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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
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
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
"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.