Does inhibition of P-glycoprotein lead to drug–drug interactions?

Permeability-glycoprotein (Pgp) is a drug transporter responsible for the efflux of xenobiotics out of cells that influence the pharmacokinetics of numerous drugs. However, the role of this transporter in drug-drug interactions is still poorly studied even though a lot of P-glycoprotein substrates and P-glycoprotein inhibitors are identified among drugs of standard usage. On one hand, Pgp is distributed within a lot of organs and tissues implicated in the absorption or excretion of xenobiotics, and drug-drug interactions with this protein may increase the bioavailability of simultaneously administered active drugs. On the other hand, Pgp is linked to the integrity of blood-tissue barriers, such as the blood-brain barrier or placenta, and a partial blockage of Pgp could be responsible for a new drug distribution in the organism with possible increase of drug rates in organs behind these barriers. Therefore, concomitant administration of substrates and Pgp inhibitors would modify drug pharmacokinetics by increasing bioavailability and organ uptake, leading to more adverse drug reactions and toxicities. Consequently, the identification and comprehension of these drug-drug interactions remain important keys to risk assessment.     

Drug-drug interactions affected by the transporter protein, P-glycoprotein (ABCB1, MDR1) II. Clinical aspects

The simultaneous use of several drugs (polypharmacy) for the treatment of cancer, HIV, and other diseases and for multiple ailments of the elderly is a common practice in modern medicine. Co-administration of drugs may result in unwanted side effects. One reason for these side effects can be the altered function of transport proteins, especially of P-glycoprotein (Pgp), due to its simultaneous interaction with several drugs. We describe here some of the observed, unexpected side effects of polypharmacy in the clinic. We also describe intentional modulation of the function of Pgp that is introduced when facilitation of absorption of a drug through the intestines is needed and in cancer chemotherapy. In addition, we mention some methods of testing and ways by which doctors and patients can be alerted to potential side effects.     

In vivo animal models for investigating potential CYP3A- and Pgp-mediated drug-drug interactions

With the advent of polytherapy it has become prudent to minimize, as much as possible, the potential for drug-drug interactions. Towards this end, the metabolic and transporter pathways involved in the disposition of a drug candidate (phenotyping) are evaluated in vitro employing available human tissue and specific reagents. Likewise, in vitro screening for inhibition and induction of drug-metabolizing enzymes and transporters is conducted also. Such in vitro human data can be made available prior to human dosing and enable in vitro to in vivo-based predictions of clinical outcomes. Despite some success, however, in vitro systems are not dynamic and sometimes fail to predict drug-drug interactions for a variety of reasons. In comparison, relatively less effort has been made to evaluate predictions based on data derived from in vivo animal models. This review will attempt to summarize different examples from the literature where animal models have been used to predict cytochrome P450 3A (CYP3A)- and P-glycoprotein (Pgp)-based drug-drug interactions. When employing data from animal models one needs to be aware of species differences in pharmacokinetics, clearance pathways and selectivity and affinity of probe substrates and inhibitors. Because of these differences, in vivo animal studies alone, cannot be predictive of human drug-drug interactions. Despite these caveats, the information obtained from validated in vivo animal models may prove useful when used in conjunction with in vitro-in vivo extrapolation methods. Such an integrated data set can be used to select drug candidates with a reduced drug interaction potential.     

Macrocyclic lactones and cellular transport-related drug interactions: a perspective from in vitro assays to nematode control in the field

