Effects of grapefruit juice on intestinal P-glycoprotein: evaluation using digoxin in humans

Interactions between grapefruit juice and psychotropic medications: an update of the literature and an original case series

INTRODUCTION

There is a large body of preclinical data implicating that grapefruit juice (GJ) inhibits many CYP 450 isoforms. The potential of GJ-to-drug is of high relevance to clinical psychiatry, because a wide range of psychotropic medicines undergo CYP 450 metabolism and P-gp transport.

AREAS COVERED

Relevant data were identified by searching the electronic databases up to February 2024. This work constitutes a summary of preclinical and clinical data on GJ impact on CYP 450 metabolism, P-glycoprotein, and organic anion-transporting polypeptides (OATPs), with focus on studies that assessed GJ-to-psychotropic drug interactions. Additionally, an unpublished case series of nine patients is provided.

EXPERT OPINION

The impact of GJ on CYP 3A4 appears to be the critical mechanism for the majority of GJ-to-psychopharmacotherapy interactions described in human studies or case reports. However, there are studies and cases of patients clearly showing that this is not the only route explaining the GJ effect, and at times, this particular is of no relevance and that other CYP 450 isoforms as well as drug transporting proteins might be involved. The risk of GJ-to-psychotropic drugs needs to be further evaluated in a 'real-world' setting and apply not only measures of pharmacokinetics but also treatment effectiveness and safety.

Physiologically Based Pharmacokinetic Modeling of Bergamottin and 6,7-Dihydroxybergamottin to Describe CYP3A4 Mediated Grapefruit-Drug Interactions

Grapefruit is a moderate to strong inactivator of CYP3A4, which metabolizes up to 50% of marketed drugs. The inhibitory effect is mainly attributed to furanocoumarins present in the fruit, irreversibly inhibiting preferably intestinal CYP3A4 as suicide inhibitors. Effects on CYP3A4 victim drugs can still be measured up to 24 hours after grapefruit juice (GFJ) consumption. The current study aimed to establish a physiologically-based pharmacokinetic (PBPK) grapefruit-drug interaction model by modeling the relevant CYP3A4 inhibiting ingredients of the fruit to simulate and predict the effect of GFJ consumption on plasma concentration-time profiles of various CYP3A4 victim drugs. The grapefruit model was developed in PK-Sim and coupled with previously developed PBPK models of CYP3A4 substrates that were publicly available and already evaluated for CYP3A4-mediated drug-drug interactions. Overall, 43 clinical studies were used for model development. Models of bergamottin (BGT) and 6,7-dihydroxybergamottin (DHB) as relevant active ingredients in GFJ were established. Both models include: (i) CYP3A4 inactivation informed by in vitro parameters, (ii) a CYP3A4 mediated clearance estimated during model development, as well as (iii) passive glomerular filtration. The final model successfully describes interactions of GFJ ingredients with 10 different CYP3A4 victim drugs, simulating the effect of the CYP3A4 inactivation on the victims' pharmacokinetics as well as their main metabolites. Furthermore, the model sufficiently captures the time-dependent effect of CYP3A4 inactivation as well as the effect of grapefruit ingestion on intestinal and hepatic CYP3A4 concentrations.

Food effects on gastrointestinal physiology and drug absorption

Food ingestion affects the oral absorption of many drugs in humans. In this review article, we summarize the physiological factors in the gastrointestinal (GI) tract that affect the in vivo performance of orally administered solid dosage forms in fasted and fed states in humans. In particular, we discuss the effects of food ingestion on fluid characteristics (pH, bile concentration, and volume) in the stomach and small intestine, GI transit of water and dosage forms, and microbiota. Additionally, case examples of food effects on GI physiology and subsequent changes in oral drug absorption are provided. Furthermore, the effects of food, especially fruit juices (e.g., grapefruit, orange, apple) and green tea, on transporter-mediated permeation and enzyme-catalyzed metabolism of drugs in intestinal epithelial cells are also summarized comprehensively.

Food constituent- and herb-drug interactions in oncology: Influence of quantitative modelling on Drug labelling

AIMS

Herbal products, spices and/or fruits are perceived as inherently healthy; for instance, St. John's wort (SJW) is marketed as a natural antidepressant and patients often self-administer it concomitantly with oncology medications. However, food constituents/herbs can interfere with drug pharmacokinetics, with risk of altering pharmacodynamics and efficacy. The objective of this work was to develop a strategy to prioritize herb- or food constituent-drug interactions (FC-DIs) to better assess oncology drug clinical risk.

METHODS

Physiologically based pharmacokinetic (PBPK) models were developed by integrating in vitro parameters with the clinical pharmacokinetics of food constituents in grapefruit juice (bergamottin), turmeric (curcumin) or SJW (hyperforin). Perpetrator files were linked to verified victim PBPK models through appropriate interaction mechanisms (cytochrome P450 3A, breast cancer resistance protein, P-glycoprotein) and applied in prospective PBPK simulations to inform the likelihood and magnitude of changes in exposure to osimertinib, olaparib or acalabrutinib.

RESULTS

Reported FC-DIs with oncology drugs were well recovered, with absolute average fold error values of 1.10 (bergamottin), 1.05 (curcumin) and 1.01 (hyperforin). Prospective simulations with grapefruit juice and turmeric showed clinically minor to insignificant changes in exposure (<1.50-fold) to acalabrutinib, osimertinib and olaparib, but predicted 1.57-fold FC-DI risk between acalabrutinib and curcumin. Moderate DDI risk was expected when acalabrutinib, osimertinib or olaparib were dosed with SJW.

CONCLUSIONS

A model-informed decision tree based on mechanistic understanding of transporter and/or enzyme-mediated FC-DI is proposed based on bergamottin, curcumin and hyperforin FC-DI clinical data. Adopting this quantitative modelling approach should streamline herbal product safety assessments, assist in FC-DI management, and ultimately promote safe clinical use of oncology drugs.

