Grapefruit juice increases the plasma levels of many drugs which are likely metabolized by
intestinal CYP3A4 activity (Table 19.3). Furthermore, grapefruit juice did not affect metabolism
of quinine, which is mainly metabolized by CYP3A4 in the liver, thus supporting the
view that grapefruit affects intestinal cytochrome P450 [63]. However, while a single intake
of grapefruit juice may alter intestinal CYP3A4 [55, 62, 64], further investigations have found
that repeated consumption may proceed to inhibit hepatic CYP3A4 [65, 66]. In contrast,
grapefruit juice did not affect the pharmacokinetics or hemodynamics, such as elevated heart
rate or blood pressure, of caffeine [67], which is a CYP1A2 substrate.
While inhibition of CYP3A4 delays drug metabolism and consequently increases plasma
levels of affected drugs, grapefruit juice may also affect drug transport. In regard to drug
efflux, grapefruit juice did not significantly affect digoxin pharmacokinetics, and thus is
unlikely to be an important inhibitor of P-gp function [68]. However, one in vitro study has
demonstrated impaired uptake of the antihistamine fexofenadine in epithelial cells transfected
with OATP1A2 [69]. The possible role of grapefruit juice as an inhibitor of OATP
was further extended in clinical studies [70e72]. Indeed, this pathway of inhibition by grapefruit
juice may play an important role in regard to drugs, such as fexofenadine, that are not
metabolized by either hepatic or intestinal CYP3A4 [73].
juice-induced increase in felodipine may have depended on the route of administration
where drug levels were affected by oral intake as compared to intravenous injection [62].