Fulvestrant is a drug treatment of hormone receptor-positive metastatic breast cancer in post-menopausal women with disease progression following anti-estrogen therapy. It is an estrogen receptor antagonist with no agonist effects, which works both by down-regulating and by degrading the estrogen receptor. Fulvestrant competitively and reversibly binds to estrogen receptors present in cancer cells and achieves its anti-estrogen effects through two separate mechanisms. First, fulvestrant binds to the receptors and downregulates them so that estrogen is no longer able to bind to these receptors. Second, fulvestrant degrades the estrogen receptors to which it is bound. Both of these mechanisms inhibit the growth of tamoxifen-resistant as well as estrogen-sensitive human breast cancer cell lines. Fulvestrant is used for the treatment of hormone receptor positive metastatic breast cancer in postmenopausal women with disease progression following anti-estrogen therapy. Fulvestrant is marketed under the trade name Faslodex, by AstraZeneca.

Tamoxifen (brand name Nolvadex), is selective estrogen receptor modulators (SERM) with tissue-specific activities for the treatment and prevention of estrogen receptor positive breast cancer. Tamoxifen itself is a prodrug, having relatively little affinity for its target protein, the estrogen receptor (ER). It is metabolized in the liver by the cytochrome P450 isoform CYP2D6 and CYP3A4 into active metabolites such as 4-hydroxytamoxifen (4-OHT) (afimoxifene) and N-desmethyl-4-hydroxytamoxifen (endoxifen) which have 30–100 times more affinity with the ER than tamoxifen itself. These active metabolites compete with estrogen in the body for binding to the ER. In breast tissue, 4-OHT acts as an ER antagonist so that transcription of estrogen-responsive genes is inhibited. Tamoxifen has 7% and 6% of the affinity of estradiol for the ERα and ERβ, respectively, whereas 4-OHT has 178% and 338% of the affinity of estradiol for the ERα and ERβ. The prolonged binding of tamoxifen to the nuclear chromatin of these results in reduced DNA polymerase activity, impaired thymidine utilization, blockade of estradiol uptake, and decreased estrogen response. It is likely that tamoxifen interacts with other coactivators or corepressors in the tissue and binds with different estrogen receptors, ER-alpha or ER-beta, producing both estrogenic and antiestrogenic effects. Tamoxifen is currently used for the treatment of both early and advanced estrogen receptor (ER)-positive (ER+) breast cancer in pre- and post-menopausal women. Additionally, it is the most common hormone treatment for male breast cancer. Patients with variant forms of the gene CYP2D6 (also called simply 2D6) may not receive full benefit from tamoxifen because of too slow metabolism of the tamoxifen prodrug into its active metabolites. Tamoxifen is used as a research tool to trigger tissue-specific gene expression in many conditional expression constructs in genetically modified animals including a version of the Cre-Lox recombination technique. Tamoxifen has been shown to be effective in the treatment of mania in patients with bipolar disorder by blocking protein kinase C (PKC), an enzyme that regulates neuron activity in the brain. Researchers believe PKC is over-active during the mania in bipolar patients.

ARN-810 (GDC-0810) is a novel, orally bioavailable, estrogen receptor antagonist that induces proteasomal estrogen receptor degradation in breast cancer cell lines at picomolar concentrations and tumor regression in tamoxifen-sensitive and resistant BC xenograft models. Results from a first-in-human phase I/IIa study of ARN-810 indicate that it is tolerable and may benefit some postmenopausal women with advanced estrogen receptor-positive breast cancer. Development of ARN-810 was discontinued.

Panomifene (also known as GYKI-13504 or EGIS-5660), a triphenyl-alkene derivative that was studied as an antiestrogen. This drug binds to specific estrogen receptors and exhibits inhibitory effects on experimental mammary tumors both in vitro and in vivo. Panomifene reached phase II clinical trials for the treatment of breast cancer before development was terminated.

Pipendoxifene (ERA-923) is a selective estrogen receptor modulator with a distinct profile compared with tamoxifen. In particular, unlike tamoxifen, ERA-923 is devoid of uterotropic activity and does not stimulate the growth of endometrial tumors in the EnCa-101 (human endometrial adenocarcinoma that is continuously passaged in animals in the presence of estrogen) experimental mice model. These data may indicate that, in patients, ERA-923 will not increase the incidence of endometrial hyperplasia or cancer. ERA-923 inhibits estrogen-stimulated ER-alpha-dependent tumor growth with equal effects compared with tamoxifen in models sensitive to tamoxifen. ERA-923 partially or completely overcomes tamoxifen resistance. In vivo combination of temsirolimus and ERA-923 at certain doses and schedules completely inhibited breast carcinoma growth, while individual agents were only partially effective. Pipendoxifene had been in phase II clinical trial for the treatment of refractory metastatic breast cancer. However, this research has been discontinued.