A regulatory science viewpoint on botanical-drug interactions

There is a continued predisposition of concurrent use of drugs and botanical products. Consumers often self-administer botanical products without informing their health care providers. The perceived safety of botanical products with lack of knowledge of the interaction potential poses a challenge for providers and both efficacy and safety concerns for patients. Botanical–drug combinations can produce untoward effects when botanical constituents modulate drug metabolizing enzymes and/or transporters impacting the systemic or tissue exposure of concomitant drugs. Examples of pertinent scientific literature evaluating the interaction potential of commonly used botanicals in the US are discussed. Current methodologies that can be applied to advance our efforts in predicting drug interaction liability is presented. This review also highlights the regulatory science viewpoint on botanical–drug interactions and labeling implications.

Transporter-mediated natural product-drug interactions for the treatment of cardiovascular diseases

The growing use of natural products in cardiovascular (CV) patients has been greatly raising the concerns about potential natural product–CV drug interactions. Some of these may lead to unexpected cardiovascular adverse effects and it is, therefore, essential to identify or predict potential natural product–CV drug interactions, and to understand the underlying mechanisms. Drug transporters are important determinants for the pharmacokinetics of drugs and alterations of drug transport has been recognized as one of the major causes of natural product–drug interactions. In last two decades, many CV drugs (e.g., angiotensin II receptor blockers, beta-blockers and statins) have been identified to be substrates and inhibitors of the solute carrier (SLC) transporters and the ATP-binding cassette (ABC) transporters, which are two major transporter superfamilies. Meanwhile, in vitro and in vivo studies indicate that a growing number of natural products showed cardioprotective effects (e.g., gingko biloba, danshen and their active ingredients) are also substrates and inhibitors of drug transporters. Thus, to understand transporter-mediated natural product–CV drug interactions is important and some transporter-mediated interactions have already shown to have clinical relevance. In this review, we review the current knowledge on the role of ABC and SLC transporters in CV therapy, as well as transporter modulation by natural products used in CV diseases and their induced natural product–CV drug interactions through alterations of drug transport. We hope our review will aid in a comprehensive summary of transporter-mediated natural product–CV drug interactions and help public and physicians understand these type of interactions.

Phytotherapeutics: The Emerging Role of Intestinal and Hepatocellular Transporters in Drug Interactions with Botanical Supplements

In herbalism, botanical supplements are commonly believed to be safe remedies, however, botanical supplements and dietary ingredients interact with transport and metabolic processes, affecting drug disposition. Although a large number of studies have described that botanical supplements interfere with drug metabolism, the mode of their interaction with drug transport processes is not well described. Such interactions may result in serious undesired effects and changed drug efficacy, therefore, some studies on interaction between botanical supplement ingredients and drug transporters such as P-gp and OATPs are described here, suggesting that the interaction between botanical supplements and the drug transporters is clinically significant.

Intestinal and Hepatocellular Transporters: Therapeutic Effects and Drug Interactions of Herbal Supplements

Herbal supplements are generally considered safe; however, drug disposition is influenced by the interactions of herbal supplements and food constituents with transport and metabolic processes. Although the interference of herbal supplements with drug metabolism has been studied extensively, knowledge of how they interact with the drug transport processes is less advanced. Therefore, we describe here specific examples of experimental and human interaction studies of herbal supplement components with drug transporters addressing, for example, organic anion transporting polypeptides or P-glycoprotein, as such interactions may lead to severe side effects and altered drug efficacy. Hence, it is clearly necessary to increase the awareness of the clinical relevance of the interference of herbal supplements with the drug transport processes.

Transporters and drug-drug interactions: important determinants of drug disposition and effects

Uptake and efflux transporters determine plasma and tissue concentrations of a broad variety of drugs. They are localized in organs such as small intestine, liver, and kidney, which are critical for drug absorption and elimination. Moreover, they can be found in important blood-tissue barriers such as the blood-brain barrier. Inhibition or induction of drug transporters by coadministered drugs can alter pharmacokinetics and pharmacodynamics of the victim drugs. This review will summarize in particular clinically observed drug-drug interactions attributable to inhibition or induction of intestinal export transporters [P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)], to inhibition of hepatic uptake transporters [organic anion transporting polypeptides (OATPs)], or to inhibition of transporter-mediated [organic anion transporters (OATs), organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATEs), P-gp] renal secretion of xenobiotics. Available data on the impact of nutrition on transport processes as well as genotype-dependent, transporter-mediated drug-drug interactions will be discussed. We will also present and discuss data on the variable extent to which information on the impact of transporters on drug disposition is included in summaries of product characteristics of selected countries (SPCs). Further work is required regarding a better understanding of the role of the drug metabolism-drug transport interplay for drug-drug interactions and on the extrapolation of in vitro findings to the in vivo (human) situation.

Grapefruit-Drug Interactions: Can Interactions With Drugs Be Avoided?

Grapefruit is rich in flavonoids, which have been demonstrated to have a preventive influence on many chronic diseases, such as cancer and cardiovascular disease. However, since the early 1990s, the potential health benefits of grapefruit have been overshadowed by the possible risk of interactions between drugs and grapefruit and grapefruit juice. Several drugs interacting with grapefruit are known in different drug classes, such as HMG-CoA reductase inhibitors, calcium antagonists, and immunosuppressives. Currently known mechanisms of interaction include the inhibition of cytochrome P450 as a major mechanism, but potential interactions with P-glycoprotein and organic anion transporters have also been reported. This review is designed to provide a comprehensive summary of underlying mechanisms of interaction and human clinical trials performed in the area of grapefruit drug interactions and to point out possible replacements for drugs with a high potential for interactions.

