamoxifen citrate is a nonsteroidal agent that has demonstrated potent antiestrogenic properties in animal test systems. The antiestrogenic effects may be related to its ability to compete with estrogen for binding sites in target tissues such as breast. Tamoxifen inhibits the induction of rat mammary carcinoma induced by dimethylbenzanthracene (DMBA) and causes the regression of already established DMBA-induced tumors. In this rat model, Tamoxifen appears to exert its antitumor effects by binding the estrogen receptors. Return to top

Absorption and distribution
Following a single oral dose of 20 mg Tamoxifen, an average peak plasma concentration of 40 ng/mL (range 35 to 45 ng/mL) occurred approximately 5 hours after dosing. The decline in plasma concentrations of Tamoxifen is biphasic with a terminal elimination half-life of about 5 to 7 days. The average peak plasma concentration of N-desmethyl Tamoxifen is 15 ng/mL (range 10 to 20 ng/mL). Chronic administration of 10 mg Tamoxifen given twice daily for 3 months to patients results in average steady-state plasma concentrations of 120 ng/mL (range 67-183 ng/mL) for Tamoxifen and 336 ng/mL (range 148-654 ng/mL) for N-desmethyl Tamoxifen. The average steady-state plasma concentrations of Tamoxifen and N-desmethyl Tamoxifen after administration of 20 mg Tamoxifen once daily for 3 months are 122 ng/mL (range 71-183 ng/mL) and 353 ng/mL (range 152-706 ng/mL), respectively. After initiation of therapy, steady-state concentrations for Tamoxifen are achieved in about 4 weeks and steady-state concentrations for N-desmethyl Tamoxifen are achieved in about 8 weeks, suggesting a half-life of approximately 14 days for this metabolite. In a steady-state, crossover study of 10 mg Tamoxifen citrate tablets given twice a day vs. a 20 mg Tamoxifen citrate tablet given once daily, the 20 mg Tamoxifen citrate tablet was bioequivalent to the 10 mg Tamoxifen citrate tablets. Return to top

Metabolism
Tamoxifen is extensively metabolized after oral administration. N-desmethyl Tamoxifen is the major metabolite found in patients' plasma. The biological activity of N-desmethyl Tamoxifen appears to be similar to that of Tamoxifen. 4-HydroxyTamoxifen and a side chain primary alcohol derivative of Tamoxifen have been identified as minor metabolites in plasma. Tamoxifen is a substrate of cytochrome P-450 3A, 2C9 and 2D6, and an inhibitor of P-glycoprotein. Return to top

Excretion
Studies in women receiving 20 mg of 14C Tamoxifen have shown that approximately 65% of the administered dose was excreted from the body over a period of 2 weeks with fecal excretion as the primary route of elimination. The drug is excreted mainly as polar conjugates, with unchanged drug and unconjugated metabolites accounting for less than 30% of the total fecal radioactivity. Return to top

Special populations
The effects of age, gender and race on the pharmacokinetics of Tamoxifen have not been determined. The effects of reduced liver function on the metabolism and pharmacokinetics of Tamoxifen have not been determined. Return to top