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.
