New Research Findings with Tamoxifen

A new study by researchers at the Fred Hutchinson Cancer Research Center in Seattle appears to have identified a new risk factor for tamoxifen. The researchers, led by Dr. Christopher Li, confirmed earlier studies that tamoxifen decreases the risk of a second breast cancer. However, they also found that if the cancer does come back, it tends to be in a more aggressive form.

Specifically, they concluded that tamoxifen may cause a fivefold increased risk of estrogen-receptor (ER)-negative breast cancer-a cancer that is more difficult to treat-in the breast opposite, or contralateral, to the initial tumor.

The majority of breast tumors-about two thirds of those initially diagnosed-are ER-positive, which means they need the hormone estrogen to grow. Such tumors respond well to anti-estrogen drugs, such as tamoxifen, which block estrogen receptors ("docking sites") on the surface of cancer cells.

ER-negative tumors, on the other hand, can thrive without estrogen and are therefore unresponsive to estrogen blockers such as tamoxifen. In addition to being more difficult to treat, ER-negative tumors are associated with a higher mortality rate than ER-positive tumors.

The researchers followed nearly 9,000 women in western Washington who were diagnosed with cancer in one breast between 1990 and 1998. They reported their findings in the Journal of the National Cancer Institute.

"Among women who take tamoxifen after an initial breast-cancer diagnosis, our study and numerous random trials suggest that tamoxifen will decrease a woman's risk of developing cancer in the other breast," Li said. "But if they do develop a tumor on the opposite side, our study suggests that the cancer is more likely to be ER-negative, which is associated with a higher mortality risk."

Li cautioned that these findings should not change current clinical practice. "Tamoxifen has clearly been shown to reduce the risk of recurrence and improve survival among women diagnosed with breast cancer," he emphasized.

SOURCES:
Journal of the National Cancer Institute, July 4, 2001; 93:963-965, 1008-1013
Fred Hutchinson Cancer Research Center (http://www.fhcrc.org)

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