Abstract

Introduction

The grapefruit, thought to be a cross between an orange and a shaddock, was developed in the West Indies in the early 1700s and first introduced to Florida in the 1820s. Since the early part of the 20th century, mutant strains of white grapefruit have appeared with pink to slightly reddish colour, and have been propagated by citriculturists into several strains of grapefruit. The three major types of grapefruit that exist today are white, pink/red and ruby/rio red varieties. Grapefruit juice combines the sweet and tangy flavour of the orange and shaddock and also provides up to 69% of the RDA for vitamin C along with as many as 250 mg of Potassium [1].

However, the wide consumption of grapefruit juice cannot entirely be attributed to its taste, and nutritive value. In fact, much of the enthusiasm in its use stems from medical research that has suggested that grapefruit juice reduces atherosclerotic plaque formation [2] and inhibits breast cancer cell proliferation and mammary cell tumorigenesis [3,4]. Traditionally grapefruit juice has been found to contain antioxidant, antinitrosaminic, antiseptic, aperitif, cardiotonic, detoxicant, hypocholesterolemic, sedative and stomachic activities. In the light of its above activities, it has been traditionally indicated throughout time for anorexia, bacteria, benign prostatic hypertrophy, cancers (breast, colon, prostate, lung, skin and throat), candida, cold, diabetes, dysuria, high cholesterol, infection, insomnia, mycobacterium, mycosis, nervousness, pseudomonas, rheumatism, staphylococcus and yeast.

However, as many as fifteen years ago, investigators found that grapefruit juice can markedly augment oral drug bioavailability. This was an unexpected observation from an interaction study between the dihydropyridine calcium channel antagonist, felodipine, and ethanol in which grapefruit juice was used as a flavour supplement to mask the taste of the ethanol [5]. Studies that followed, confirmed that grapefruit juice significantly increased the oral bioavailability of felodipine [6,7]. Subsequent studies probed the constituents of grapefruit juice, its interaction with various other drugs and the mechanisms of action of those interactions. Several grapefruit juice-drug interactions were discovered and these remain a potential concern especially since the juice and drugs are often consumed together at breakfast. An increasing number of adverse drug reactions might be avoided on the basis of knowledge about the interaction of grapefruit juice and relevant drugs. Therefore, patients need to be educated about the hazards (and advantages) of grapefruit interaction with medication. In recent years, more drugs have been investigated for their interaction with grapefruit juice and new models have been proposed for the mechanism of such interaction. This article presents a simplistic summary of most examples of such interactions and also explores the phytochemistry and possible mechanisms of action involved in drug-grapefruit juice interactions in light of recent studies on this subject.

Mechanism of action

The mechanism of action of this interaction involves inhibition of the CYP 3A4, a member of the cytochrome P 450 (CYP) enzyme system. CYP is a large multigene family of heme-containing enzymes located in the endoplasmic reticulum of cells throughout the body. It is especially concentrated in the liver and intestinal wall where it is involved in oxidative biotransformation of various endogenous and exogenous substances. CYP 3A isoforms constitute 70% of CYP enzymes in enterocytes [8,9]. P-glycoprotein (Pgp), a member of the ABC (adenosine triphosphate-binding cassette), is another membrane transporter located in the apical brush border of enterocytes. Once taken up by the enterocytes, a lipophilic drug may be metabolized by CYP 3A4 or be pumped back into the lumen by the Pgp. Hence the oral delivery of many drugs is limited by the actions of CYP 3 A4 or Pgp. Metabolism by the CYP 3A4 will also occur in the liver before the drug finally enters the systemic circulation. Grapefruit juice causes inhibition of CYP 3A4 and thus serves to increase the bioavalability of the drug by decreasing its pre-systemic metabolism [10]. This action is in essence, similar to that caused by CYP-inhibiting drugs like itraconazole, ketoconazole and erythromycin [11–13].