Macrocyclic lactones (MLs) are antiparasitic drugs used against endo-ectoparasites. Regarding the wide use of MLs in different species, it is likely that drug-drug interactions may occur after their co-administration with other compounds. A new paradigm was introduced in the study of the pharmacology of MLs during the last years since the interactions of MLs with ATP-binding cassete (ABC) transporters have been described. The current review article gives an update on the available information concerning drug-drug interactions involving the MLs. The basis of the methodological approaches used to evaluate transport interactions, and the impact of the pharmacology-based modulation of drug transport on the MLs disposition kinetics and clinical efficacy, are discussed in an integrated manner. A different number of in vitro and ex vivo methods have been reported to study the characterization of the interactions between MLs and ABC transporters. The production of the ABC transporters knockout mice has provided valuable in vivo tools to study this type of drug-drug interaction. In vivo trials performed in different species corroborated the effects of ABC transporter modulators on the pharmacokinetics behaviour of MLs. Important pharmacokinetic changes on plasma disposition of MLs have been observed when these compounds are co-administered with P-glycoprotein modulators. The modulation of the activity of P-glycoprotein was evaluated as a strategy not only to increase the systemic availability of MLs but also to improve their clinical efficacy. The understanding of the MLs interactions may supply relevant information to optimize their use in veterinary and human therapeutics.     

Inhibition of P-glycoprotein (ABCB1)- and multidrug resistance-associated protein 1 (ABCC1)-mediated transport by the orally administered inhibitor, CBT-1((R))

Cellular expression of ATP-binding cassette (ABC) transport proteins, such as P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP1), or ABCG2, is known to confer a drug-resistant phenotype. Thus, the development of effective transporter inhibitors could be of value to cancer treatment. CBT-1 is a bisbenzylisoquinoline plant alkyloid currently in development as a Pgp inhibitor. We characterized its interactions with the three major ABC transporters associated with drug resistance - Pgp, MRP1 and ABCG2 - and compared it to other known inhibitors. CBT-1 completely inhibited rhodamine 123 transport from Pgp-overexpressing cells at a concentration of 1muM. Additionally, 1 microM completely reversed Pgp-mediated resistance to vinblastine, paclitaxel and depsipeptide in SW620 Ad20 cells. CBT-1 was found to compete [(125)I]-IAAP labeling of Pgp with an IC(50) of 0.14 microM, and low concentrations of CBT-1 (<1 microM) stimulated Pgp-mediated ATP hydrolysis. In MRP1-overexpressing cells, 10 microM CBT-1 was found to completely inhibit MRP1-mediated calcein transport. CBT-1 at 25 microM did not have a significant effect on ABCG2-mediated pheophorbide a transport. Serum levels of CBT-1 in samples obtained from eight patients receiving CBT-1 increased intracellular rhodamine 123 levels in CD56+ cells 2.1- to 5.7-fold in an ex vivo assay. CBT-1 is able to inhibit the ABC transporters Pgp and MRP1, making it an attractive candidate for clinical trials in cancers where Pgp and/or MRP1 might be overexpressed. Further clinical studies with CBT-1 are warranted.     

Cardiovascular Ion Channel Inhibitor Drug-Drug Interactions with P-glycoprotein

P-glycoprotein (Pgp) is an ATP-binding cassette (ABC) transporter that plays a major role in cardiovascular drug disposition by effluxing a chemically and structurally diverse range of cardiovascular therapeutics. Unfortunately, drug-drug interactions (DDIs) with the transporter have become a major roadblock to effective cardiovascular drug administration because they can cause adverse drug reactions (ADRs) or reduce the efficacy of drugs. Cardiovascular ion channel inhibitors are particularly susceptible to DDIs and ADRs with Pgp because they often have low therapeutic indexes and are commonly coadministered with other drugs that are also Pgp substrates. DDIs from cardiovascular ion channel inhibitors with the transporter occur because of inhibition or induction of the transporter and the transporter’s tissue and cellular localization. Inhibiting Pgp can increase absorption and reduce excretion of drugs, leading to elevated drug plasma concentrations and drug toxicity. In contrast, inducing Pgp can have the opposite effect by reducing the drug plasma concentration and its efficacy. A number of in vitro and in vivo studies have already demonstrated DDIs from several cardiovascular ion channel inhibitors with human Pgp and its animal analogs, including verapamil, digoxin, and amiodarone. In this review, Pgp-mediated DDIs and their effects on pharmacokinetics for different categories of cardiovascular ion channel inhibitors are discussed. This information is essential for improving pharmacokinetic predictions of cardiovascular therapeutics, for safer cardiovascular drug administration and for mitigating ADRs emanating from Pgp.     

