The herb St. John’s Wort, used as an alternative therapy for mild depression, may hinder the effectiveness of a newer type of cancer drug, according to researchers.

Their study of 10 healthy volunteers found that the herbal remedy lowered blood levels of the cancer drug Gleevec (known in some countries as Glivec) by as much as 42 percent.

In a cancer patient, the study authors warn, this could mean the difference between success or failure on Gleevec, an oral drug used mainly to treat the blood cancer chronic myeloid leukemia.

The findings, published in the journal Pharmacology, add to the list of prescription drugs that don’t mix well with St. John’s Wort. Past studies have shown that the drug may alter blood levels of some other chemotherapy drugs, as well as certain cardiovascular drugs, HIV medications, antidepressants and birth control pills.

To be safe, people taking any medication should talk to their doctor or pharmacist about any potential drug interactions before starting an herbal product, said the new study’s lead author, Dr. Patrick F. Smith of the State University of New York at Buffalo.

“A lot of times people don’t think of these herbals as medications,” Smith noted in an interview with Reuters Health.

Gleevec received much attention when U.S. regulators gave it an accelerated approval in 2001. It was the first in a new wave of “targeted” cancer drugs designed to zero in on molecular abnormalities on cancer cells and spare healthy cells.

The drug’s speedy approval, however, meant a relative lack of information on potential interactions with other medications, Smith and his colleagues note in their report.

Many agents are known to affect the activity of the enzyme that is primarily responsible for metabolizing Gleevec. St. John’s Wort is one of these agents, and the herb appears to speed the body’s clearance of Gleevec, Smith said.

In his team’s study, 10 healthy adult volunteers took a dose of the cancer drug before and after two weeks on St. John’s Wort. The researchers found that participants’ blood levels of Gleevec were typically one-third lower after they had been on the herb.

Individuals differ in how St. John’s Wort affects their metabolism of drugs, and some Gleevec patients “would do fine” if they took the herb, Smith pointed out.

“But a certain percentage,” he added, “would not.”

SOURCE: Pharmacotherapy, November 2004.