Tamoxifen - How It Works
Clinical pharmacology details from the US FDA-approved label: how Tamoxifen works in your body, how it's absorbed, how long it stays active, and how it's eliminated.
Mechanism of Action
12.1 Mechanism of Action Tamoxifen is an estrogen agonist/antagonist. Tamoxifen competes with estrogen for binding to the estrogen receptor, which can result in a decrease in estrogen receptor signaling-dependent growth in breast tissue. Tamoxifen has demonstrated antitumor activity against human breast cancer cell lines xenografted in mice. The drug has been shown to inhibit the induction of rat mammary carcinoma induced by dimethylbenzanthracene (DMBA) and to cause the regression of already established DMBA-induced tumors.
12 CLINICAL PHARMACOLOGY 12.1 Mechanism of Action Tamoxifen is an estrogen agonist/antagonist.
Tamoxifen competes with estrogen for binding to the estrogen receptor, which can result in a decrease in estrogen receptor signaling-dependent growth in breast tissue.
Tamoxifen has demonstrated antitumor activity against human breast cancer cell lines xenografted in mice.
The drug has been shown to inhibit the induction of rat mammary carcinoma induced by dimethylbenzanthracene (DMBA) and to cause the regression of already established DMBA-induced tumors.
12.3 Pharmacokinetics 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, the major metabolite, 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 to 183 ng/mL) for tamoxifen and 336 ng/mL (range 148 to 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 to 183 ng/mL) and 353 ng/mL (range 152 to 706 ng/mL), respectively.
The steady-state plasma concentrations of endoxifen and 4-hydroxytamoxifen are 29.1 (95% CI 24.6 to 33.6) and 3.7 (95% CI 3.3 to 4.1) 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 tablets given twice a day vs. a 20 mg tamoxifen tablet given once daily, the 20 mg tamoxifen tablet was bioequivalent to the 10 mg tamoxifen tablets.
A pharmacokinetic study was performed in healthy perimenopausal and postmenopausal female subjects to evaluate the bioavailability of SOLTAMOX (n=30) in comparison with the commercially available tamoxifen citrate tablets (n=33) under fasting conditions.
A third arm evaluated the effect of food on SOLTAMOX (n=16).
The rate and extent of absorption of SOLTAMOX was found to be bioequivalent to that of tamoxifen citrate tablets under fasting conditions.
There was no difference in bioavailability (C max and AUC) of SOLTAMOX oral solution between fed and fasting states, and therefore SOLTAMOX can be given without regard to meals.
Metabolism Tamoxifen is extensively metabolized by CYP450 enzymes, including CYP3A, CYP2D6, CYP2C9, CYP2C19, and CYP2B6.
N-desmethyltamoxifen, formed predominantly by CYP3A, is the major metabolite found in plasma.
The pharmacological activity of N-desmethyltamoxifen is similar to that of tamoxifen.
Endoxifen and 4-hydroxytamoxifen, identified as minor metabolites, have 100-fold greater affinity for the estrogen receptor and 30 to 100-fold greater potency in suppressing estrogen-dependent cell proliferation than tamoxifen.
Pharmacokinetics
12.3 Pharmacokinetics 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, the major metabolite, 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 to 183 ng/mL) for tamoxifen and 336 ng/mL (range 148 to 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 to 183 ng/mL) and 353 ng/mL (range 152 to 706 ng/mL), respectively. The steady-state plasma concentrations of endoxifen and 4-hydroxytamoxifen are 29.1 (95% CI 24.6 to 33.6) and 3.7 (95% CI 3.3 to 4.1) 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 tablets given twice a day vs. a 20 mg tamoxifen tablet given once daily,