Citrus paradisi is the botanical name of grapefruit, whose juice is widely known to affect drug bioavailability in humans. Grape-
fruit juice contains flavonoids, a large class of plant polyphenolic secondary metabolites that have various pigmental and antimicro- bial functions, and which are also found in many other plants and vegetables. They are chemically classified as flavones, isoflavones, flavan-3-ols and anthocyanidins, and are studied for their antiox- idant, anti-inflammatory and antimicrobial activity and ability to prevent cardiovascular disease [113,114].
The fact that grapefruit juice raises the bioavailability of many drugs has been attributed to the irreversible inhibition of intestinal CYP3A4 and P-gp, in view of the fact that intravenously adminis- tered drugs are not affected. A large number of flavonoid studies have revealed in vitro pharmacokinetic interference [115]. How- ever, the main inhibitory effect on intestinal CYP3A4 and P-gp is attributed to some furanocoumarins and bergamottin, and the inhi- bition of OATP1A2 to naringin, which together could lead to some clinically important pharmacokinetic alterations [75,116–120].
Many studies have confirmed the high risk of drug interactions with grapefruit juice [121–124]. Gleaser et al. analysed the effect of grapefruit juice on intestinal transporters and CYP3A4, and found it to change the availability of fexofenadine (an OATP1A2 and P-gp probe drug) in human volunteers, but the expression of OATP1A2 and P-gp was unaffected [125].
The clinical consequences described by Rashid et al. are also of interest: these authors showed that the co-administration of a 10 mg nifedipine capsule and 200 ml of double-strength grapefruit juice led to significantly greater absolute nifedipine bioavailability as a result of a reduction in pre-systemic metabolism. Furthermore, their results indicated that the AUC of nifedipine is larger, and its plasma concentration higher in Southern Asians than in Caucasians, confirming ethnic differences in the pharmacokinetics of nifedipine [126].
Other fruit juices do not appear to change drug pharmacoki- netics in any similar clinically significant way, but many in vitro experiments have indicated that the absorption or metabolism of drugs may be affected; this should be tested in clinical trials [127].
Diosmin is a flavonoid found in many citrus species, and sig- nificantly inhibits P-gp-mediated efflux in vitro [128]. In fact, diosmin intake may increase the absorption of co-administered P- gp substrate drugs. Diosmin has also been shown to enhance the absorption of the poorly absorbed