Cancer patients who take St. John's wort while undergoing chemotherapy may be reducing the ability of the cancer treatment to do its job, according to a study reported April 8 at the 93rd Annual Meeting of the American Association for Cancer Research.

About half of all commonly used chemotherapy drugs are metabolized (altered) by the liver through the action of a particular liver enzyme.

Ron H. J. Mathijssen, MD, and colleagues from the Rotterdam Cancer Institute studied the effect of St. John’s wort on this enzyme among a group of three patients undergoing treatment with the chemotherapy drug irinotecan (Camptosar). The enzyme normally changes irinotecan in the body into SN-38, which is responsible for its anti-cancer effects.

Patients were given a standard dose of St. John’s wort, 300 mg three times a day, in addition to irinotecan for one cycle of chemotherapy. The patients also underwent another cycle of treatment with irinotecan without St. John’s wort.

After each treatment cycle, researchers measured blood levels of SN-38, the active anti-cancer agent.

“These blood levels indicate how rapidly the body eliminates the drug,” said Mathijssen. When irinotecan was combined with St. John’s wort the circulating levels of SN-38 decreased by about 40%, he said.

Even though there was a lag between the treatment cycles, the effect of St. John’s wort was still apparent after three weeks. “We know that it is not enough to simply stop St. John’s wort for three weeks before beginning chemotherapy,” said Mathijssen.

Mathijssen did not report any clinical effect of St. John’s wort on chemotherapy but said, “we can assume that with lower circulating levels of SN-38 the therapeutic effect is less."

Moreover, Karen Antman, MD, professor of medicine and pharmacology at Columbia University College of Physicians & Surgeons, said that because about half of all cancer drugs are metabolized in the liver “it is very likely that St. John’s wort will have this same effect on other chemotherapy agents.”

Antman, who was not involved in the Rotterdam study, said that as many as 50% of cancer patients are taking vitamins or herbal supplements — often without the knowledge of their doctors.

Antman said when the diagnosis of cancer is given, often a patient starts taking vitamins. But she said that it is crucial that patients tell their doctors “exactly what vitamins or supplements they are taking.”

This is particularly important when the herbal supplement is St. John’s wort. “We know this drug — and it is a drug — has interactions with many other drugs,” she said.

Currently the Food and Drug Administration (FDA) does not regulate St. John’s wort and other supplements, but Antman said that many clinicians have urged the federal government to institute some type of regulation, “particularly for this agent.”

She said that cancer patients might take St. John’s wort for treatment of depression that often accompanies a cancer diagnosis.

“But this is a case where it would really be better to use a prescription antidepressant because we know what the interactions are with those agents,” Antman said. When doctors know that a drug will affect the potency of the chemotherapy agent, “we can increase the dose of the chemotherapy drug to compensate for the interaction,” she said.