Vanguard Neurologist

From Bloomberg.com:

U.S. regulators may warn patients taking tamoxifen, a breast cancer medicine used by millions of women, not to use certain types of antidepressants because they block the tumor-fighting drug’s effectiveness.

The Food and Drug Administration is “looking at adding new information to the tamoxifen label” to advise women taking the cancer drug against using some antidepressants, agency spokeswoman Karen Riley said today in an e-mailed statement. Research has shown that Eli Lilly & Co.’s Prozac, Pfizer Inc.’s Zoloft and GlaxoSmithKline Plc’s Paxil can stop tamoxifen from attacking cancer cells.

Tumors were more than twice as likely to return after two years in women taking the antidepressants while on the cancer drug, compared with those taking tamoxifen alone, in a study presented May 30 at a meeting of the American Society of Clinical Oncology in Orlando. Prozac, Paxil and Zoloft are from a family of antidepressants called selective serotonin reuptake inhibitors, or SSRIs.

Doctors commonly prescribe antidepressants to curb hot flashes caused by tamoxifen, even though the medicines aren’t approved for this use, said Powel Brown, director of cancer prevention at the Lester and Sue Smith Breast Cancer Center at Baylor College of Medicine in Houston. Other types of antidepressants, such as Wyeth’s Effexor, may be safer for women on tamoxifen, he said.

“Effexor doesn’t interfere with tamoxifen so that is the preferred drug for oncologists to treat hot flashes,” said Brown, who wasn’t involved in the study, in an interview before the data were presented. “We need to get that message out to primary care doctors and psychiatrists and gynecologists so they will be aware that antidepressants like Paxil have a risk of inhibiting tamoxifen.”

Breast cancer is the second most common malignancy in women, after skin tumors, according to the Atlanta-based U.S. Centers for Disease Control and Prevention. About 187,000 women were diagnosed with breast cancer and 41,000 women died from it in 2005, the most recent year for which CDC data are available.

Tamoxifen can’t combat tumors until it mixes inside the body with a liver enzyme called CYP2D6 to morph into an active tumor fighter called endoxifen. Many antidepressants use the same enzyme, sapping supplies needed for tamoxifen to do its work.

Risk of Recurrence

The FDA may add details on CYP2D6 to the tamoxifen label, including information on which drugs are known to deprive tamoxifen of the enzyme, Riley said. She didn’t comment on how long the change might take.

The research presented at the cancer meeting was funded by Medco Health Solutions Inc., a pharmacy benefits company based in Franklin Lakes, New Jersey. Medco searched medical and pharmacy records of 10.7 million people to see what happened to women taking tamoxifen along with antidepressants known to interfere with the CYP2D6 enzyme.

After two years, patients taking both SSRIs and tamoxifen had a 14 percent risk of tumors recurring, compared with 7.5 percent for women taking the cancer drug alone. The risk increased to 16 percent among those using Paxil, Prozac or Zoloft, the data showed.

The study included 945 women on tamoxifen, and an additional 353 also using an antidepressant.

“This is the first large outcomes-based study to reinforce earlier research questioning the use of Paxil and Prozac in patients taking tamoxifen,” said Robert Epstein, a study author and chief medical officer for Medco, in an interview before the study was presented.

Outside science advisers to the FDA in October 2006 reviewed the potential for certain antidepressants to render tamoxifen ineffective. The current label doesn’t have any statements about this risk.