Dangerous interactions between medications arise frequently from one drug impairing the elimination of another, most commonly via inhibition of one or more cytochrome P450 (CYP) enzymes. Consequently, regulatory agencies recommend thorough characterization of CYP inhibition properties of new chemical entities, at least during preclinical development [1
]. In contrast, CYP inhibition potential of most herbal remedies/dietary supplements is not evaluated prior to marketing, since such products are not regulated in the same manner as drugs, even though they consist of mixtures of diverse chemical entities [2
Grapefruit juice (Citrus paradisi
, Macf.) is an extensively studied dietary substance shown to inhibit enteric (i.e., gut) CYP3A in a mechanism-based, irreversible (so-called “suicide substrate”) manner [2
]. Increased drug systemic exposure can lead to untoward effects. Consequently, package inserts of several drug products contain cautionary statements regarding concomitant grapefruit juice intake. Furanocoumarins, including bergamottin and 6′,7′-dihydrobergamottin, have been identified as major candidate CYP3A inhibitors using various human-derived in vitro
]. A clinical study involving a “furanocoumarin-free” grapefruit juice and the model CYP3A substrate, felodipine, established furanocoumarins, in aggregate, as key mediators of the felodipine-grapefruit juice interaction [5
]. These observations may be extended to other drugs that undergo extensive first-pass metabolism by enteric CYP3A, including some calcium channel antagonists (e.g., felodipine, verapamil), HMG CoA reductase inhibitors (e.g., simvastatin, lovastatin), and immunosuppressants (e.g., cyclosporine, tacrolimus).
Protocols, regulations, or even general strategies to investigate interactions between drugs and dietary substances are in nascent stages of development [2
]. Recognition of this knowledge gap prompted our research group to investigate potential drug-diet interactions prospectively. Inhibition of enteric CYP3A-mediated metabolism has been a primary focus, as dietary substances enter the body by the oral route and are most likely to alter drug absorption/elimination processes in the intestine [2
]. Moreover, dietary substances used as herbal remedies were selected, since patients often mix folkloric and prescription medications, often unbeknownst to their physicians and/or pharmacists [6
]. Finally, we have approached drug-diet interactions from the vantage points of both natural products and drug metabolism sciences, melding bioactivity-directed fractionation with in vitro
, and ultimately, in vivo
CYP phenotyping methods. This merger of disciplines should allow identification of causative ingredients, as well as underlying mechanisms, that contribute to clinically relevant drug-dietary substance interactions in a time-efficient manner.
To test the aforementioned strategy, a natural products/drug metabolism approach was used to investigate five different cranberry juices as inhibitors of intestinal CYP3A activity [8
]. Cranberry juice is used commonly, and often prophylactically, for urinary tract infections (UTIs), particularly by women and the aged. This folkloric treatment of UTIs has been ascribed to proanthocyanidins with A-type linkages [9
], which have been shown to inhibit adhesion of bacterial fimbriae to uroepithelial cells (reviewed in [10
]). In addition, some data have emerged regarding benefits of cranberry products to mitigate some cancers and vascular and dental diseases [11
], likely stimulating cranberry product consumption. Yet, only a few drug-cranberry juice interaction studies have been reported, of which results were inconclusive. For example, in rats, cranberry juice was as effective as grapefruit juice in enhancing systemic exposure of the CYP3A substrate nifedipine [14
]. Alternatively, a clinical study involving a different CYP3A substrate (cyclosporine) and cranberry juice indicated no interaction [15
]. Similarly, a separate clinical study reported that cranberry juice had no effect on the pharmacokinetics of the CYP3A probe substrate midazolam [16
]. Although caveats to these studies were discussed previously [8
], until a year ago, the literature suggested that cranberry juice has a drug interaction liability for rats, but not humans, thus having no clinical concerns. While these inconsistencies may demonstrate a lack of a drug-cranberry juice interaction, an alternative explanation, as described and demonstrated [8
], is that inherent variability in chemical constituents in study materials could lead to disparate results. Similarly, inconsistency in study materials has been cited as a reason why studies of clinical benefits of cranberry have been inconclusive [17
]. Indeed, as reported by our research group [8
], a commercial product was identified that showed a significant interaction with midazolam via both in vitro
assays and a proof-of-concept clinical study.
Inhibition of enteric CYP3A by some cranberry juices suggests a potential for pharmacokinetic interactions with medications that undergo extensive intestinal first-pass metabolism by CYP3A. To identify potential intestinal CYP3A inhibitory constituents from cranberry in vitro, dried whole cranberries [Vaccinium macrocarpon (Ericaceae)] were carried through a bioactivity-directed fractionation approach involving midazolam and human intestinal microsomes (HIM). Three compounds were characterized, and IC50 values were determined using HIM and recombinant CYP3A4.