Grapefruit juice is rich in a number of phytochemicals, including flavonoids and furanocoumarins. The most abundant flavonoid in the juice is naringin, with reported concentrations ranging from approximately 200 to 2000 μmol/L [11
]. Due to their abundance and in vitro
inhibition of CYP3A, the flavonoids were originally presumed to be the GFJ constituents responsible for mediating drug interactions. However, administration of naringin capsules, in the same amounts found in GFJ, failed to alter the pharmacokinetics of the CYP3A substrates, felodipine and nisoldipine [13
]. A similar study involving another flavonoid, quercetin, also failed to demonstrate an effect on nifedipine disposition [15
]. Based on these findings and more recent work, the flavonoids are no longer thought to play a major role in GFJ-drug interactions involving CYP3A substrates.
The furanocoumarins (FCs, ) are a structurally distinct class of compounds found in GFJ. All FCs contain a three-ring “head” and, except for bergaptol and bergapten, an aliphatic “tail.” The FCs differ in the presence and composition of their aliphatic tail, which in turn influences their inhibitory potency toward CYP3A. In addition, the furan ring moiety has an important role in the generation of a reactive intermediate – a furanoepoxide or γ-ketoenal – that irreversibly binds to CYP apoprotein and eliminates enzymatic activity [16
Figure 1 Structures of GFJ constituents implicated in clinical drug interactions. Left panel: The furanocoumarins are potent, mechanism-based, inhibitors of CYP3A. Mechanism-based inhibition of CYP3A is thought to result after binding of CYP protein to either (more ...)
Of the various FCs present in GFJ, bergamottin and 6′,7′-dihydroxybergamottin (DHB) have been the most-extensively studied regarding their capacity to mediate GFJ-drug interactions (). Reported concentration ranges for bergamottin and DHB in GFJ are 1-37 μM and 0.2-52.5 μM, respectively [11
]. In addition, GFJ contains FC dimers – also known as spiroesters or paradisins – that are formed through either head-to-tail or tail-to-tail linkage of DHB to itself, or to bergamottin. Although present at lower concentrations than bergamottin and DHB, these dimers are potent inhibitors of CYP3A in vitro
The variability in the concentrations of flavonoids and FCs in GFJ may result from factors such as (1) the type, origin, and quality of the grapefruits used to make the juice; (2) the manufacturing process; and, (3) storage conditions [11
]. Exposure of GFJ to ultraviolet (UV) light or heat has been shown to alter the concentrations of FCs in the juice [22
]. The concentrations of bergamottin and DHB rapidly declined after being irradiated with UV light, whereas the bergaptol levels decreased more slowly. After 6 hours of UV exposure, the concentrations of bergaptol, DHB, and bergamottin were reduced to 6%, 2%, and 2% of their baseline values, respectively [22
]. The same investigators also demonstrated that bergamottin and DHB are unstable at elevated temperatures. Although exposure to 4°C and 37°C for one hour failed to change the bergamottin and DHB concentrations in the juice, their levels declined when the juice was heated to 62°C, 72°C, and 95°C [23
]. More specifically, 95°C exposure for one hour caused the bergamottin concentration to decrease from 17.9 μM to 3.14 μM, and the DHB concentration to fall from 7.85 μM to 0.16 μM. Bergaptol concentrations concurrently increased by 14.1 μM under the same treatment conditions, suggesting that bergamottin and DHB are degraded to bergaptol by exposure to heat. The UV- and heat-treated GFJ did not interact with nifedipine after administration to rats [22
We evaluated the effect of prolonged storage at room temperature on the furanocoumarin content and the CYP3A-inhibiting capacity of a single GFJ sample. Aliquots were removed and analyzed at multiple time points over a one year period. Furanocoumarin concentrations were determined by liquid chromatography-mass spectroscopy, and in vitro inhibition of CYP3A in human liver microsomes was determined using the index substrate triazolam. Concentrations of DHB and paradisin C declined substantially with time (). Bergamottin concentrations were relatively stable, while bergaptol concentrations increased. In the same samples, reversible and irreversible CYP3A inhibition declined with time in parallel with the decrement in DHB and paradisin C concentrations (). The findings confirm the assumption that DHB and paradisins in GFJ are mainly responsible for CYP3A inhibition. The results also suggest that drug interactions with GFJ are less likely with GFJ that has undergone extensive storage at room temperature.
Time-dependent changes in concentrations of bergaptol, bergamottin, DHB, and paradisin C in aliquots of a GFJ sample stored at room temperature for one year.
Figure 3 In the same GFJ sample described in , time-dependent changes in the capacity of the aliquots to inhibit CYP3A activity – represented as triazolam hydroxylation activity – by human liver microsomes in vitro. Lower values on the (more ...)
The complex chemistry of GFJ makes its use in drug-interaction studies challenging. The majority of investigations to date have not provided a phytochemical analysis of the juice, instead reporting that “the same lot of juice was used throughout the study.” This assures some degree of product quality control, but does not allow comparison of results from different studies. Ideally, future studies should report the chemical composition of the GFJ being administered, especially the bergamottin, DHB, and paradisin concentrations when CYP3A inhibition is anticipated. When the mechanism of interaction is presumed to be OATP inhibition, quantifying the amount of naringin in the juice appears warranted [10