(October 6, 2009 - Insidermedicine)

Individuals with breast cancer whose bodies can break down the drug tamoxifen most effectively respond best to tamoxifen treatment, according to research published in the latest issue of the Journal of the American Medical Association.

Here is some information about tamoxifen and its role in breast cancer:

•    Tamoxifen works by disrupting the activity of estrogen, preventing it from feeding breast cancer

•    The drug is only effective after it is broken down, or metabolized, in the body into its component parts

•    The enzyme responsible for metabolizing tamoxifen is known as CYP2D6, but the activity level of this enzyme varies from person to person and depends on genetic factors

Researchers from the Dr. Margarete Fischer-Bosch-Institute of Clinical Pharmacology in Stuttgart conducted genetic testing on tumor tissue or blood samples taken from over 1,000 women with breast cancer treated with tamoxifen in order to determine the activity level of their CYP2D6 enzyme pathway. The investigators then analyzed the activity of this pathway against the outcomes of the patients.

Women whose CYP2D6 enzyme pathways were most active had a lower cancer recurrence rate than those whose CYP2D6 activity levels were intermediate or poor. High activity of CYP2D6 was also associated with higher rates of surviving without experiencing a breast cancer event and surviving without experiencing a recurrence of their disease. Overall survival rates were similar among all three groups, however.

Today's research suggests that genetic testing of the CYP2D6 enzyme system could be used to determine which breast cancer patients might be the best candidates for tamoxifen and which patients might do better on another therapy.