Grapefruit juice, lyophilized grapefruit juice, and powdered whole grapefruit inhibit cytochrome P450-mediated triazolam hydroxylation by beagle dog liver microsomes

Quantifying four-probe metabolites in a single UPLC–MS/MS run to explore the effects of cooked rhubarb on cytochrome P450 isozymes

Background: Cytochrome P450 (CYP) enzymes primarily account for the oxidation of xenobiotics. A single run UPLC–MS/MS method was developed to simultaneously detect four metabolites from a probe cocktail to investigate four CYP isozymes. Paracetamol was detected to investigate CYP1A2, 4-hydroxytolbutamide for CYP2C6, 6-hydroxychlorzoxazone for CYP2E1 and dehydronifedipine for CYP3A1. 6-hydroxychlorzoxazone was detected in ESI-negative mode, while the other three were detected in ESI-positive mode. Results: The method was rapid (2.5 min) and validated to be specific, accurate and precise. It was successfully applied to evaluate the effects of cooked rhubarb on the activities of the four isozymes in rat microsomes. Cooked rhubarb inhibited the activity of CYP2C6 but induced both CYP2E1 and CYP3A1. Conclusion: This convenient UPLC–MS/MS method was applicable. It could improve the efficiency of future studies estimating the influence of herbs, herbal products or drug entities on the activities of CYP1A2, CYP2C6, CYP2E1 and CYP3A1.

Mechanism-based inhibition of human cytochrome P450-3A activity by grapefruit hybrids having low furanocoumarin content

A citrus breeding program aimed at developing low furanocoumarin (FC) grapefruit cultivars provided 40 grapefruit juice (GFJ) samples containing variable concentrations of FC derivatives, established as being mechanism-based (irreversible) inhibitors of human CYP3A isoforms. The principal inhibitory FCs were identified as 6',7'-dihydroxybergamottin, along with a series of dimeric compounds (spiroesters) having high inhibitory potency. A random subset of the GFJ samples (n = 25) were tested as CYP3A inhibitors using an in vitro model based on human liver microsomal metabolism of the index substrate triazolam. The reciprocal values of in vitro 50% inhibitory concentrations (IC(50)) were highly correlated with concentrations of inhibitory FCs in the GFJ samples (r(2) = 0.96). However the correlations were driven mainly by a few samples having high FC content and high reciprocal IC(50) (corresponding to low IC(50)). Among the rest of the samples, the relationship was less robust. Further study is needed to determine how low the FC content needs to be (or how high the IC(50) needs to be) to assure minimal risk of clinical interactions involving GFJ and CYP3A substrate drugs.

Effects of powdered whole grapefruit and metoclopramide on the pharmacokinetics of cyclosporine in dogs

OBJECTIVE

To determine whether oral administration of metoclopramide or a commercially available powdered whole grapefruit (PWG) nutraceutical in combination with cyclosporine enhances systemic availability of cyclosporine in dogs.

SAMPLE

8 healthy mixed-breed dogs in part 1 and 6 of these 8 dogs in part 2.

PROCEDURES

Cyclosporine pharmacokinetics were determined over the course of 24 hours after oral administration of cyclosporine (5 mg/kg) alone, cyclosporine with metoclopramide (0.3 to 0.5 mg/kg), cyclosporine with 2 g of PWG, or cyclosporine combined with both metoclopramide and 2 g of PWG by use of a Latin square crossover study with a 14-day washout period between treatments. Sixty days later, 6 of the 8 dogs were given 10 g of PWG followed by cyclosporine, and pharmacokinetic parameters were compared with those previously obtained after administration of cyclosporine alone.

RESULTS

Although metoclopramide or coadministration of metoclopramide and 2 g of PWG had no effect on the pharmacokinetic parameters of cyclosporine, compared with results for cyclosporine alone, the higher (10-g) dose of PWG resulted in 29% faster mean time to maximal plasma cyclosporine concentration, 54% larger area under the curve, and 38% lower apparent oral clearance.

CONCLUSIONS AND CLINICAL RELEVANCE

Adjustment of the cyclosporine dose may not be needed when metoclopramide is coadministered orally to prevent common adverse effects of cyclosporine. Powdered whole grapefruit has the potential to reduce the required orally administered dose of cyclosporine but only when PWG is used in an amount (at least 10 g) that is currently not cost-effective.

The effect of grapefruit juice on drug disposition

INTRODUCTION

Since their initial discovery in 1989, grapefruit juice (GFJ)-drug interactions have received extensive interest from the scientific, medical, regulatory and lay communities. Although knowledge regarding the effects of GFJ on drug disposition continues to expand, the list of drugs studied in the clinical setting remains relatively limited.

AREAS COVERED

This article reviews the in vitro effects of GFJ and its constituents on the activity of CYP enzymes, organic anion-transporting polypeptides (OATPs), P-glycoprotein, esterases and sulfotransferases. The translational applicability of the in vitro findings to the clinical setting is discussed for each drug metabolizing enzyme and transporter. Reported AUC ratios for available GFJ-drug interaction studies are also provided. Relevant investigations were identified by searching the PubMed electronic database from 1989 to 2010.

EXPERT OPINION

GFJ increases the bioavailability of some orally administered drugs that are metabolized by CYP3A and normally undergo extensive presystemic extraction. In addition, GFJ can decrease the oral absorption of a few drugs that rely on OATPs in the gastrointestinal tract for their uptake. The number of drugs shown to interact with GFJ in vitro is far greater than the number of clinically relevant GFJ-drug interactions. For the majority of patients, complete avoidance of GFJ is unwarranted.