Tamoxifen citrate is an estrogen receptor antagonist and partial agonist which has been shown to induce apoptosis in human malignant glioma cells and to block VEGF production in breast cancer cells. The structure of tamoxifen citrate is showed in figure 1.
About one-fifth of cancer patients suffer from breast cancer worldwide. Breast cancer is cancer that develops from breast tissue. Signs of breast cancer may include a lump in the breast, a change in breast shape, dimpling of the skin, fluid coming from the nipple, a newly inverted nipple, or a red or scaly patch of skin. In those with distant spread of the disease, there may be bone pain, swollen lymph nodes, shortness of breath, or yellow skin. Figure 2 shows the types of breast cancer.
Various chemotherapeutic agents are used to treat the breast cancer, such as letrozole, anastrozole, exemestane, goserelin, leuprorelin, etc. Among the plenty of APIs, tamoxifen citrate is a nonsteroidal weak estrogen that has found successful applications for each stage of breast cancer in the treatment of selected patients.
In 1973, tamoxifen citrate was approved by the UK Committee on the Safety of Medicines for the treatment of breast cancer. Tamoxifen citrate subsequently became available in more than 110 countries as first-line endocrine therapy for the treatment of breast cancer.
Tamoxifen is a type of hormonal therapy known as a selective estrogen receptor modulator (SERM). The drug attaches to hormone receptors (specific proteins) in breast cancer cells. Once the medication is inside the cells, it stops the cancer from accessing the hormones they need to multiply and grow.
Although tamoxifen exhibits direct anti-estrogenic activity, it is best thought of as a pro-drug. Metabolism generates several metabolites of tamoxifen. Two of these metabolites, 4-hydroxy-tamoxifen and 4-hydroxy-N-desmethyltamoxifen, exhibit much greater anti-estrogenic effects compared with tamoxifen. The minor metabolites – metabolite-Y, metabolite-Z, and 4-hydroxy-N-desmethyltamoxifen – all contribute to the antitumor actions of tamoxifen because they are all antiestrogens that inhibit the binding of estradiol to the estrogen receptor.
The results from the in vivo DMBA studies demonstrated that a 1-month course of tamoxifen therapy in rats given 1 month after the carcinogenic insult only delayed the appearance of mammary tumors; continuous therapy for 6 months, on the other hand, resulted in 90% of the animals remaining tumor free. Indeed, tumors appeared whenever tamoxifen therapy was stopped. Thus, tamoxifen was shown to have a tumoristatic component to its mode of action and the laboratory results indicated that long-term (up to 5 years) or indefinite therapy might be the best clinical strategy for adjuvant tamoxifen treatment.
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