Competitive and allosteric interactions in ligand binding to P-glycoprotein as observed on an immobilized P-glycoprotein liquid chromatographic stationary phase

A liquid chromatographic stationary phase containing immobilized P-glycoprotein (Pgp) was synthesized using cell membranes obtained from Pgp-expressing cells. The resulting Pgp-stationary phase was used in frontal and zonal chromatographic studies to investigate the binding of vinblastine (VBL), doxorubicin (DOX), verapamil (VER), and cyclosporin A (CsA) to the immobilized Pgp. The compounds were added individually to the chromatographic system with or without ATP in the running buffer. Using this approach, dissociation constants were calculated for VBL (23.5 +/- 7.8 nM), DOX (15.0 +/- 3.2 microM), VER (54.2 +/- 4.7 microM), and CsA [97.9 +/- 19.4 nM (without ATP) and 62.5 +/- 4.6 nM (with ATP)]. The compounds were also added in pairs using standard competitive chromatography procedures. The results of the study demonstrate that competitive interactions occurred between VBL and DOX, cooperative allosteric interactions occurred between VBL and CsA and ATP and CsA, and anticooperative allosteric interactions occurred between ATP and VBL and VER. The chromatographic studies indicate that the immobilized Pgp was modified by ligand and cofactor binding and that the stationary phase can be used to study drug-drug binding interactions on the Pgp molecule.     

Drug-drug interactions of anti-infective drugs: utility of fluorescence cyp inhibition assays in drug discovery

Drug interactions are often a result of induction or inhibition of cytochrome P450 (CYP) enzymes by co-administered drugs. A high throughput fluorescence assay using cDNA-expressed human CYP isozymes and fluorogenic substrates has been reported for the study of CYP inhibition. We used this assay to evaluate CYP inhibition profiles of 21 marketed anti-infective drugs. We found that six of the eight potent inhibitors identified in this screen (IC50 <10 microM against at least one CYP isozyme) correlated with significant drug-drug interactions in the clinic. In contrast, the intermediate and weak inhibitors (IC50 >10 microM) did not indicate clinically significant drug interactions. Furthermore, we observed that results obtained in the fluorescence assay correlated with conventional, well-established, low throughput methods that utilize human liver microsomes. These data suggest that in the early stages of drug discovery, the fluorescence assay for CYP inhibition could be used in conjunction with a human liver microsomal assay to identify new chemical entities with a potential for drug-drug interactions.     

Caco-2 permeability,P-glycoprotein transport ratios and brain penetration of heterocyclic drugs

In this study the gastrointestinal absorption and P-glycoprotein (Pgp) efflux transport of heterocyclic drugs was investigated with the Caco-2 cell model. Based on the calculation of the physico-chemical properties a good oral absorption was predicted for all the drugs tested in this study which corresponded well with the measured Caco-2 permeabilities (Papp). Generally a high permeability of the tested heterocyclic drugs was measured being in agreement with earlier published human in vivo absorption data. Based on the transport data of domperidone and verapamil it was found that the Pgp efflux transporter was expressed in the Caco-2 cells. Many of the drugs tested were indicated to be potential Pgp efflux substrates. Since Pgp is expressed at the Blood Brain Barrier (BBB) as well, it was expected that CNS penetration will be impaired if a drug is a Pgp substrate. However, no correlation could be found between brain penetration in rats and the Pgp efflux ratio as measured with the Caco-2 cells. From the data it is concluded that Pgp efflux ratio's as determined in in vitro High Throughput Screening (HTS) tests, where the transport conditions are fixed (pH gradient, concentration, etc.), cannot routinely be used to predict a possible limited brain penetration.     

