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UI study shows grapefruit juice inactivates enzyme, helps drug absorption

IOWA CITY, Iowa -- There is a secret ingredient in grapefruit juice that helps the body absorb some medications better, even if the form of the drugs have been altered, according to a University of Iowa College of Pharmacy study.

During their investigation, UI researchers also found how the grapefruit juice magic works: The juice inactivates a gastrointestinal tract enzyme that destroys various therapeutic drugs -- including those for warding off organ transplant rejection and others for treating hypertension and anxiety.

Armed with this latest information and data from previous studies, David Min, Pharm.D., UI associate professor of pharmacy, and his colleagues now plan to move toward identifying the active component that causes the grapefruit juice absorption effect. They then hope to develop a way to incorporate the active agent into many drugs.

"This could reduce the required dosage for many treatments and as a result decrease the costs," said Min, who expects researchers to pinpoint the key juice component within the next two years.

Investigators nationwide have studied the grapefruit juice effect for the past several years. In the most recent study, the UI team wanted to test whether grapefruit juice would have an effect on the new, improved cyclosporine -- an anti-rejection medication for transplant patients. Two years ago Min looked at the old version of cyclosporine and found that taking the drug with the juice increased the medication's absorption between 30 and 70 percent depending on the patient population. Even though manufacturers improved the old version to enable the body to absorb cyclosporine more rapidly and completely, the latest UI study showed that there still is a grapefruit juice effect. On average, taking the drug with the juice increased absorption by 45 percent.

The juice inactivates the enzyme CYP3A4 in the intestine. It is this intestinal enzyme that may be responsible for destroying many important therapeutic drugs before they have a chance to enter the bloodstream and perform their intended functions. To offset the effect, more drugs are needed, which can be expensive. The average annual cost for cyclosporine is $6,000.

Although the grapefruit juice effect might tempt some people to begin taking their medication with the juice, Min warned against the practice because the effect is varied, depending on the brand of juice and the concentration of its ingredients. Physicians prescribe dosages based on the normal, decreased absorption rates.

"The problem is grapefruit juice is not a drug so we can't standardize it," Min said. "That is why it is so important to isolate the active agent and attempt to somehow standardize it and incorporate it into the drugs themselves."

The UI team studied 12 individuals. On three occasions, spaced at least seven days apart so as to allow the body to rid itself of the previously administered drug, each subject received one of three treatments: one intravenous dose of the new cyclosporine or an oral dose of cyclosporine either with grapefruit juice or with water. In 10 of the 12 subjects, taking the drug with the juice elevated the average peak concentration of the drug from 2,013 to 2,224 ng/mL and increased the average total amount of drug exposure in the body from 8,073 to 11,902 ng-hr/mL.

Min and his colleagues' latest work appeared in the October issue of the Journal of Clinical Pharmacology.

12/1/98