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Patients taking certain medications are usually told to avoid grapefruit juice because it can boost drug absorption, sometimes leading to a serious, even deadly, overdose.

But a new study suggests the bittersweet fruit can also make at least one medication a more potent cancer fighter.

"It really looks like [grapefruit juice] can modify this drug and we think this could be a potentially effective regimen going forward," said the lead researcher, Ezra Cohen, a cancer specialist at the University of Chicago.

More than two decades ago, doctors discovered that grapefruit juice inhibits the action of enzymes that normally break down certain drugs as they pass through the intestines. That essentially means the medication remains intact longer and more of it is available to be absorbed into the bloodstream – potentially triggering a drug overdose.

But in the case of a cancer drug called sirolimus, "we saw this potential interaction not as a problem but as an opportunity," recalled Dr. Cohen.

Sirolimus is usually given in very high doses because it is not easily absorbed by the body. And the high dose can cause debilitating side effects including nausea and diarrhea.

Dr. Cohen and colleagues carried out a series of experiments to see if grapefruit juice could be used in a controlled fashion to increase the absorption of sirolimus – while also reducing the dose.

They recruited 138 cancer patients who were treated with different levels of the medication, some in combination with the juice.

Their findings, published in the journal Clinical Cancer Research, revealed that a glass of grapefruit juice lets patients derive the same benefits from sirolimus as they would get from three times as much of the drug by itself.

Dr. Cohen said the grapefruit-sirolimus combo could help patients avoid the side effects associated with taking high doses of the drug. What's more, it brings down the overall cost of treatment because less medication is required.

This represents the first time that a food-drug interaction has been harnessed for cancer treatment. The same approach may also work for other medications, Dr. Cohen speculated.

Despite these promising initial results, the researchers are having trouble raising additional money to do followup work on sirolimus.

The drug, also known as rapamycin, has been on the market for decades and its patent for use in cancer has expired. So it's not a big money maker.

It was the first in a series of drugs, known as mTOR inhibitors, developed to prevent organ rejection in transplant patients. It was later found that the drug also has anti-cancer properties.

Several newer drugs, which are chemically similar to sirolimus, have been developed in recent years to treat a variety of malignancies, including cancers of the kidney and gastrointestinal tract.

Some of these newer medications cost as much as $5,000 a month to treat a cancer patient, compared with $1,000 a month for sirolimus.

The relatively lower financial return adds to the challenge of attracting investors. "This is what we are struggling with now," said Dr. Cohen. "We have approached several public and government organizations including the NIH [U.S. National Institutes of Health] and Health Canada, but have not been successful getting funding."