Study shows how St. 's wort blocks drugs

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Study shows how St. 's wort blocks drugs

By Amy Norton

NEW YORK, Apr 12 (Reuters Health) - The herbal mood-booster St. 's wort

is known to interfere with certain drugs such as HIV medications, and now

researchers think they have discovered why. The finding could help

scientists more easily identify other drugs that should not be mixed with

St. 's wort.

Although St. 's wort is sold as a dietary supplement and promoted as a

natural antidepressant, it does carry some drug-like risks. Chronic use of

the herb can block the action of vital medications such as protease

inhibitors for HIV and anti-rejection drugs used after organ transplants.

The exact reason for these interactions has been unclear, but researchers at

Tufts University in Boston, Massachusetts, have found that St. 's wort

appears to boost the activity of a " drug transporter " in the intestines

called P-glycoprotein. The transporter moves drugs from areas of absorption

to regions where they are more likely to be excreted, resulting in lower

blood levels of the drug.

This revved-up drug transporter may prevent certain drugs from being

sufficiently absorbed by the body, Dr. Greenblatt explained in an

interview.

He and his colleagues presented their findings last week in Orlando,

Florida, at a meeting of the Federation of American Societies for

Experimental Biology.

In experiments with intestinal cells that were exposed to St. 's wort,

Greenblatt's team found that P-glycoprotein levels rose in tandem with

increasing doses of the herb. At the highest St. 's wort concentration,

the cells' P-glycoprotein shot up 300%.

Moreover, when these cells were exposed to drugs that are moved by

P-glycoprotein, the drug transporter ushered away far more of the drugs than

it normally would. Although a one-time dose of St. 's wort would not

produce such effects, Greenblatt noted, chronic use over a couple of weeks

could.

And while HIV and transplant patients are already warned not to take St.

's wort because of its known interactions, Greenblatt said the herb may

also interfere with other drugs transported by P-glycoprotein. These include

certain painkillers called opiates, a number of cancer drugs and some

antibiotics.

Knowing precisely how St. 's wort blocks drugs should allow researchers

to screen particular medications for interactions with the herb, rather than

catching such effects in people, according to Greenblatt.