Grapefruit juice causes quick and irreversible sustained inhibition of the CYP system, possibly by greatly accelerating the degradation of these enzymes while also reducing translation from its mRNA. However, the process of transcription of mRNA from the cell DNA is not affected. Overall, grapefruit juice reduces the levels of CYP 3A4 in the cells by as much as 47% within four hours of ingestion of grapefruit juice with the resultant increased bioavailability being maintained for as long as 24 hours, by which time 30% of its effect is still detectable [14–17]. It has been observed that decreased content of CYP3A4 was not associated with increased CYP3A4 mRNA, probably indicating the absence of a feedback mechanism for CYP3A4 expression. Restoration of CYP3A4 activity would therefore require denovo synthesis or enterocyte replacement, accounting for the prolonged duration of the actions of grapefruit juice [18].

Grapefruit juice shows a high variability of the magnitude of effect among individuals. This variability is dependent upon inherent differences in enteric CYP3A4 protein expression such that individuals with highest baseline CYP3A4 have the highest proportional increase [19,20]. However, the effects of grapefruit juice are predominantly on the intestinal CYP rather than hepatic CYP. This is shown by the fact that most of the drugs that are involved in interaction with grapefruit juice undergo their primary metabolism at the intestinal level and in usual quantity, grapefruit juice does not affect the pharmacokinetics of these drugs when they are administered intravenously. Furthermore, while it increases the area under the plasma concentration-time curve (AUC), it has no significant effect on the half life of the drugs [10,21–23].

In contrast to the clear inhibitory effects of grapefruit juice on CYP 3A4, the effects of grapefruit juice on Pgp are controversial, ranging from activation to inhibition. Earlier results have shown grapefruit juice to cause activation of Pgp in vitro [24]. Any such activation in vivo will mean a greater efflux of the drug back into the lumen, thereby decreasing the oral bioavailability of that drug and at least partially, if not completely offsetting the effects produced by the inhibition of CYP system of enzymes. This is taken as an explanation for the less-than-expected increase in the bioavailability of drugs that are established substrates of Pgp [24]. However, grapefruit juice does not change the absorption of digoxin, a prototypical P-glycoprotein substrate, likely because it has high inherent oral bioavailability [17,25]. However, recent studies have demonstrated the inhibition of Pgp by grapefruit juice both by its down-regulation and inhibition of function [26,27]. For example, grapefruit juice increases the bioavailability of cyclosporine. This effect is thought to be primarily though Pgp inhibition (instead of CYP3A4 inhibition) since orange juice mediated reduction in enterocyte CYP3A4 concentrations did not produce a similar increase in bioavailability [17]. In fact, grapefruit juice has also shown inhibition of multidrug resistant protein 2 (MRP2), an efflux protein closely related to Pgp in terms of its expression and function [26].

Yet, in spite of all what is known, the mechanism of action of grapefruit juice-drug interaction requires further investigation. Investigators still need to determine for certainty any in vivo effect of grapefruit juice on Pgp. One study [28] has also reported the action of grapefruit juice independent of its actions on Pgp and CYP 3A4. This also requires further investigation. Similarly, grapefruit and even orange juice have also recently been shown to be potent in vitro inhibitors of a number of organic anion-transporting polypeptides (OATPs) that are involved in apical-to-basal transport of drugs in the small intestine [17,18,25,29]. They were also found to decrease the absorption of the non-metabolized OATP substrate, fexofenadine hence pointing towards inhibition of intestinal uptake transporters by fruit juices to decrease drug bioavailability. This newly proposed mechanism of action and its effect vis a vis various medications also demands further investigation [25,29].

