タモキシフェン

用途

男性または女性ホルモン製剤そのままでは男性化ないし女性化の副作用が現れることが多いため，それらの副作用の少ない薬剤として乳癌治療にタモキシフェン(ノルバデックス)やメピチオスタン(チオデロン)が最近開発され，用いられている。

効能

抗悪性腫瘍薬, エストロゲン受容体調節薬

説明

In 1966, ICI Pharmaceuticals (now AstraZeneca) first synthesized tamoxifen in the hope of developing a morning-after contraceptive pill. The UK patent for this compound was in place in 1962, whereas the US patent was repeatedly denied until the 1980s. Tamoxifen was approved for a fertility treatment but it was not proven as useful in regulating human contraception. Even though there was a link between estrogen and breast cancer, developing a cancer treatment was not a priority at the time. In 1971, the first clinical study showed a convincing effect of tamoxifen in treating advanced breast cancer. From 1971 to 1977, this drug was neither clinically nor financially remarkable. In 1980s, however, publications first showed that tamoxifen, in addition to chemotherapy, improved survival for patients with early stage breast cancer. In 1998, the meta-analysis by the Oxford-based Early Breast Cancer Trialists’ Collaborative Group showed that tamoxifen did indeed save lives in early breast cancer. In 2001, tamoxifen sales were over $1.024 billion. Since the expiration of the patent in 2002, it is now widely available as a generic drug. By 2004, tamoxifen was the best selling hormonal drug for the treatment of breast cancer.

化学的特性

White Crystalline Solid

使用

A nonsteroidal estrogen antagonist of interest in the treatment of some forms of breast cancer. Tamoxifen is a Protein Kinase C inhibitor, and induces apoptosis in human malignant glioma cell lines

適応症

Tamoxifen (Nolvadex) is a synthetic antiestrogen used in the treatment of breast cancer. Normally, estrogens act by binding to a cytoplasmic protein receptor, and the resulting hormone–receptor complex is then translocated into the nucleus, where it induces the synthesis of ribosomal RNA (rRNA) and messenger RNA (mRNA) at specific sites on the DNA of the target cell. Tamoxifen also avidly binds to estrogen receptors and competes with endogenous estrogens for these critical sites. The drug–receptor complex has little or no estrogen agonist activity.Tamoxifen directly inhibits growth of human breast cancer cells that contain estrogen receptors but has little effect on cells without such receptors.

世界保健機関（WHO）

Tamoxifen is an anti-estrogen agent used mainly to treat breast cancer. Tamoxifen is listed in the WHO Model List of Essential Drugs.

一般的な説明

Tamoxifen is a selective estrogen response modifier (SERM), protein kinase C inhibitor and anti-angiogenetic factor. Tamoxifen is a prodrug that is metabolized to active metabolites 4-hydroxytamoxifen (4-OHT) and endoxifen by cytochrome P450 isoforms CYP2D6 and CYP3A4. In breast cancer, the gene repressor activity of tamoxifen against ERBB2 is dependent upon PAX2. Blocks estradiol-stimulated VEGF production in breast tumor cells.

作用機序

Tamoxifen is slowly absorbed, and maximum serum levels are achieved 4 to 7 hours after oral administration. The drug is concentrated in estrogen target tissues, such as the ovaries, uterus, vaginal epithelium, and breasts. Hydroxylation and glucuronidation of the aromatic rings are the major pathways of metabolism; excretion occurs primarily in the feces.

薬物動態学

Circulating levels of the demethylated metabolite at steady state are up to twice the level of the parent drug, because the elimination half-life of N-demethyl tamoxifen is 14 days, compared with 7 days for tamoxifen. Tamoxifen demonstrates only weak estrogenic effects at several sites, including the endometrium and bone, and on the lipid profile. Tamoxifen undergoes rapid N-dem ethylation to its major metabolite, N-dem ethyltamoxifen, by CYP3A4 and via CYP2D6 to its minor metabolite, 4-hydroxytam oxifen. Evidence suggests that 4-hydroxytamoxifen is the active metabolite of tamoxifen, with a higher binding affinity than the parent drug for the ER

臨床応用

Tamoxifen is a SERM that is used as an antiestrogen in the treatment of estrogen-dependent breas Tcancer following prim ary treatment (c hemotherapy and/or surgery).

副作用

Tamoxifen administration is associated with few toxic side effects, most frequently hot flashes (in 10–20% of patients) and occasionally vaginal dryness or discharge. Mild nausea, exacerbation of bone pain, and hypercalcemia may occur.

安全性プロファイル

Confirmed human carcinogen. Moderately toxic by ingestion and intraperitoneal routes. Human systemic effects by an unspecified route: nausea or vomiting, leukopenia, thrombocytopenia, and skin changes. An experimental teratogen. Other experimental reproductive effects. Human mutation data reported. When heated to decomposition it emits toxic fumes of NOx.

発がん性

Tamoxifen is known to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in humans.