Effects of grapefruit juice and SLCO1B1 388A>G polymorphism on the pharmacokinetics of pitavastatin

Pitavastatin undergoes little hepatic metabolism but it is a substrate for uptake and efflux transporters, particularly OATP1B1 (gene SLCO1B1). A previous study in 8 Japanese healthy subjects showed that co-administration with grapefruit juice (GFJ) resulted in a small increase in systemic exposure to pitavastatin. We examined whether common polymorphisms in SLCO1B1 might influence the pharmacokinetics of pitavastatin or the interaction with GFJ. Twelve Chinese healthy male volunteers took pitavastatin 2 mg orally with water or with GFJ on separate occasions and plasma concentrations of pitavastatin acid and lactone were measured over 48 h. GFJ increased the mean area under the plasma concentration-time curve (AUC0-48 h) for both pitavastatin acid and lactone by 14% (p<0.05). Subjects with SLCO1B1 *1b/*1b haplotype (388GG-521TT) had 47% and 44% higher systemic exposure for pitavastatin acid and lactone than the SLCO1B1 *1a carriers (388AA/AG-521TT, p<0.05 and p=0.005, respectively). The SLCO1B1 388A>G polymorphism, which increases transporter activity for some statins, was associated with higher plasma levels of pitavastatin acid and lactone in subjects with the homozygous variant indicating decreased hepatic uptake. Co-administration of pitavastatin with GFJ resulted in a small but significant increase in plasma levels in healthy Chinese subjects.

Intestinal OATP1A2 inhibition as a potential mechanism for the effect of grapefruit juice on aliskiren pharmacokinetics in healthy subjects

PURPOSE

To conduct a mechanistic investigation of the interaction between aliskiren and grapefruit juice in healthy subjects.

METHODS

Twenty-eight subjects received an oral dose of aliskiren 300 mg (highest recommended clinical dose) with 300 mL of either water or grapefruit juice in a two-way crossover design. Safety and pharmacokinetic analyses were performed. In vitro studies were performed in HEK293 cells to investigate the role of organic anion transporting polypeptide (OATP) transporter-mediated uptake of aliskiren.

RESULTS

Co-administration of a single dose of aliskiren with grapefruit juice decreased the plasma concentration of aliskiren, with mean decreases in the AUC(inf), AUC(last), and C(max) of 38, 37, and 61%, respectively. The uptake of [¹⁴C]aliskiren into OATP2B1-expressing cells was essentially the same as that into control cells, and the inhibitor combination atorvastatin and rifamycin had no effect on [¹⁴C]aliskiren accumulation in either cell type. The uptake of [¹⁴C]aliskiren and [³H]fexofenadine was linear in OATP1A2-expressing cells and was reduced by naringin, with IC₅₀ values of 75.5 ± 11.6 and 24.2 ± 2.0 μM, respectively.

CONCLUSIONS

Grapefruit juice decreases exposure of aliskiren partially via inhibition of intestinal OATP1A2.

A novel method using confocal laser scanning microscopy for sensitive measurement of P-glycoprotein-mediated transport activity in Caco-2 cells

OBJECTIVES

The aim of this study was to use time-lapse confocal laser scanning microscopy to establish a more sensitive and specific method for evaluating P-glycoprotein activity in Caco-2 cells.

METHODS

The change in the fluorescence of residual rhodamine 123 at the apical and central regions of Caco-2 cells was measured in the presence of digoxin or St John's wort by using time-lapse confocal laser scanning microscopy. The data were compared with measurements made using conventional techniques, a fluorescence microplate reader and a fluorescence microscope.

KEY FINDINGS

The percentage decrease of rhodamine 123 caused by 10 µm digoxin or 0.1 µg/ml St John's wort was significantly larger in the apical region of the Caco-2 cell than in the central region or in the whole cell. The digoxin-induced inhibition in the apical region as measured by time-lapse confocal laser scanning microscopy was greater than that measured in the whole cell by a microplate reader or a fluorescence microscope.

CONCLUSIONS

The assay of residual rhodamine 123 in the apical region of Caco-2 cells by confocal laser scanning microscopy was more sensitive than the conventional methods using a microplate reader or fluorescence microscopy. It will be a valuable screening tool for studying both the inhibition and induction of P-glycoprotein activity.

Grapefruit-drug interactions

Grapefruit juice and grapefruit product consumption have potential health benefits; however, their intake is also associated with interactions with certain drugs, including calcium channel blockers, immunosuppressants and antihistamines. The primary mechanism through which interactions are mediated is mechanism-based intestinal cytochrome P450 3A4 inhibition by furanocoumarins resulting in increased bioavailability of administered medications that are substrates. Grapefruit products have also been associated with interactions with P-glycoprotein (P-gp) and uptake transporters (e.g. organic anion-transporting polypeptides [OATPs]). Polyphenolic compounds such as flavonoids have been proposed as the causative agents of the P-gp and OATP interactions. The mechanisms and magnitudes of the interactions can be influenced by the concentrations of furanocoumarins and flavonoids in the grapefruit product, the volume of juice consumed, and the inherent variability of specific enzymes and transporter components in humans. It is therefore challenging to predict the extent of grapefruit product-drug interactions and to compare available in vitro and in vivo data. The clinical significance of such interactions also depends on the disposition and toxicity profile of the drug being administered. The aim of this review is to outline the mechanisms of grapefruit-drug interactions and present a comprehensive summary of those agents affected and whether they are likely to be of clinical relevance.