Assessment of the in vitro CYP inhibition potential for these natural products has important implications for predicting the likelihood of natural product-drug interactions if these products are taken concomitantly. The susceptibility of CYP3A4 to modulation by food constituents may be related to its high level of expression in the intestine, as well as its broad substrate specificity. Reported ethnic differences in the activity of this enzyme may be partly due to dietary factors. Food-drug interactions involving CYP1A2, CYP2E1, glucuronosyltransferases and glutathione S-transferases have also been documented, although most of these interactions are modest in magnitude and clinically relevant only for drugs that have a narrow therapeutic range. Recently, interactions involving drug transporters, including P-glycoprotein and the organic anion transporting polypeptide, have also been identified. Hence a lot of food varieties have the potential to require dosage adjustment to maintain drug concentrations within their therapeutic windows, especially with drugs that have a high first pass degradation [30]. Further research is needed to determine the scope, magnitude and clinical importance of food effects on drug metabolism and transport.

Relevant phytochemistry

Another area in which the search for definite answers continues, is the quest to find the active constituents of grapefruit juice that are responsible for its actions on CYP enzyme systems and Pgp. The components of grapefruit juice that are responsible for clinical drug interactions have yet to be fully determined but the compounds thought to be responsible for this action include flavonoid glycosides (narirutin, naringin, naringinen, quercetin, kaemferol, hesperidin, neohesperidin, didymin, and poncirin) [8,31–34], furanocoumarins (6',7'-dihydroxybergamottin, bergamottin) and sesquiterpen (nootkatone)[8,22,32,35,36].

Flavanoids exist in grapefruit juice in the form of glycosides, with naringin being the most abundant. Upon ingestion, these are converted to aglycones and sugars by the action of intestinal flora. Being polyphenolic and electron rich, these compounds can theoretically inhibit the CYP enzymes. However, studies have at most shown an in vitro effect by these compounds on the these enzymes and have failed to identify any in vivo effect by them [37,38], leading to an implication that they are probably not the main active ingredients of grapefruit juice [1,39]. Studies have even failed to demonstrate any sort of activity in naringin although its metabolite naringinin was observed to be active in vitro. Yet, because of their huge quantities in grapefruit juice, and the fact that naringin is not present in other citrus juices, flavanoids remain a subject of research.

The main focus at present, however, is on furanocoumarins. This group includes Bergamottin, its derivative 6' 7' dihydroxybergamottin (DHB) and a host of other compounds [40]. Controversy still exists on the degree of their role in the inhibitory effects of grapefruit juice. Several studies have shown DHB [23,35,40] and to an extent Bergamottin [23] to be important contributors to the grapefruit juice effect. In one study, the inhibitory potency of DHB and four recently isolated furanocoumarins, when mixed with one another, almost approached that of grapefruit juice. Omission of any of the components resulted in decreased potency, suggesting that all major furanocoumarins contribute to the inhibitory effects of grapefruit juice [40]. However, others have suggested that DHB and Bergamottin are not the primary substances responsible for inhibition of CYP activity clinically [41,42]. For now, this topic also remains a subject of intense research.

Drug-grapefruit juice interactions

Conclusion

In light of the wide ranging effects of grapefruit juice on the pharmacokinetics of various drugs, physicians need to be aware of these interactions and should make an attempt to warn and educate their patients regarding potential consequences of concomitant ingestion of these two items. Patient-to-patient variability should be kept in mind and elderly should be particularly warned about these interactions since they are more prone to grapefruit juice-drug interactions [17]. Physicians should also consider using these effects to their own advantage in order to reduce the dosage requirements of certain drugs. However, since further research is required into the mechanism of action of grapefruit juice, it is still premature to recommend it as an adjunctive booster with other drugs.

Abbreviations

AUC = area under the plasma concentration-time curve

CYP = cytochrome P -450

DEA = desethylamiodarone

DHB = dihydroxybergamottin

HMG – CoA = 3 – hydroxyl – 3 – methylglutaryl coenzyme A

NFP = nifedipine

OATP = organic anion-transporting polypeptides

Pgp = P – glycoprotein

RDA = Recommended daily allowance

SV = simvastatin

Competing interests

The author(s) declare that they have no competing interests.

Authors' contributions

Both authors contributed equally.

Acknowledgements