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FRIDAY, March 26 (HealthDayNews) -- St. John's wort, an herb often taken for mild depression, may interfere with the cancer-fighting ability of Gleevec, a drug prescribed for leukemia patients.

That's what University of Florida researcher Reginald Frye will tell his colleagues on March 27 at the American Society for Clinical Pharmacology and Therapeutics' annual meeting in Miami Beach.

Previously, St. John's wort has been found to disrupt the metabolism of other drugs, including a protease inhibitor used to treat HIV, and cyclosporine, a drug used to reduce the risk of rejection in organ transplant patients.

Frye and his research colleagues from the University of Pittsburgh, where he was previously on faculty, gave 12 healthy subjects, aged 20 to 51, a 400-milligram tablet of Gleevec on the first day, then gave them 900 milligrams of St. John's wort in three daily doses on days three to 17. On day 15, they again got 400 milligrams of Gleevec.

On the first day, they measured how fast Gleevec was eliminated from the body, to get a picture of normal metabolism. Those blood tests were repeated after taking St. John's wort.

"You need a particular concentration [of Gleevec] to prevent recurrence of the leukemia," Frye explains.

However, it turned out the herb interfered with the blood levels of the anti-cancer drug. "The blood concentration of Gleevec the second time they took it was 30 percent lower than the first time," Frye says. "By taking the St. John's wort, the concentration was below the ideal threshold for at least one third of the day."

With Gleevec at such low blood levels, cancer growth could recur, he adds.

Gleevec was granted approval in 2001 by the U.S. Food and Drug Administration for the treatment of chronic myeloid leukemia (CML), gastrointestinal stromal cancer and pediatric leukemia. About 40,000 people in the United States have CML, according to the FDA.

According to Novartis Pharmaceuticals Corp., which makes Gleevec and funded the study, the anti-cancer drug disrupts the molecular process of CML. In patients with the disease, there is a constant signal that tells the body to keep making abnormal white blood cells. The signal comes about after a change in the person's DNA forms a chromosome called the Philadelphia chromosome. In turn, the chromosome creates an abnormal protein that tells the body to transmit the signal to keep making abnormal white blood cells. Gleevec is thought to work by preventing the abnormal protein from transmitting the message.

The finding isn't a surprise to Eric Yarnell, a naturopathic physician in Seattle and an adjunct faculty member at Bastyr University in Seattle. "It's certainly fitting in with the pattern we are seeing with St. John's wort," says Yarnell, referring to other reports of adverse effects when the herb is taken with other drugs.

"This is the 10th study I am aware of that concludes there are adverse effects of St. John's wort on the metabolism of other drugs," Yarnell says. Regarding the latest study, he says the small sample size is one potential drawback. "We really need to see more studies," he adds.

Yarnell prescribes St. John's wort and is involved with a botanical company that distributes it, along with hundreds of other botanicals. It's crucial, he says, to first find out which drugs a patient is taking to avoid adverse interactions with the herb and to avoid the herb if a problem is likely.

Frye agrees. "Either do not take St. John's wort or talk with your doctor or pharmacist and be sure they are aware of everything you are taking," he says.

More information

For more on leukemia, visit the American Cancer Society. To read a public alert on St. John's wort, visit the National Institute of Mental Health.

(SOURCES: Reginald Frye, Ph.D., Pharm.D., associate professor, pharmacy, University of Florida College of Pharmacy, Gainesville; Eric Yarnell, N.D., naturopathic physician, adjunct faculty, Bastyr University, Seattle; March 27, 2004, presentation, American Society for Clinical Pharmacology and Therapeutics' annual meeting, Miami Beach, Fla. )

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