DNA Matters When Treating Breast Cancer

MIAMI (CBS4) -- Millions of women take the drug Tamoxifen to prevent their breast cancer from coming back or, if they are at high risk they may take it to avoid getting breast cancer. But a new study indicates that some women may not process the drug properly, making it is less effective at warding off cancer.

"Tamoxifen is metabolized from an inactive drug into the active drug by enzymes called CYP2D6," explained Dr. Elisabeth McKeen, a Medical Oncologist at PB Cancer Institute.

If people have the normal CYP2D6 genes then they rapidly convert Tamoxifen to the active drug and Tamoxifen works for them, but problems arise when they have a CYP2D6 mutation.

"If they inherit one or two mutations then they make the drug less active," said Dr. McKeen. "And studies have shown that these women are more likely to have their cancer come back."

A simple blood test determines whether patients have the CYP2D6 mutation and whether they should take a different drug such as Arimidex, Femara, or Aromasin.

There is also a second concern with Tamoxifen. Drugs used to treat depression may also make Tamoxifen less effective.

"The most common drugs that compete for these enzymes are the anti-depressants. They are serotonin, the SSRIs, the serotonin reuptake inhibitors like Zoloft and Prozac," said Dr. McKenn who explained, "Women on Tamoxifen should not take Zoloft or Prozac."

A hint that Tamoxifen may not be working for you or that you're not metabolizing it into the active drug would be that you're not having hot flashes.

The National Cancer Institute says Tamoxifen has been used for almost ten years to reduce the risk of breast cancer in women who have an increased risk of developing the disease. Two studies have confirmed the benefit of taking the drug daily for five years.

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