Fruit juice inhibition of uptake transport: a new type of food-drug interaction

A new type of interaction in which fruit juices diminish oral drug bioavailability through inhibition of uptake transport is the focus of this review. The discovery was based on an opposite to anticipated finding when assessing the possibility of grapefruit juice increasing oral fexofenadine bioavailability in humans through inhibition of intestinal MDR1-mediated efflux transport. In follow-up investigations, grapefruit or orange juice at low concentrations potentially and selectively inhibited in vitro OATP1A2-mediated uptake compared with MDR1-caused efflux substrate transport. These juices at high volume dramatically depressed oral fexofenadine bioavailability. Grapefruit was the representative juice to characterize the interaction subsequently. A volume-effect relationship study using a normal juice amount halved average fexofenadine absorption. Individual variability and reproducibility data indicated the clinical interaction involved direct inhibition of intestinal OATP1A2. Naringin was a major causal component suggesting that other flavonoids in fruits and vegetables might also produce the effect. Duration of juice clinical inhibition of fexofenadine absorption lasted more than 2 h but less than 4 h indicating the interaction was avoidable with appropriate interval of time between juice and drug consumption. Grapefruit juice lowered the oral bioavailability of several medications transported by OATP1A2 (acebutolol, celiprolol, fexofenadine, talinolol, L-thyroxine) while orange juice did the same for others (atenolol, celiprolol, ciprofloxacin, fexofenadine). Juice clinical inhibition of OATP2B1 was unresolved while that of OATP1B1 seemed unlikely. The interaction between grapefruit juice and etoposide also seemed relevant. Knowledge of both affected uptake transporter and drug hydrophilicity assisted prediction of the clinical interaction with grapefruit or orange juice.

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.

Highly sensitive measurement of P-glycoprotein-mediated transport activity in Caco-2 cells

We established a highly sensitive method for evaluating P-glycoprotein activity in Caco-2 cells. Using time-lapse confocal laser-scanning microscopy, we measured the change in fluorescence of residual rhodamine 123 in Caco-2 cells. Horizontal fluorescence images of rhodamine 123 in the apical and central parts of these cells were captured for 90 min. A continuous and significant decrease in the fluorescent intensity of rhodamine 123 in the apical part of the Caco-2 cells occurred during the measurement period, while the decrease in the central part was mild. The decrease in rhodamine 123 intensity in the apical part of Caco-2 cells were abolished in the presence of 10 microM digoxin, but the decrease in the central part was not. The decrease in total rhodamine 123 over whole cells was no significant influence of digoxin was observed. This residual rhodamine 123 assay for evaluating P-glycoprotein in the apical part of Caco-2 cells imaged by confocal laser scanning microscopy is more sensitive than conventional methods and will be a valuable screening tool for studying both the inhibition and induction of P-glycoprotein activity and expression. This method may also be useful for predicting P-glycoprotein-mediated alterations in the intestinal absorption of drugs.

Evaluation of in vivo P-glycoprotein phenotyping probes: a need for validation

Drug transporters are involved in clinically relevant drug-drug interactions. P-glycoprotein (P-gp) is an efflux transporter that displays genetic polymorphism. Phenotyping permits evaluation of real-time, in vivo P-gp activity and P-gp-mediated drug-drug interactions. Digoxin, fexofenadine, talinolol and quinidine are commonly used probe drugs for P-gp phenotyping. Although current regulatory guidance documents highlight methodologies for evaluating transporter-based drug-drug interactions, whether current probe drugs are suitable for phenotyping has not been established, and validation criteria are lacking. This review proposes validation criteria and evaluates P-gp probes to determine probe suitability. Based on these criteria, digoxin, fexofenadine, talinolol and quinidine have limitations to their use and are not recommended for P-gp phenotyping.

The role of transporters in the pharmacokinetics of orally administered drugs

Drug transporters are recognized as key players in the processes of drug absorption, distribution, metabolism, and elimination. The localization of uptake and efflux transporters in organs responsible for drug biotransformation and excretion gives transporter proteins a unique gatekeeper function in controlling drug access to metabolizing enzymes and excretory pathways. This review seeks to discuss the influence intestinal and hepatic drug transporters have on pharmacokinetic parameters, including bioavailability, exposure, clearance, volume of distribution, and half-life, for orally dosed drugs. This review also describes in detail the Biopharmaceutics Drug Disposition Classification System (BDDCS) and explains how many of the effects drug transporters exert on oral drug pharmacokinetic parameters can be predicted by this classification scheme.

Potential role of intestinal first-pass metabolism in the prediction of drug-drug interactions

BACKGROUND

The contribution of intestine to the magnitude of drug-drug interactions (DDI) may be significant, considering high levels of inhibitors in the gut lumen achieved during absorption and the abundance of metabolic enzymes in the mature enterocytes. Intestinal inhibition is incorporated in the DDI prediction models as the ratio of the intestinal wall availability in the presence and absence of the inhibitor (F(G)(') and F(G), respectively).

OBJECTIVE

This review will focus on the ability of the current approaches to estimate the extent of intestinal DDI accurately, addressing predominantly the most abundant intestinal P450 enzyme, CYP3A4.

METHODS

Considering the sensitivity of the DDI prediction models to the accuracy of the F(G) estimates, the current study focuses on 3 different in vitro and in vivo approaches to assess this parameter.

RESULTS/CONCLUSION

The advantages and limitations of each of F(G) methods are outlined. Accurate assessment of this parameter is essential for the prediction of human drug clearance and drug-drug interaction potential.

Predicting drug disposition, absorption/elimination/transporter interplay and the role of food on drug absorption

The ability to predict drug disposition involves concurrent consideration of many chemical and physiological variables and the effect of food on the rate and extent of availability adds further complexity due to postprandial changes in the gastrointestinal (GI) tract. A system that allows for the assessment of the multivariate interplay occurring following administration of an oral dose, in the presence or absence of meal, would greatly benefit the early stages of drug development. This is particularly true in an era when the majority of new molecular entities are highly permeable, poorly soluble, extensively metabolized compounds (BDDCS Class 2), which present the most complicated relationship in defining the impact of transporters due to the marked effects of transporter-enzyme interplay. This review evaluates the GI luminal environment by taking into account the absorption/transport/elimination interplay and evaluates the physiochemical property issues by taking into account the importance of solubility, permeability and metabolism. We concentrate on the BDDCS and its utility in predicting drug disposition. Furthermore, we focus on the effect of food on the extent of drug availability (F), which appears to follow closely what might be expected if a significant effect of high fat meals is inhibition of transporters. That is, high fat meals and lipidic excipients would be expected to have little effect on F for Class 1 drugs; they would increase F of Class 2 drugs, while decreasing F for Class 3 drugs.

