Tamoxifen citrate is a drug used in breast cancer. It is in the anti-estrogen class. It inhibits estrogen receptors competitively. In other words, before the estrogen hormones can bind to the receptor, the tamoxifen molecule comes and binds to the receptor. In this way, the effects of estrogen hormones are eliminated. So, tamoxifen is an estrogen antagonist. This situation is similar to enzyme inhibition and generally works on the same principle, however, an agonist site (pocket) may also be present in the binding sites of the receptors.
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When we look at the estrogen hormones, we see that they have a steroid structure. Another steroid structure is testosterone, which is the male sex hormone, passed as one of the androgen hormones. Steroid structures play important roles in human biology. They play important roles in bio signaling by interacting with core receptors. However, when it comes to cancer, the task of binding to the receptor has become a problem. Because, to give an example of breast cancer, there are estrogen receptors on the surface of cancer cells (an increase in the number of these receptors has been reported during cancer). This means that estrogen receptors work for the growth of cancer. So cancer has become dependent on estrogen to grow. For more information What is tamoxifen citrate?