药物诱导CYP450的体外检测方法研究进展

在联合用药时,药物之间时常发生的代谢动力学相互作用是药物疗效发生改变的重要因素之一。细胞色素P450酶被诱导是影响药物代谢动力学的一个主要原因。近年来,CYP450酶诱导的体外检测方法被广泛运用,根据核受体能激活酶基因的转录,建立了一些核受体[芳香烃受体（AhR）、孕烷X受体（PXR）和组成型雄甾烷受体（CAR）]测定方法;体外肝细胞能有效地反映酶诱导效应,是最常用的检测工具。通过体外酶诱导的检测数据预测体内药物相互作用,为进一步的在体和临床研究提供指导意见。本文主要综述了近年来常用的几种测定方法的原理、应用、数据分析以及发展现状。     

Signalling possible drug–drug interactions in a spontaneous reporting system: delay of withdrawal bleeding during concomitant use of oral contraceptives and itraconazole

AIMS: In spontaneous adverse drug reaction reporting systems, there is a growing need for methods facilitating the automated detection of signals concerning possible adverse drug reactions. In addition, special attention is needed for the detection of adverse drug reactions resulting from possible drug-drug interactions. We describe a method for detecting possible drug-drug interactions using logistic regression analysis to calculate ADR reporting odds ratios. METHODS: To illustrate this method, we analysed the adverse drug reaction 'delayed withdrawal bleeding' resulting from a possible interaction between itraconazole and oral contraceptives in reports received by the Netherlands Pharmacovigilance Foundation LAREB between 1991 and 1998. RESULTS: In total 5,503 reports were included in the study. The odds ratio, adjusted for year of reporting, age and source of the reports, for a delayed withdrawal bleeding in women who used both drugs concomitantly compared with women who used neither oral contraceptives, nor itraconazole, was 85 (95% CI: 32-230). CONCLUSIONS: Since spontaneous reporting systems can only generate signals concerning possible relationships, this association needs to be analysed by other methods in more detail in order to determine the real strength of the relationship. This approach might be a promising tool for the development of procedures for automated detection of possible drug-drug interactions in spontaneous reporting systems.     

天然药对CYP3A的影响

天然药在我国及东南亚国家应用相当广泛，在欧洲及美国，天然药正日益受到重视。天然药成分虽复杂，但其单体或有效成分与合成药一样，大多数是通过细胞色素P450酶（CYP450）代谢。CYP3A是很重要的CYP450酶系，临床上一半以上药物由其代谢。目前很多体外、体内实验证明多种天然药及其成分广泛影响CYP3A，从而在合并用药时，导致药物间的相互作用。研究天然药对CYP3A的调控不仅对安全合理用药有指导作用，而且对于新药研发和揭示药物相互作用机制都有指导意义。     

Factors confounding the successful extrapolation of in vitro CYP3A inhibition information to the in vivo condition.

For the most part, the majority of adverse drug-drug interactions, which are pharmacokinetic in origin, can be understood in terms of alterations of cytochrome P450-catalyzed reactions. Much is known about the human P450 enzymes, and in many cases it is possible to apply this information to clinically related issues. Of the relatively small subset of the total number of human P450s, CYP3A4 appears to be responsible for the largest fraction of the drug oxidation reactions. As a consequence many important drug-drug interactions observed in the clinic are associated with drugs which are principally metabolized by CYP3A4. The two major reasons for drug-drug interactions involving CYP3A4 are induction and inhibition, with inhibition appearing to be the more important in terms of known clinical problems. Fortunately, with the available knowledge of human P450s and in vitro reagents, it is possible to do experiments with drugs to predict the in vivo condition. The goal of these studies is not only to improve predictions about which drugs might show serious P450 interaction problems, but also to decrease the number of in vivo interaction studies that must be performed in drug development. The focus of the current report is to describe some of the confounding factors associated with in vitro drug inhibition studies and the impact of these issues on in vitro/in vivo extrapolations.     

Population-based assessments of clinical drug-drug interactions: qualitative indices or quantitative measures?

