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The Pharmacogenetics of Tamoxifen Therapy

January 2007

Breast cancer is the most common cancer in women. Tamoxifen is the most commonly prescribed drug for patients with breast cancer. Tamoxifen therapy has significantly impacted breast cancer patients, decreasing breast cancer deaths and recurrences. Estrogen stimulates the growth of some cell types, including malignant breast tissue. After binding with estrogen receptors, tamoxifen blocks the action of estrogen, acting as an antiestrogen.

Tamoxifen is effective only in women who have estrogen receptor positive (ER+) breast cancer, but not all ER+ patients benefit from tamoxifen therapy. Tamoxifen failure is often due to a patient’s inability to metabolize the drug to endoxifen. Cytochrome P450 genotyping results may assist physicians in selecting the appropriate medications for their patients. Specifically, for breast cancer patients, identification of a patient’s CYP2D6genotype may help identify patients who are less likely to benefit from tamoxifen, and for whom an alternate drug may be indicated.

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