Effect of NXY-059, a novel neuroprotectant, on the pharmacokinetics of a single dose of digoxin in healthy subjects

OBJECTIVE

NXY-059 is a novel free-radical trapping neuroprotectant. Digoxin treatment is common in acute ischaemic stroke, the intended patient population for NXY-059. Since both digoxin and NXY-059 are eliminated primarily renally, with a substantial contribution by active renal secretion, and because digoxin has a narrow therapeutic window, this open, randomised, crossover, two-period study investigated whether NXY-059 affects the pharmacokinetics (PK) of digoxin.

RESEARCH DESIGN AND METHODS

Twenty-two healthy subjects received 0.5 mg oral digoxin 2 h after the start of 60-h intravenous infusions of NXY-059 and placebo separated by a 14-day washout. Blood and urine were collected for 60 h. Digoxin concentrations were measured by a novel liquid chromatography-mass spectrometry assay.

MAIN OUTCOME MEASURES

The ratio of the geometric mean (90% confidence interval) between NXY-059 and placebo for the digoxin area under the concentration-versus-time curve was 0.91 (0.83-0.99) and was within the predefined range for no interaction (0.80-1.25). No safety concerns were raised in the study. No serious adverse events were recorded. The most common adverse event was headache with similar frequencies in the two treatments.

CONCLUSIONS

NXY-059 had no clinically significant effect on the PK of digoxin.

Clinical relevance of the small intestine as an organ of drug elimination: drug-fruit juice interactions

Most drugs are taken orally. For those intended to act systemically, a significant fraction of the dose can be eliminated during its first passage through a sequence of organs before entry into the general circulation. For some drugs, the degree of first-pass elimination can be large enough such that oral bioavailability is significantly reduced, with the consequent potential for a reduced clinical response. Of these first-pass eliminating organs, the small intestine and liver are the most commonly implicated, in part because they express the highest levels of drug-metabolizing enzymes. For several drugs whose major route of elimination occurs via CYP3A-mediated metabolism, the extent of first-pass metabolism in the small intestine can rival that in the liver. As such, alterations in enteric CYP3A activity alone can significantly influence oral bioavailability. The most extensively studied xenobiotic shown to inhibit only enteric CYP3A is grapefruit juice, the consequences of which can be clinically significant. Although much information has been gained regarding the grapefruit juice effect, progress in the relatively understudied area of drug-diet interactions continues to be sluggish and reactive. In stark contrast, the potential for drug-drug interactions involving any new therapeutic agent must be evaluated, prospectively, before market introduction. To prospectively elucidate mechanisms underlying drug-diet interactions, a multidisciplinary, translational research approach is required, which capitalizes on the collective expertise of drug metabolism scientists and natural products chemists. Such an approach would allow proper between-study comparisons, and ultimately provide conclusive information as to whether specific dietary substances can be taken safely with certain medications.

Membrane transporter proteins: a challenge for CNS drug development

Drug transporters are membrane proteins present in various tissues such as the lymphocytes, intestine, liver, kidney, testis, placenta, and central nervous system. These transporters play a significant role in drug absorption and distribution to organic systems, particularly if the organs are protected by blood-organ barriers, such as the blood-brain barrier or the maternal-fetal barrier. In contrast to neurotransmitters and receptor-coupled transporters or other modes of interneuronal transmission, drug transporters are not directly involved in specific neuronal functions, but provide global protection to the central nervous system. The lack of capillary fenestration, the low pinocytic activity, and the tight junctions between brain capillary and choroid plexus endothelial cells represent further gatekeepers limiting the entrance of endogenous and exogenous compounds into the central nervous system. Drug transport is a result of the concerted action of efflux and influx pumps (transporters) located both in the basolateral and apical membranes of brain capillary and choroid plexus endothelial cells. By regulating efflux and influx of endogenous or exogenous substances, the blood-brain barrier and, to a lesser extent, the blood-cerebrospinal barrier in the ventricles, represents the main interface between the central nervous system and the blood, ie, the rest of the body. As drug distribution to organs is dependent on the affinity of a substrate for a specific transport system, membrane transporter proteins are increasingly recognized as a key determinant of drug disposition. Many drug transporters are members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter superfamily or the solute-linked carrier (SLC) class. The multidrug resistance protein MDR1 (ABCB1), also called P-glycoprotein, the multidrug resistance-associated proteins MRP1 (ABCC1) and MRP2 (ABCC2), and the breast cancer-resistance protein BCRP (ABCG2) are ATP-dependent efflux transporters expressed in the blood-brain barrier. They belong to the superfamily of ABC transporters, which export drugs from the intracellular to the extracellular milieu. Members of the SLC class of solute carriers include, for example, organic ion transporting peptides, organic cation transporters, and organic ion transporters. They are ATP-independent polypeptides principally expressed at the basolateral membrane of brain capillary and choroid plexus endothelial cells that also mediate drug transport through central nervous system barriers.

The impact of CYP2C8 polymorphism and grapefruit juice on the pharmacokinetics of repaglinide

AIMS

The primary aim of the study was to investigate the possible effect of the CYP2C8*3 allele and of grapefruit juice on the pharmacokinetics of repaglinide. Furthermore, the impact of a single dose of grapefruit juice on the pharmacokinetics of repaglinide in relation to dose.