Population-based assessments of drug-drug interactions have become more common since the introduction and acceptance of the population pharmacokinetic approach. Unlike traditional methods, population-based studies provide clinically relevant results that can be applied directly to a target patient population. Furthermore, population-based studies do not demand the traditional requirements of intensive pharmacokinetic sampling, rigorous inpatient stays, or stringent assessment schedules. As such, the population-based approach can effectively be used to confirm known drug-drug interactions and further characterize anticipated interactions. A prospectively designed analysis can also reveal drug-drug interactions that might otherwise have gone undetected with traditional methods. Ultimately, these results could help to alleviate clinicians' concerns about using widely marketed drugs in combination therapies and also reduce patients' risk of experiencing unacceptable side effects. This article intends to provide a balanced overview of the population-based approach and its merits, drawbacks, and potential utility in the assessment of drug-drug interactions during clinical drug development.     

Importance of P-glycoprotein for drug-drug interactions

P-glycoprotein (ABCB1) is one of the most extensively studied transporters regarding drug resistance and drug-drug interactions. P-glycoprotein is expressed in multiple key organs in drug disposition such as small intestine, blood-brain barrier, kidney, and liver. Therefore, P-glycoprotein mediated drug-drug interactions can occur at various organs and tissues. This chapter will mainly focus on drug-drug interactions that are mediated by the intestinal P-glycoprotein.During the last decade, many in vitro and in vivo studies reported that the induction or inhibition of P-glycoprotein can lead to drug-drug interactions. For instance, induction of the intestinal P-glycoprotein activity can cause reduced bioavailability of orally administered drugs and decreased therapeutic efficacy. On the other hand, the inhibition of the intestinal P-glycoprotein activity can lead to increased bioavailability, thus leading to an increased risk of adverse side effects.     

Assessment of the quality and quantity of drug-drug interaction studies in recent NDA submissions: study design and data analysis issues.

This report investigates the quality and quantity of drug-drug interaction studies in recent new drug applications (NDAs). Eighty-nine studies contained in 14 NDAs submitted between December 1995 and November 1996 to the U.S. Food and Drug Administration (FDA) were reviewed. The results indicated that the median number of clinical drug-drug interaction studies per NDA was 6, almost double that of a 1994-1995 survey. In vitro metabolism data were present in 70% of the submissions. More than 50% of the submissions contained interaction studies using a battery of drugs (cimetidine, digoxin, or warfarin) without optimal use of the in vitro metabolism or in vivo mass balance data. Various study designs using a median number of 12 subjects were employed in the evaluation of drug-drug interactions. Some of the important study design factors such as dose size, dosing regimen, dosing duration, and timing of coadministration were considered, although not consistently, by the sponsors in their study design. Seventy-five percent of the studies used normal, healthy male subjects, and 25% used patients for whom the new molecular entities were intended. In 33% of the studies, female subjects were also recruited. Although the majority (80%) of the submissions still used p-values to determine the significance of drug interactions, 30% used a more relevant equivalence approach with 90% confidence intervals for key pharmacokinetic and/or pharmacodynamic parameter ratios to assess the extent of drug interactions. Overall, 82% of the studies concluded no interaction. Although population pharmacokinetic analysis can be a useful tool in studying drug-drug interactions, only 21% of the submissions used this approach. In summary, this assessment reveals that the quantity and quality of drug-drug interaction studies in NDAs have improved over the years. These improvements, as well as others that can be implemented, should result in more informative labeling and better patient care. FDA guidance for industry dealing with the design, analysis, and labeling language of in vivo metabolic drug-drug interactions has been developed to assist sponsors and FDA reviewers with these issues.     

Strategies for more effective and reliable ADME. 30-31 March 2004, London, UK

This conference was organised by Vision in Business, in order to address the issue that 'more than 40% of drug preclinical failure is due to ADME-related issues and many drugs are later withdrawn because of unanticipated drug-drug interactions (C Masimirembwa, AstraZeneca M?lndal, Sweden). Academic and industrial scientists were brought together to discuss and present current knowledge/theories on subjects of importance in preclinical and clinical drug development. These included in vivo/in vitro correlations, approaches and experimental models used, in silico models and methods, metabolic databases and their application, as well as safety and financial outcomes. Participants were challenged by the issues covered and encouraged to share their own experiences and opinions during extensive discussions with other scientists.