Molecular mechanism of action
Tamoxifen competitively binds to GPCR type and intracellular type estrogen receptors (ER). Binding to the intracellular ER on tumours and other tissue targets produces a nuclear complex that decreases DNA synthesis and inhibits estrogen effects. It thus acts as an estrogen antagonists which inhibits the growth of some tumours dependent on estrogen stimulation.
Tamoxifen itself is a prodrug, having relatively little affinity for the estrogen receptor. It is metabolised in the liver by the cytochrome P450 to active metabolites such as 4-hydroxytamoxifen and N-desmethyl-4-hydroxytamoxifen.