Researchers from the Lawson Health Research Institute say the breast cancer drug tamoxifen is less effective during winter. Specifically, the lack of exposure to vitamin D during affects the body's ability to metabolize the drug.
The findings, which have been reported in the journal Breast Cancer Research and Treatment, are the first to identify this seasonal effect. Researcher Richard Kim says that during the winter months, nearly 30 percent of patients are at risk for less than optimal level of the active form of tamoxifen and therefore may not benefit as much from the therapy.
Kim explained that tamoxifen is converted in the liver to endoxifen (the active form of the drug) by a protein called CYP2D6. "We know that endoxifen is nearly 100 times more potent against the estrogen receptor relative to tamoxifen," said Kim. "Our research has shown that patients with normal CYP2D6 activity can easily convert tamoxifen to endoxifen, whereas patients with no CYP2D6 activity have a significantly limited ability to convert tamoxifen to endoxifen."
With nearly 10 percent of the population completely lacking CYP2D6 enzymatic activity, the new findings have important clinical applications. "These new clinical and genetic markers can aid physicians to better identify patients who will benefit from tamoxifen," explained Kim, "as well as those patients at risk for suboptimal benefit."
In addition to the seasonal effect, Kim found that the simultaneous use of common antidepressants can also substantially lower the body's ability to metabolize endoxifen levels. "Patients who are either partly or completely deficient in CYP2D6 and who are prescribed the SSRI class of antidepressants are at major risk for subtherapeutic endoxifen levels," warned Kim.
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Source: Lawson Health Research Institute