Some patients with breast cancer who are taking tamoxifen may not get the full benefit of the drug treatment if they are also taking antidepressants, researchers said here on March 25 at the 7th European Breast Cancer Conference (EBCC7).
Recent literature has pointed to the importance of the CYP2D6 enzyme system in the activation of tamoxifen to its more active metabolite endoxifen. In addition to some patients being poor metabolisers of tamoxifen, medications such as fluoxetine and paroxetine can lead to additional inhibition of this important enzyme and affect breast cancer therapy.
Sean Hopkins, Ottawa Hospital Regional Cancer Centre, Ottawa, Ontario, and colleagues reviewed drug claim data from 945 patients who were receiving tamoxifen or an aromatase inhibitor with a CYP2D6 inhibitor, with the goal of establishing recommendations for pharmacogenomic testing in patients with breast cancer. Of the patients, 463 had claims for at least 1 of the medications identified: 154 (29%) received tamoxifen, 321 (60%) received an aromatase inhibitor, and 68 (13%) received no hormonal therapy. The researchers found that 7 patients receiving tamoxifen and 16 receiving an aromatase inhibitor were also receiving drugs that were a strong CYP2D6 inhibitor.
One patient on tamoxifen and 6 patients receiving an aromatase inhibitor also received a moderate CYP2D6 inhibitor (sertraline, 7). All 7 patients who were receiving tamoxifen and a strong inhibitor of the CYP2D6 enzyme were contacted by the healthcare team to advise them that an alternative medication should be explored. The researchers suggested that patients receiving moderate to weak inhibitors of CYP2D6 should be tested to determine what type of metabolisers they are before starting tamoxifen treatment.
A study published recently by the Toronto group looked at how outcomes were related to treatments that the patients were taking concurrently. “Our study differs in that we have been able to take action as soon as we have discovered that a patient is taking interacting medication[s] by contacting the prescribing doctor in order to get them to switch to a different medication with no deleterious effect on tamoxifen metabolism,” said Hopkins.
While infrequent, patients with breast cancer receive medications that can have an adverse effect on tamoxifen therapy. “Our group will be exploring ways to incorporate this type of ongoing quality assurance to all of the over 6,000 breast cancer patients that are seen at the cancer centre on an annual basis, and expand this to other potential drug related problems and drug interactions,” said Hopkins.