Researchers have discovered that one of the most commonly prescribed drugs for breast cancer, tamoxifen, may not be as effective for women who inherit a common genetic variation.
According to researchers at the University of Michigan and the Mayo Clinic, the genetic variation affects the level of a crucial enzyme that activates tamoxifen to fight breast cancer.
The study, led by researcher James Rae, Ph.D., at the University of Michigan Comprehensive Cancer Center and Matthew Goetz, M.D., an oncologist at the Mayo Clinic, tested the most common genetic variant responsible for lowering the CYP2D6 enzyme, and found that women with this genetic variant were almost twice as likely to see their breast cancer return.
Apparently up to 10 percent of women inherit this genetic trait.
Rae, a research assistant professor of internal medicine says they have shown that CYP2D6 is responsible for activating tamoxifen to a metabolite called endoxifen that is nearly 100 times more potent as an anti-estrogen than tamoxifen itself.
He says this suggests that women who inherit a genetic variant in the CYP2D6 gene appear to be at higher risk of relapse when treated with five years of tamoxifen.
The researchers at the University of Michigan Comprehensive Cancer Center were among the group to discover CYP2D6 metabolizes tamoxifen, and they are leading ongoing work in looking at how genetic differences affect women’s response to tamoxifen.
Their research has also found the antidepressant drug Paxil can prevent tamoxifen from being activated, while the antidepressant drug Effexor does not.
These drugs, selective serotonin reuptake inhibitors or SSRI’s, are often used to treat hot flashes, a common side effect of tamoxifen.
In this current study of 256 women with breast cancer, researchers also found that women with the CYP2D6 variant were less likely to have hot flashes.
Any hot flashes among this group tended to be less severe, suggesting that this side effect could predict the gene variation.
Further studies are needed, but researchers hope one day this finding may lead to a genetic test that could help doctors determine which women are most likely to benefit from tamoxifen.
This type of test is not currently offered clinically.