Fareston Is Effective Against Some Advanced-Stage Cancers That Have Stopped Responding to Aromatase Inhibitors

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Fareston (chemical name: toremifene), a hormonal therapy medicine, is approved by the U.S. Food and Drug Administration to treat postmenopausal women diagnosed with metastatic hormone-receptor-positive breast cancer. Fareston isn’t commonly used in the United States.

Fareston is a SERM (selective estrogen receptor modulator) that blocks the effects of estrogen in breast tissue. SERMs work by sitting in the estrogen receptors in breast cells. If a SERM is in the estrogen receptor, there’s 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.

Other SERMs are:

  • tamoxifen (brand name: Nolvadex)
  • Evista (chemical name: raloxifene)

Metastatic breast cancer is cancer that has spread to parts of the body away from the breast, such as the bones or liver. Metastatic breast cancer is considered advanced-stage cancer.

A small Japanese study has found that Fareston is a good treatment option for postmenopausal women diagnosed with advanced-stage hormone-receptor-positive breast cancer that grew while being treated with the aromatase inhibitors Arimidex or Femara. The research was presented at the Breast Cancer Symposium in September 2012.

Aromatase inhibitors are hormonal therapy medicines used to treat hormone-receptor-positive breast cancer and help stop the cancer from coming back (recurrence) in postmenopausal women. The aromatase inhibitors are:

  • Arimidex (chemical name: anastrozole)
  • Aromasin (chemical name: exemestane)
  • Femara (chemical name: letrozole)

Aromatase inhibitors stop the production of estrogen in postmenopausal women. Aromatase inhibitors work by blocking the enzyme aromatase, which turns androgen hormones into small amounts of estrogen in the body. This means that less estrogen is available to stimulate the growth of hormone-receptor-positive breast cancer cells.

This study looked at 91 postmenopausal women who had been diagnosed with advanced-stage hormone-receptor-positive breast cancer that grew while the women were being treated with either Arimidex or Femara.

The women were randomly chosen to get either Aromasin or Fareston until the cancer grew or unacceptable side effects developed.

Women who got Fareston were 38% less likely to have the cancer grow than the women who got Aromasin.

About twice as many women who got Fareston got benefits from the medicine compared to women who got Aromasin:

  • 47.5% of the women who got Fareston got some benefit from it
  • 26.7% of the women who got Aromasin got some benefit from it

The women who got Fareston lived about 2 months longer than the women who got Aromasin, but this difference wasn’t statistically significant, which means that it could have been due to chance and not because of the difference in treatment.

The women who got Fareston were more likely to have side effects such as nausea, fatigue, hot flashes, and night sweats, but none of the side effects were severe.

If you’re a postmenopausal women being treated for advanced-stage, hormone-receptor-positive breast cancer that’s stopped responding to either Arimidex or Femara, you and your doctor may be considering other hormonal therapy medicines. Because Fareston isn’t commonly used in the United States, it’s likely that your doctor may recommend tamoxifen instead of Fareston. Both medicines are good options. Still, Fareston may be a better choice for women who have an abnormal CYP2D6 enzyme or are taking an antidepressant.

Your body uses the CYP2D6 enzyme to convert tamoxifen into its active form. Two things may interfere with the body’s ability to make this happen: a flaw in the CYP2D6 enzyme and certain medicines that block the enzyme’s effectiveness.

About 10% of people have a CYP2D6 enzyme that doesn’t function as well as it should. If you’re considering taking tamoxifen, you may want to ask your doctor about being tested for this enzyme abnormality. Still, CYP2D6 testing is controversial because several large studies found that an abnormal CYP2D6 enzyme didn’t affect tamoxifen’s effectiveness. Together, you and your doctor can decide if CYP2D6 testing makes sense for your unique situation.

Medicines that block CYP2D6 enzyme activity include some antidepressants known as serotonin-specific reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs). Other commonly prescribed medicines such as Cardioquin (chemical name: quinidine), Benadryl (chemical name: diphenhydramine), and Tagamet (chemical name: cimetidine), can interfere with the activation of tamoxifen -- reducing its effectiveness.

Unlike tamoxifen, Fareston isn’t affected by the CYP2D6 enzyme.

For more information on hormonal therapy medicines, including what to expect and questions to ask your doctor about hormonal therapy, visit the Breastcancer.org Hormonal Therapy section.

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