METHODS

Thirty-six healthy male subjects, genotyped for CYP2C8*3 (11 genotyped as CYP2C8*1/*3, one as CYP2C8*3/*3 and 24 as CYP2C8*1/*1), participated in a randomized, cross-over trial. In the two phases, the subjects drank 300 mL water or 300 mL grapefruit juice, in randomized order, 2 h before administration of a single dose of either 0.25 mg or 2 mg repaglinide.

RESULTS

Neither the mean AUC(0-infinity) (geometric mean ratio: 1.01; 95% CI: 0.93-1.1, P = 0.88) nor the mean C(max) (geometric mean ratio: 1.05; 95% CI: 0.94-1.2, P = 0.35) of repaglinide were statistically significantly different in the group carrying the CYP2C8*3 mutant allele compared with wild-types. Grapefruit juice caused a 19% decrease in the geometric mean ratio of the 3-hydroxyquinidine to quinidine ratio (difference: 0.81; 95% CI: 0.75-0.87, P < 0.0001), which was used as an index of CYP3A4 activity, and an increase in the mean AUC(0-infinity) of repaglinide (geometric mean ratio: 1.13; 95% CI: 1.04-1.2, P = 0.0048), but had no statistically significant effect on the t(1/2). There was no statistically significant difference in blood glucose concentration in subjects who had or had not ingested grapefruit juice. The effect was more pronounced at the low dose of repaglinide (0.25 mg) than at the therapeutic dose of 2 mg.

CONCLUSIONS

The pharmacokinetics of repaglinide in subjects carrying the CYP2C8*3 mutant allele did not differ significantly from those in the wild-types. Grapefruit juice increased the bioavailability of repaglinide, suggesting significant intestinal elimination of the drug which was assumed to be primarily mediated by CYP3A4 in the gut.

A furanocoumarin-free grapefruit juice establishes furanocoumarins as the mediators of the grapefruit juice-felodipine interaction

BACKGROUND

Grapefruit juice (GFJ) enhances the systemic exposure of numerous CYP3A4 drug substrates, including felodipine, by inhibiting intestinal (but not hepatic) first-pass metabolism. Furanocoumarins have been identified as major CYP3A4 inhibitors contained in the juice, but their contribution to the GFJ effect in vivo remains unclear.

OBJECTIVE

To ascertain whether furanocoumarins mediate the GFJ-felodipine interaction, a furanocoumarin-free GFJ was created and tested against orange juice and the original GFJ with respect to the oral pharmacokinetics of felodipine.

DESIGN

With the use of food-grade solvents and absorption resins, furanocoumarins were removed (approximately 99%) from whole GFJ, whereas other major ingredients (flavonoids) were retained. In an open, 3-way, randomized crossover design, 18 healthy volunteers ingested felodipine (10 mg) with 1 of the 3 juices (240 mL). Blood was collected over 24 h. At least 1 wk elapsed between juice treatments.

RESULTS

The median and range of the area under the curve and the maximum concentration of felodipine were significantly (P < 0.001) greater with consumption of GFJ [110 (range: 58-270) nmol . h/L and 21 (7.6-50) nmol/L, respectively] than with that of orange juice [54 (29-150) nmol . h/L and 7.6 (3.4-13.9) nmol/L, respectively] or furanocoumarin-free GFJ [48 (23-120) nmol . h/L and 8.3 (3.0-16.6) nmol/L, respectively]. GFJ, orange juice, and furanocoumarin-free GFJ did not differ significantly (P > 0.09) in median time to reach maximum plasma concentration [2.5 (1.5-6), 2.8 (1.5-4), and 2.5 (2-6) h, respectively] or terminal half-life [6.6 (4.2-13.6), 7.8 (4.4-13.2), and 6.8 (2.6-14.4) h, respectively].

CONCLUSION

Furanocoumarins are the active ingredients in GFJ responsible for enhancing the systemic exposure of felodipine and probably other CYP3A4 substrates that undergo extensive intestinal first-pass metabolism.

Meta-analysis of the influence of MDR1 C3435T polymorphism on digoxin pharmacokinetics and MDR1 gene expression

AIMS

Studies revealing conflicting results of the functional significance of MDR1 exon 26 C3435T SNP on the disposition of digoxin in different ethnic groups led us to perform a meta-analysis on published data investigating the influence of C3435T SNP on the pharmacokinetics of digoxin and the expression of MDR1.

METHODS

Meta-analysis was performed on data from published studies investigating the influence of MDR1 C3435T SNP on digoxin pharmacokinetics, as well as MDR1 expression in Caucasian and Japanese populations. The following outcomes were included: exposures to digoxin measured by area under the concentration-time curve and maximum concentration, the mean intestinal MDR1 mRNA expression and P-gp expression in the absence of digoxin administration.

RESULTS

The overall results of the meta-analysis in Caucasian and Japanese subjects suggested no major influence of the C3435T SNP on exposure levels of digoxin as determined by AUC(0-4 h) or AUC(0-24 h) although C(max) values for digoxin were lower in wild-type (CC) subjects compared with subjects harbouring TT genotypes. Subgroup analysis by ethnic populations showed the oral availability of digoxin to be lower in wild-type Caucasian populations compared with wild-type Japanese subjects. No causal relationships were detected between the C3435T SNP and MDR1 mRNA or protein expression.

