4-Hydroxytamoxifen (4-OHT), a first-generation selective estrogen receptor modulator (SERM), acts as an antagonist in breast cancer cells but exhibits estrogen-like effects in the uterus and bone. It is an active metabolite of the pro-drug tamoxifen (TAM), functioning as an estrogen receptor antagonist. 4-OHT binds to estrogen receptors to inhibit estradiol-induced cell proliferation without inducing cytotoxicity. It is a metabolite of the antiestrogen, tamoxifen and exhibits a higher affinity than tamoxifen and its other metabolites for binding to estrogen receptors, making it 50 to 100 times more potent in inhibiting cell proliferation in normal human breast cells and breast cancer cell lines in culture.
