Some breast cancer cells require estrogen to grow. Estrogen binds to and activates the estrogen receptor in these cells. Tamoxifen is metabolized into compounds that also bind to the estrogen receptor but do not activate it. Because of this competitive antagonism, tamoxifen acts like a key broken off in the lock that prevents any other key from being inserted, preventing estrogen from binding to its receptor. Hence breast cancer cell growth is blocked.

Tamoxifen was discovered by pharmaceutical company Imperial Chemical Industries (now AstraZeneca) and is sold under the trade names Nolvadex, Istubal, and Valodex. However, the drug, even before its patent expiration, was and still is widely referred to by its generic name "tamoxifen."

Tamoxifen is currently used for the treatment of both early and advanced ER+ (estrogen receptor positive) breast cancer in pre- and post-menopausal women.^[3] Additionally, it is the most common hormone treatment for male breast cancer.^[4] It is also approved by the FDA for the prevention of breast cancer in women at high risk of developing the disease.^[5] It has been further approved for the reduction of contralateral (in the opposite breast) cancer.

Comparative studies

In 2006, the large STAR clinical study concluded that raloxifene is equally effective in reducing the incidence of breast cancer, but after an average 4-year follow-up there were 36% fewer uterine cancers and 29% fewer blood clots in women taking raloxifene than in women taking tamoxifen, although the difference is not statistically significant.^[6][7][8]

In 2005, the ATAC trial showed that after average 68 months following a 5 year adjuvant treatment, the group that received anastrozole (Arimidex) had significantly better results than the tamoxifen group in measures like disease free survival, but no overall mortality benefit.^[9] Data from the trial suggest that anastrozole should be the preferred medication for postmenopausal women with localized breast cancer that is estrogen receptor (ER) positive.^[10] Another study found that the risk of recurrence was reduced 40% (with some risk of bone fracture) and that ER negative patients also benefited from switching to anastrozole.^[11][12]

Other uses

McCune-Albright syndrome

In McCune-Albright syndrome (MAS) tamoxifen has been used to treat premature puberty and the consequences of premature puberty. Tamoxifen has been seen to decrease rapid bone maturation which is the result of excessive estrogen and improve predicted adult height (PAH).^[13][14][15] The same effects have also been seen in short pubertal boys.^[16]

However, one in vitro study in 2007 and later an in vivo study in 2008 have shown that tamoxifen induces apoptosis in growth plate chondrocytes, reduces serum IGF-I levels and causes persistent retardation of longitudinal and cortical radial bone growth in young male rats, leading the researches to express concern giving tamoxifen to growing individuals.^[17][18]

Infertility

Tamoxifen is used to treat infertility in women with anovulatory disorders. A dose of 10–40 mg per day is administered in days 3–7 of a woman's cycle.^[19] In addition, a rare condition occasionally treated with tamoxifen is retroperitoneal fibrosis.^[20]

Gynecomastia

Tamoxifen is used to prevent estrogen related gynecomastia, resulting from elevated estrogenic levels. It is taken as a preventative measure in small doses, or used at the onset of any symptoms e.g. nipple soreness/sensitivity. Other drugs are taken for the similar purposeses such as clomiphene citrate and the anti-aromatase drugs which are used in order to try to avoid the hormone related adverse effects. Tamoxifen is also sometimes used to treat or prevent gynecomastia in sex offenders undergoing temporary chemical castration.^[21]

Bipolar disorder

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.^[22][23]

Angiogenesis and cancer

Tamoxifen is one of three drugs in an anti-angiogenetic protocol developed by Dr. Judah Folkman, a researcher at Children's Hospital at Harvard Medical School in Boston. Folkman discovered in the 1970s that angiogenesis – the growth of new blood vessels – plays a significant role in the development of cancer. Since his discovery, an entirely new field of cancer research has developed. Clinical trials on angiogenesis inhibitors have been underway since 1992 using myriad different drugs. The Harvard researchers developed a specific protocol for a golden retriever named Navy who was cancer-free after receiving the prescribed cocktail of celecoxib, doxycycline, and tamoxifen – the treatment subsequently became known as the Navy Protocol.^[24] Furthermore tamoxifen treatment alone has been shown to have anti-angiogenetic effects in animal models of cancer which appear to be, at least in part, independent of tamoxifen's estrogen receptor antagonist properties.^[25]

Control of gene expression

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.^[26]

Riedel's thyroiditis

Tamoxifen has been proposed as part of a treatment plan for Riedel's thyroiditis.^[27]