CONCLUSIONS

Our meta-analysis of available studies indicates that the synonymous MDR1 C3435T SNP does not affect the pharmacokinetics of digoxin and the expression of MDR1 mRNA. Future studies should focus on the impact of MDR1 haplotypes on the pharmacokinetics of MDR1 substrates rather than the C3435T SNP alone.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

Undesirable effects of citrus juice on the pharmacokinetics of drugs: focus on recent studies

It is well known that intake of grapefruit juice affects the pharmacokinetics of various kinds of drugs. It has been reported that other citrus juices also interact with certain drugs. To re-evaluate citrus juice-drug interactions based on currently available evidence, a literature search was conducted for new and updated information since the grapefruit juice-drug interaction was last reviewed in 1998. MEDLINE (1998-October 2004) was accessed and more than 200 reports were found. The effects of grapefruit juice ingestion on the pharmacokinetics of orally administered drugs have been reported for 40 drugs since the reviews published in 1998. Increases in either area under the concentration-time curve (AUC) or maximum plasma concentration (C(max)) were found with 34 of these, the major mechanism being considered to be inactivation of intestinal cytochrome P450 3A4, a so-called mechanism-based inhibition. Although recent reports point to the inhibitory effects of grapefruit juice on the function of P-glycoprotein, which transports substrates from enterocytes back into the lumen, the contribution to the bioavailability of drugs that are substrates of P-glycoprotein has not been established yet. Dramatic decreases in AUC and C(max) for two drugs in association with grapefruit juice ingestion has been reported and, in these cases, inhibitory effects on organic anion transporting polypeptide, which mediates absorption from the intestinal lumen to enterocytes, might be involved. Other citrus juices such as Seville (sour) orange juice and commonly ingested varieties of orange juice also showed significant effects on the AUC and C(max) of some drugs. Although the situation is complex and uncertainties remain, we recommend that patients avoid citrus juice intake while taking medications and that healthcare providers advise against citrus juice intake in this setting until any interactions with subject drugs can be clarified in clinical studies.

Pharmacogenetics in drug regulation: promise, potential and pitfalls

Pharmacogenetic factors operate at pharmacokinetic as well as pharmacodynamic levels-the two components of the dose-response curve of a drug. Polymorphisms in drug metabolizing enzymes, transporters and/or pharmacological targets of drugs may profoundly influence the dose-response relationship between individuals. For some drugs, although retrospective data from case studies suggests that these polymorphisms are frequently associated with adverse drug reactions or failure of efficacy, the clinical utility of such data remains unproven. There is, therefore, an urgent need for prospective data to determine whether pre-treatment genotyping can improve therapy. Various regulatory guidelines already recommend exploration of the role of genetic factors when investigating a drug for its pharmacokinetics, pharmacodynamics, dose-response relationship and drug interaction potential. Arising from the global heterogeneity in the frequency of variant alleles, regulatory guidelines also require the sponsors to provide additional information, usually pharmacogenetic bridging data, to determine whether data from one ethnic population can be extrapolated to another. At present, sponsors explore pharmacogenetic influences in early clinical pharmacokinetic studies but rarely do they carry the findings forward when designing dose-response studies or pivotal studies. When appropriate, regulatory authorities include genotype-specific recommendations in the prescribing information. Sometimes, this may include the need to adjust a dose in some genotypes under specific circumstances. Detailed references to pharmacogenetics in prescribing information and pharmacogenetically based prescribing in routine therapeutics will require robust prospective data from well-designed studies. With greater integration of pharmacogenetics in drug development, regulatory authorities expect to receive more detailed genetic data. This is likely to complicate the drug evaluation process as well as result in complex prescribing information. Genotype-specific dosing regimens will have to be more precise and marketing strategies more prudent. However, not all variations in drug responses are related to pharmacogenetic polymorphisms. Drug response can be modulated by a number of non-genetic factors, especially co-medications and presence of concurrent diseases. Inappropriate prescribing frequently compounds the complexity introduced by these two important non-genetic factors. Unless prescribers adhere to the prescribing information, much of the benefits of pharmacogenetics will be squandered. Discovering highly predictive genotype-phenotype associations during drug development and demonstrating their clinical validity and utility in well-designed prospective clinical trials will no doubt better define the role of pharmacogenetics in future clinical practice. In the meantime, prescribing should comply with the information provided while pharmacogenetic research is deservedly supported by all concerned but without unrealistic expectations.

Mechanistic basis of adverse drugreactions: the perils of inappropriate dose schedules

Adverse drug reactions (ADRs) have long been recognised as a significant cause of morbidity and mortality. They account for a substantial number of clinical consultations, hospital admissions and extended duration of in-patient stay as well as mortality. By far the most common ADRs are the concentration-dependent pharmacological reactions, the majority of which ought to be preventable. As a result of high concentrations of the parent drug and/or its metabolite(s), there is an augmentation of primary pharmacological activity and/or appearance of new and undesirable secondary pharmacological activity. Typically, these high concentrations result from administration of high doses in an attempt to maximise efficacy and/or modulation of the pharmacokinetics of a drug by either genetic or non-genetic factors. High plasma concentrations of parent drug may result from inherited impairment or drug-induced inhibition of its pharmacokinetic disposition. Conversely, inherited overcapacity or drug-induced induction of the metabolism of a drug may result in low concentrations of parent drug and frequently, rapid accumulation of its metabolites. Environmental, dietary and phytochemical factors may also influence the activity of drug metabolising enzymes. As with inherited polymorphisms of acetylation and cytochrome P450-based drug metabolising enzymes, polymorphisms of other conjugation reactions, such as glucuronidation, increasingly appear to be associated with drug toxicity. Diseases of organs involved in elimination of a drug also alter its pharmacokinetics, plasma concentration and, therefore, the profile of its concentration-dependent ADRs. Inherited mutations, concurrently administered drugs or presence of certain diseases may also alter the sensitivity of some pharmacological targets, accounting for a substantial number of ADRs and interactions. When there is enhanced pharmacodynamic sensitivity, plasma drug concentrations that are apparently within the normal 'non-toxic' range give rise to ADRs. Recent advances have also provided important insights into the wider scope of drug-drug interactions. Interactions that occur at P-glycoproteins, drug transporters and efflux pumps, at various transmembrane interfaces such as the gastrointestinal wall, renal tubules, hepatobiliary border and blood-brain barrier, are beginning to explain many non-metabolic interactions. These alter the systemic exposure to drugs and have so far, begun to explain unexpected neurotoxicity and hepatotoxicity. The function of these transporters is also genetically modulated. These advances, together with continued increased awareness and education of prescribers and pharmacists, offer great opportunities for substantially minimising concentration-related ADRs.

