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Grapefruit Juice Components Involved in Drug Interactions

A number of constituents in grapefruit juice are thought to be involved in the interaction between grapefruit juice and drugs. Current research suggests two different components may be involved. One such component is a flavonoid. Flavonoids are polyphenolic compounds possessing fifteen carbon atoms. They possess two benzene rings joined by a linear carbon chain as seen below:

The flavonoids have aroused recent interest because of their potential beneficial effects on human health. Flavonoids have been reported to have antiviral, anti-allergic, anti-platelet, anti-inflammatory, anti-tumor and antioxidant activities. The flavonoid thought to cause drug interactions in grapefruit juice is naringin. Naringin is the compound that gives grapefruit its characteristic bitter flavor. In commercial grapefruit juice preparations, naringin is often lower than normal concentrations. This is because grapefruit processors attempt to select fruits with a low naringin content, and often blend juices obtained from different grapefruit varieties to obtain the a lesser degree of bitterness (Guo et al, 2000).

The major furanocoumarin in grapefruit is called bergamottin. Its metabolite is 6',7'-dihydroxybergamottin and has also been implicated in inhibiting CYP3A4.

It is likely that no single component of grapefruit juice is entirely responsible for all the grapefruit drug interactions observed in vivo. There is perhaps a synergistic effect of several grapefruit juice components on the drug metabolism system. The identification of key elements responsible for effects on drug metabolism remains a subject for further research.

The furanocoumarins are all present in the peel of grapefruit and can be isolated using standard chemical extraction procedures (Guo et al, 2000). Most furanocoumarins that have been isolated show some efficacy in inhibiting CYP3A4.

Figure 20: Furanocoumarins of grapefruit and their effects on CYP3A4 in the presence of the substrate testosterone (Guo et al, 2000)

Chemists measured CYP3A4 activity in the presence of two concentrations of testosterone, a substrate of CYP3A4. As shown in Figure 20, whole grapefruit juice has the largest overall inhibitory effect on CYP3A4. The furanocoumarins as a group also have a large inhibitory effect on CYP3A4.

This study suggests that all the major furanocoumarins are necessary for maximal inhibition of CYP3A4 activity. The contribution of each component varies in inhibitory potency and natural abundance in the peel. It has also been shown that furanocoumarins derived from other citrus fruits other than grapefruit can elicit the same inhibitory response on CYP3A4.

Another study was conducted looking specifically at grapefruit oil. Grapefruit oil is derived from the peel of grapefruits and is often used as a flavor enhancer in commercial preparations of grapefruit juice. However, the oil is rich in furanocoumarins, and may account largely for the inhibition of CYP3A4 by commercial grapefruit juices.

Figure 21: Three components of grapefruit juice and their effects on CYP3A4 versus ketoconazole, a known inhibitor of CYP3A4 (Schmiedlin-Ren et al, 1997)

Using intestinal cells that express CYP3A4, researchers used grapefruit oil, a metabolite of bergamottin, and a furanocoumarin dimer to observe effects against a known inhibitor of CYP3A4, ketoconazole. The immunoblots in Figure 21 that measure the levels of CYP3A4 after several different dosages of each substance show that even in very small amounts, grapefruit oil is a potent inhibitor of intestinal CYP3A4.

The addition of grapefruit oil to frozen concentrates of grapefruit juice may mean that fresh juice and reconstituted juices would have very different effects on drug grapefruit interactions. This data emphasizes the importance in standardizing grapefruit preparations in experiments involving grapefruit juice and drug interactions. Furthermore, the variability of grapefruit juice preparations in clinical studies or further experiments should be taken into account and the concentration of active ingredients such as bergamottin and other furanocoumarins should be standardized in the future research.

Flavonoids are abundant in grapefruit juice. A recent comprehensive study examined the effects of a wide range of flavonoids on CYP3A4. Results showed that the flavonoids galangin, naringin, and pruning had the greatest inhibitory effects. Each of these flavonoids inhibited CYP3A4 in a concentration-dependent manner, with 200uM naringenin showing the highest inhibition.

Figure 22: General structure of flavonoids isolated from grapefruit juice (Ho et al, 2001)

The molecular conformation of the flavonoids that demonstrated the highest inhibitory effects is important. The position of the hydroxyl groups on the A and B rings seem to be important for interacting with the enzyme. A hydroxyl group at position 7 in Figure 22 (which is present in galangin, naringin, and prunin), may interact with the Fe(III) heme iron of CYP3A4.

Flavonoids lacking a free hydroxyl substituent on ring A at the R position in Figure 20 had no inhibitory effects on the enzyme.
This type of chemical information could be vital to custom designing future drugs to inhibit CYP3A4. The Ho et al study confirms that the flavonoids in grapefruit juice contribute to the inhibitory effects of grapefruit juice on CYP3A4.