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Selective Estrogen Receptor Modulators (SERMs)

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Selective estrogen receptor modulators, called SERMs for short, block the effects of estrogen in the breast tissue. SERMs work by sitting in the estrogen receptors in breast cells. If a SERM is in the estrogen receptor, there is no room for estrogen and it can't attach to the cell. If estrogen isn't attached to a breast cell, the cell doesn't receive estrogen's signals to grow and multiply.

Cells in other tissues in the body, such as bones and the uterus, also have estrogen receptors. But each estrogen receptor has a slightly different structure, depending on the kind of cell it is in. So breast cell estrogen receptors are different from bone cell estrogen receptors and both of those estrogen receptors are different from uterine estrogen receptors. As their name says, SERMs are "selective" — this means that a SERM that blocks estrogen's action in breast cells can activate estrogen's action in other cells, such as bone, liver, and uterine cells.

There are three SERMs:

  • tamoxifen in pill form (also called tamoxifen citrate; brand name: Nolvadex) and in liquid form (brand name: Soltamox)
  • Evista (chemical name: raloxifene)
  • Fareston (chemical name: toremifene)

Each is a pill, usually taken once a day, except for tamoxifen in liquid form. Tamoxifen is the oldest, most well-known, and most-prescribed SERM.

SERMs can be used to treat women both before and after menopause.

Benefits of SERMs

Because tamoxifen is the most commonly used SERM, most of the studies comparing SERMs to aromatase inhibitors have looked at tamoxifen versus aromatase inhibitors. Several studies have compared tamoxifen with aromatase inhibitors to see which type of medicine was more effective in treating early-stage, hormone-receptor-positive breast cancer in postmenopausal women. Based on the results, most doctors go by the following recommendations:

  • An aromatase inhibitor is the best type of hormonal therapy to start with for postmenopausal women. When treating early-stage, hormone-receptor-positive breast cancer, aromatase inhibitors have more benefits and fewer serious side effects than tamoxifen.
  • Switching to an aromatase inhibitor after taking tamoxifen for 2 to 3 years (for a total of 5 years of hormonal therapy) offers more benefits than 5 years of tamoxifen. Taking an aromatase inhibitor for 5 years after taking tamoxifen for 5 years continues to reduce the risk of the cancer coming back, compared to no treatment after tamoxifen.
  • Taking an aromatase inhibitor for 5 years after taking tamoxifen for 5 years continues to reduce the risk of the cancer coming back, compared to no treatment after tamoxifen.

For premenopausal women diagnosed with hormone-receptor-positive breast cancer, the SERM tamoxifen is the hormonal therapy treatment standard.

Side effects of SERMs

SERMs may cause some serious side effects, including blood clots, stroke, and endometrial cancer. If you and your doctor are considering tamoxifen or another SERM as part of your treatment plan, tell your doctor if you smoke or have a history of blood clots or heart attack. If you're taking a SERM, call your doctor immediately if you have any of these symptoms:

The most common side effects of SERMs are:

You should not take a SERM if you are breastfeeding, pregnant, trying to get pregnant, or if there is any chance that you could be pregnant. These medicines may cause damage to developing embryos. You should use an effective non-hormonal type of birth control — such as condoms, a diaphragm along with spermicide, or a non-hormonal I.U.D. — while you are taking a SERM. Ask your doctor which type of non-hormonal birth control would be best for you, as well as how long you should use this type of birth control after you stop taking a SERM.

As a benefit, SERMs also can improve bone density, which reduces the risk of osteoporosis.

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