Evaluation of first-pass cytochrome P4503A (CYP3A) and P-glycoprotein activities using alfentanil and fexofenadine in combination

Cytochrome P4503A (CYP3A) and P-glycoprotein (P-gp) are major determinants of oral bioavailability. Development of in vivo probe(s), for both CYP3A and P-gp, which could be administered in combination, is a current goal. Nevertheless, there is considerable overlap in CYP3A and P-gp substrate selectivities; there are few discrete probes. Alfentanil is a selective CYP3A probe but not a P-gp substrate. Fexofenadine is a P-gp probe but not a CYP3A substrate. This investigation tested the hypothesis that alfentanil and fexofenadine could be administered in combination to probe first-pass CYP3A and P-gp activities in humans. Two 3-way crossover studies were conducted in healthy volunteers. In the first protocol, subjects received oral alfentanil alone, fexofenadine alone, or fexofenadine 1 hour after alfentanil. In the second protocol, subjects abstained from citrus and apple products for 5 days and received fexofenadine alone, fexofenadine 1 hour after alfentanil, or alfentanil 4 hours after fexofenadine. An assay using solid-phase extraction and electrospray liquid chromatography/mass spectrometry was developed for the simultaneous quantification of plasma alfentanil and fexofenadine. In both protocols, alfentanil plasma concentrations and area under the concentration versus time curve (AUC) were unaffected by fexofenadine or meal composition. Fexofenadine given 1 hour after alfentanil and followed 1 hour later by a meal containing orange or apple juice had a somewhat lower AUC compared with fexofenadine alone (geometric mean ratio with and without the interacting drug = 0.73, 90% confidence interval [CI] = 0.59-1.04). Fexofenadine given 1 hour after alfentanil and followed 2 hours later by a meal not containing citrus or apple products had an AUC that was unchanged compared with fexofenadine alone (ratio = 0.91, 90% CI = 0.70-1.35). These results show that alfentanil disposition was not affected by fexofenadine. A dosing regimen was identified in which fexofenadine disposition was not affected by alfentanil. The timing and content of meals after fexofenadine had a significant effect on fexofenadine disposition. Alfentanil and fexofenadine in combination appear to be a useful probe for evaluating both first-pass CYP3A and P-gp activities in humans.

Mechanisms of resistance to anticancer drugs: the role of the polymorphic ABC transporters ABCB1 and ABCG2

ATP-binding cassette (ABC) genes play a role in the resistance of malignant cells to anticancer agents. The ABC gene products, including ABCB1 (P-glycoprotein) and ABCG2 (breast cancer-resistance protein [BCRP], mitoxantrone-resistance protein [MXR], or ABC transporter in placenta [ABCP]), are also known to influence oral absorption and disposition of a wide variety of drugs. As a result, the expression levels of these proteins in humans have important consequences for an individual’s susceptibility to certain drug-induced side effects, interactions, and treatment efficacy. Naturally occurring variants in ABC transporter genes have been identified that might affect the function and expression of the protein. This review focuses on recent advances in the pharmacogenetics of the ABC transporters ABCB1 and ABCG2, and discusses potential implications of genetic variants for the chemotherapeutic treatment of cancer.

Herbal modulation of P-glycoprotein

P-glycoprotein (Pgp) is a 170 kDa phosphorylated glycoprotein encoded by human MDR1 gene. It is responsible for the systemic disposition of numerous structurally and pharmacologically unrelated lipophilic and amphipathic drugs, carcinogens, toxins, and other xenobiotics in many organs, such as the intestine, liver, kidney, and brain. Like cytochrome P450s (CYP3A4), Pgp is vulnerable to inhibition, activation, or induction by herbal constituents. This was demonstrated by using an ATPase assay, purified Pgp protein or intact Pgp-expressing cells, and proper probe substrates and inhibitors. Curcumin, ginsenosides, piperine, some catechins from green tea, and silymarin from milk thistle were found to be inhibitors of Pgp, while some catechins from green tea increased Pgp-mediated drug transport by heterotropic allosteric mechanism, and St. John's wort induced the intestinal expression of Pgp in vitro and in vivo. Some components (e.g., bergamottin and quercetin) from grapefruit juice were reported to modulate Pgp activity. Many of these herbal constituents, in particular flavonoids, were reported to modulate Pgp by directly interacting with the vicinal ATP-binding site, the steroid-binding site, or the substrate-binding site. Some herbal constituents (e.g., hyperforin and kava) were shown to activate pregnane X receptor, an orphan nuclear receptor acting as a key regulator of MDR1 and many other genes. The inhibition of Pgp by herbal constituents may provide a novel approach for reversing multidrug resistance in tumor cells, whereas the stimulation of Pgp expression or activity has implication for chemoprotective enhancement by herbal medicines. Certain natural flavonols (e.g., kaempferol, quercetin, and galangin) are potent stimulators of the Pgp-mediated efflux of 7,12-dimethylbenz(a)-anthracene (a carcinogen). The modulation of Pgp activity and expression by these herb constituents may result in altered absorption and bioavailability of drugs that are Pgp substrates. This is exemplified by increased oral bioavailability of phenytoin and rifampin by piperine and decreased bioavailability of indinavir, tacrolimus, cyclosporine, digoxin, and fexofenadine by coadministered St. John's wort. However, many of these drugs are also substrates of CYP3A4. Thus, the modulation of intestinal Pgp and CYP3A4 represents an important mechanism for many clinically important herb-drug interactions. Further studies are needed to explore the relative role of Pgp and CYP3A4 modulation by herbs and the mechanism for the interplay of these two important proteins in herb-drug interactions.