The study found that women with the CYP2D6 variant had only mild hot flashes, if any at all. According to Rae, this may suggest that the side effect could predict whether a woman has the CYP2D6 gene variation and will therefore have little to no benefit from Tamoxifen treatment. But Rae said he hopes this study will encourage more research into the use of genotype tests that can identify which patients have the CYP2D6 gene variant before assigning them to Tamoxifen treatment, rather than depending on whether or not a patient gets hot flashes after they begin taking the drug.
"Overall, the results we've found have held up, but we'd like to see these results reproduced in a bigger study before we say every woman should go out and get genotyped," Rae said.
The genotype test used by researchers in this study — the AmpliChip Cytochrome P450 — was developed by Roche and approved by the Food and Drug Administration in 2004. Despite the availability of the gene tests, Rae and medical centers throughout the country remain cautious about using this test on every breast cancer patient.
"Oftentimes past performances don't predict future results. I would prefer to have solid evidence that by using these tools today, you'll see improvement in treatment outcomes … and that would require prospective studies," said Herbert Kim Lyerly, breast surgeon and director of the Comprehensive Cancer Center at Duke University.
Moreover, because each genotype test costs about $500, a great deal of research is required before doctors will be convinced that the cost-to-benefit ratio of using these tests is good enough.
According to Lyerly, this study is just the tip of the iceberg in terms of what kind of research needs to be done before each woman is offered a genotype test to determine what kind of breast cancer treatment will work best for them.
"If you're looking at 10-year survival rates, it would take at least 10 years to do those kinds of prospective studies, and once we get that kind of data we'll start seeing more global acceptance that these [genotype] tests are valid we'll see greater uptake on acceptance of these tools," Lyerly explained.
However, Lyerly believes the development of these tools represents an important paradigm shift in the world of medicine.
"I think that the concept of defining whose going [to] benefit from therapy goes beyond just the one drug, Tamoxifen. It gets to the core of what's happening in medicine now," said Lyerly. "Rather than saying everyone with breast cancer whose hormone positive should take this drug, [this study] is providing very tangible evidence that there's a subset of people that benefit and subset of people that don't, so we are selecting the right medicines for the right patients. This has profound implications for how we develop drugs and how we use them."