Arimidex vs. Tamoxifen

How Arimidex and Tamoxifen Work

While they are both hormone therapies, these medications do not work in the same way. Arimidex is part of a group of medications called aromatase inhibitors. Aromatase is an enzyme found in various places in the body. Aromatase enzymes help produce estrogens (in particular, a certain estrogen called estradiol). In postmenopausal women, most of the estrogen in the body is made by aromatase enzymes. By blocking these enzymes, Arimidex helps to decrease the amount of estrogen in the body. Because aromatase enzymes are not the primary source of estrogen in premenopausal women, the drug may not be as effective in women who have not gone through menopause.
 
Tamoxifen is an estrogen receptor blocker. Instead of causing less estrogen to be produced (like Arimidex), tamoxifen prevents estrogen from binding to receptors. Because the drug directly blocks estrogen receptors, it can be effective for premenopausal women, postmenopausal women, and men.
 

Arimidex Versus Tamoxifen: What Does the Research Say?

When most medications are studied, they are compared to a placebo (or "sugar pill"). Because it is rare for a new medication to be compared to other medications, it is usually impossible to make direct comparisons. However, because it would be unethical to not treat breast cancer (that is, to give someone a placebo), most clinical studies compared Arimidex to tamoxifen (instead of a placebo).
 
One large study compared Arimidex and tamoxifen for the adjuvant treatment of early breast cancer in postmenopausal women. Adjuvant therapy for breast cancer is treatment that follows breast cancer surgery with or without radiation. It is given to help prevent the cancer from returning. These studies compared Arimidex with tamoxifen in postmenopausal women with hormone receptor-positive cancer. After five years, up to 17 percent more women taking Arimidex were cancer-free, compared to those taking tamoxifen. In this particular study, women taking Arimidex were also less likely to stop the medicine because of side effects when compared to women taking tamoxifen. This study also showed that taking both breast cancer medications together did not offer any advantage.
 
Two other large studies compared Arimidex and tamoxifen in postmenopausal women with advanced or metastatic breast cancer. These women had tumors that were either hormone receptor-positive or the receptor status was not known. In these studies, Arimidex was at least as effective as tamoxifen in how the cancer responded to the medication (either by shrinking the tumor or stopping its growth) and time to progression. "Time to progression" is a medical term that means from the time that the cancer is treated until the disease starts to get worse, such as when the cancer spreads (metastasizes) or the tumor increases in size. For these studies, time to progression was, on average, up to 11.1 months for Arimidex and, on average, up to 8.3 months for tamoxifen.
 
Two other studies showed that Arimidex may be effective for treating advanced breast cancer that has not responded to tamoxifen in postmenopausal women. These studies included women who had breast cancer that was hormone receptor-positive. Women with hormone receptor-negative breast cancer were included only if they had originally responded to treatment with tamoxifen.
 
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Arimidex Medication Information

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