Tamoxifen was discovered by ICI Pharmaceuticals (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."
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 than in women taking tamoxifen.
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. Anastrozole vs tamoxifen: deaths - 575 vs 651, recurrences - 402 vs 498, distant metastases - 324 vs 375, second breast cancer - 35 vs 59 (42% reduction). The trial suggested that anastrozole should be the preferred medication for postmenopausal women with localized breast cancer that is estrogen receptor positive.
In men, tamoxifen is sometimes used to treat gynecomastia that arises for example as a side effect of antiandrogen prostate cancer treatment. Tamoxifen is also used by bodybuilders to prevent or reduce drug-induced gynecomastia caused by the estrogenic metabolites of anabolic steroids. Tamoxifen is also sometimes used to treat or prevent gynecomastia in sex offenders undergoing treatment by temporary chemical castration.
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.
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 a myriad of 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. 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.
Finally 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 competitively binds to estrogen receptors on tumors and other tissue targets, producing a nuclear complex that decreases DNA synthesis and inhibits estrogen effects. It is a nonsteroidal agent with potent antiestrogenic properties which compete with estrogen for binding sites in breast and other tissues. Tamoxifen causes cells to remain in the G0 and G1 phases of the cell cycle. Because it prevents (pre)cancerous cells from dividing but does not cause cell death, tamoxifen is cytostatic rather than cytocidal.
Tamoxifen itself is a prodrug, having relatively little affinity for its target protein, the estrogen receptor. It is metabolized in the liver by the cytochrome P450 isoform CYP2D6 and CYP3A4 into active metabolites such as 4-hydroxytamoxifen and N-desmethyl-4-hydroxytamoxifen (endoxifen) which have 30-100 times more affinity with the estrogen receptor than tamoxifen itself. These active metabolites compete with estrogen in the body for binding to the estrogen receptor. In breast tissue, 4-hydroxytamoxifen acts as an estrogen receptor antagonist so that transcription of estrogen-responsive genes is inhibited.
Tamoxifen is a selective estrogen receptor modulator. Even though it is an antagonist in breast tissue it acts as partial agonist on the endometrium and has been linked to endometrial cancer in some women. Therefore endometrial changes, including cancer, are among tamoxifen's side effects.
The American Cancer Society lists tamoxifen as a known carcinogen, stating that it increases the risk of some types of uterine cancer while lowering the risk of breast cancer recurrence. The ACS states that its use should not be avoided in cases where the risk of breast cancer recurrence without the drug is higher than the risk of developing uterine cancer with the drug.
For some women, tamoxifen can cause a rapid increase in triglyceride concentration in the blood. In addition there is an increased risk of thromboembolism especially during and immediately after major surgery or periods of immobility. Tamoxifen is also a cause of fatty liver, otherwise known as steatorrhoeic hepatosis or steatosis hepatis.
A significant number of tamoxifen treated breast cancer patients experience a reduction of libido.
A beneficial side effect of tamoxifen is that it prevents bone loss by inhibiting osteoclasts by acting as an estrogen receptor agonist (i.e., mimicking the effects of estrogen) in this cell type, and therefore it prevents osteoporosis. When tamoxifen was launched as a drug, it was thought that tamoxifen would act as an estrogen receptor antagonist in all tissue, including bone, and therefore it was feared that it would contribute to osteoporosis. It was therefore very surprising that the opposite effect was observed clinically. Hence tamoxifen's tissue selective action directly lead to the formulation of the concept of selective estrogen receptor modulators (SERMs).
The first clinical study took place at the Christie Hospital in 1971, and showed a convincing effect in advanced breast cancer, but nevertheless ICI's development programme came close to termination when it was reviewed in 1972. It appears to have been Walpole again who convinced the company to market tamoxifen for late stage breast cancer in 1973. He was also instrumental in funding V. Craig Jordan to work on tamoxifen. Approval in the US followed in 1977, but the drug was competing against other hormonal agents in a relatively small marketplace and was not at this stage either clinically or financially remarkable.
1980 saw the publication of the first trial to show that tamoxifen given in addition to chemotherapy improved survival for patients with early breast cancer. In advanced disease, tamoxifen is now only recognised as effective in estrogen receptor positive (ER+) patients, but the early trials did not select ER+ patients, and by the mid 1980s the clinical trial picture was not showing a major advantage for tamoxifen. Nevertheless, tamoxifen had a relatively mild side-effect profile, and a number of large trials continued. It was not until 1998 that the meta-analysis of the Oxford based Early Breast Cancer Trialists' Collaborative Group showed definitively that tamoxifen saved lives in early breast cancer.