Selective Estrogen Receptor Modulators

SERMs, or selective estrogen-receptor modulators, block the action of estrogen in the breast and certain other tissues by occupying estrogen receptors inside cells.

With a SERM sitting in the estrogen receptor, there is no place for the real estrogen to "sit down"�like a game of musical chairs. The SERM blocks the more powerful estrogen signals from getting into the estrogen receptor and telling the cell to grow and spread.

SERMs do not affect all estrogen receptors in the same way, because, as the name states, they are "selective":

• SERMs block (or selectively inhibit) estrogen receptors in breast cells. Therefore, cells don't get the signals they need to grow and multiply.
• SERMs stimulate estrogen receptors in other organs, with good and bad results. For example, the SERM tamoxifen
  • stimulates liver cells, lowering cholesterol levels.
  • stimulates bone cells, resulting in stronger bones and reduced risk of bone breaks.
  • stimulates growth of uterine cells (cells in the uterus), slightly increasing the risk of uterine cancer.

A SERM may also weakly stop the formation of new blood vessels that supply the nutrients the cancer needs to grow. (This is called an "anti-angiogenic" effect.) Although this action would never be enough to stop making all new blood vessels, it may starve some cancer cells, which need extra blood vessels to grow.

As long as a SERM is sitting inside all the estrogen receptors, the cancer cells remain quiet and relatively harmless. After a long period of not being stimulated, the cancer cells may die off. SERMs may even cause breast cancer cells to destroy themselves, a process called "apoptosis," or programmed cell death.

There are three SERMs, each usually taken once a day by pill:

• The most prescribed SERM is tamoxifen (the brand name is Nolvadex, but it's also now a generic drug called tamoxifen citrate).
• Raloxifene (brand name: Evista) hasn't been used to treat women with breast cancer. But it does lower the risk of breast cancer in women who take it to treat osteoporosis. An ongoing study in women who are at high risk for breast cancer but have no personal history of the disease is comparing how well tamoxifen and raloxifene reduce the risk of breast cancer.
• The third SERM, toremifene (brand name: Fareston), is relatively new and not often used in the United States.