Mutually co-operative interactions between modulators of P-glycoprotein

Applicability of MDR1 Overexpressing Abcb1KO-MDCKII Cell Lines for Investigating In Vitro Species Differences and Brain Penetration Prediction

Implementing the 3R initiative to reduce animal experiments in brain penetration prediction for CNS-targeting drugs requires more predictive in vitro and in silico models. However, animal studies are still indispensable to obtaining brain concentration and determining the prediction performance of in vitro models. To reveal species differences and provide reliable data for IVIVE, in vitro models are required. Systems overexpressing MDR1 and BCRP are widely used to predict BBB penetration, highlighting the impact of the in vitro system on predictive performance. In this study, endogenous Abcb1 knock-out MDCKII cells overexpressing MDR1 of human, mouse, rat or cynomolgus monkey origin were used. Good correlations between ERs of 83 drugs determined in each cell line suggest limited species specificities. All cell lines differentiated CNS-penetrating compounds based on ERs with high efficiency and sensitivity. The correlation between in vivo and predicted Kp,uu,brain was the highest using total ER of human MDR1 and BCRP and optimized scaling factors. MDR1 interactors were tested on all MDR1 orthologs using digoxin and quinidine as substrates. We found several examples of inhibition dependent on either substrate or transporter abundance. In summary, this assay system has the potential for early-stage brain penetration screening. IC50 comparison between orthologs is complex; correlation with transporter abundance data is not necessarily proportional and requires the understanding of modes of transporter inhibition.

Active transport of rhodamine 123 by the human multidrug transporter P-glycoprotein involves two independent outer gates

Human P-glycoprotein (P-gp) is a multispecific drug-efflux transporter, which plays an important role in drug resistance and drug disposition. Recent cryo-electron microscopy structures confirmed its rotationally symmetric architecture, which allows dual interaction with ATP and substrates. We here report the existence of two distinct, symmetry-related outer gates. Experiments were aided by availability of the X-ray structure of a homodimeric eukaryotic homolog of P-gp from red alga (CmABCB1), which defined the role of an apical tyrosine residue (Y358) in outer gate formation. We mutated analogous tyrosine residues in each half of the human full-length transporter (Y310, Y953) to alanine. These mutants were introduced in engineered transporters which bind rhodamine 123 in one of two symmetry-related binding modes only. Outer gate dysfunction was detected by a loss of active transport characteristics, while these mutants retained the ability for outward downhill transport. Our data demonstrate that symmetric tyrosine residues Y310 and Y953 are involved in formation of two distinct symmetry-related outer gates, which operate contingent on the rhodamine 123 binding mode. Hence, the rotationally symmetric architecture of P-gp, which determines duality in ATP binding and rhodamine 123 interaction, also forms the basis for the existence of two independently operating outer gates.

Role of P-glycoprotein in the brain disposition of seletalisib: Evaluation of the potential for drug-drug interactions

Seletalisib is an orally bioavailable selective inhibitor of phosphoinositide 3-kinase delta (PI3Kδ) in clinical development for the treatment of immune-mediated inflammatory diseases. The present study investigated the role of P-gp in seletalisib disposition, especially brain distribution, and the associated risks of interactions. Seletalisib was found to be actively transported by rodent and human P-gp in vitro (transfected LLC-PK1 cells; K_m of ca. 20 µM), with minimal or no affinity for the other tested transporters. A distribution study in knockout rats (single oral dosing at 750 mg kg^-1) showed that P-gp restricts the brain disposition of seletalisib while having minimal effect on its intestinal absorption. Restricted brain penetration was also observed in cynomolgus monkeys (single oral dosing at 30 mg kg^-1) using brain microdialysis and cerebrospinal fluid sampling (K_p,uu of 0.09 and 0.24, respectively). These findings opened the question of potential pharmacokinetic interaction between seletalisib and P-gp inhibitors. In vitro, CsA inhibited the active transport of seletalisib with an IC₅₀ of 0.13 µM. In rats, co-administration of high doses of CsA (bolus iv followed by continuous infusion) increased the brain distribution of seletalisib (single oral dosing at 5 mg kg^-1). The observed data were found aligned with those predicted by in vitro-in vivo extrapolation. Based on the same extrapolation method combined with literature data, only very few P-gp inhibitors (i.e. CsA, quinine, quinidine) were predicted to increase the brain disposition of seletalisib in the clinical setting (maximal 3-fold changes).

Structure-activity relationships, ligand efficiency, and lipophilic efficiency profiles of benzophenone-type inhibitors of the multidrug transporter P-glycoprotein

The drug efflux pump P-glycoprotein (P-gp) has been shown to promote multidrug resistance (MDR) in tumors as well as to influence ADME properties of drug candidates. Here we synthesized and tested a series of benzophenone derivatives structurally analogous to propafenone-type inhibitors of P-gp. Some of the compounds showed ligand efficiency and lipophilic efficiency (LipE) values in the range of compounds which entered clinical trials as MDR modulators. Interestingly, although lipophilicity plays a dominant role for P-gp inhibitors, all compounds investigated showed LipE values below the threshold for promising drug candidates. Docking studies of selected analogues into a homology model of P-glycoprotein suggest that benzophenones show an interaction pattern similar to that previously identified for propafenone-type inhibitors.

Polymorphisms within PfMDR1 alter the substrate specificity for anti-malarial drugs in Plasmodium falciparum

Resistance to several anti-malarial drugs has been associated with polymorphisms within the P-glycoprotein homologue (Pgh-1, PfMDR1) of the human malaria parasite Plasmodium falciparum. Pgh-1, coded for by the gene pfmdr1, is predominately located at the membrane of the parasite's digestive vacuole. How polymorphisms within this transporter mediate alter anti-malarial drug responsiveness has remained obscure. Here we have functionally expressed pfmdr1 in Xenopus laevis oocytes. Our data demonstrate that Pgh-1 transports vinblastine, an established substrate of mammalian MDR1, and the anti-malarial drugs halofantrine, quinine and chloroquine. Importantly, polymorphisms within Pgh-1 alter the substrate specificity for the anti-malarial drugs. Wild-type Pgh-1 transports quinine and chloroquine, but not halofantrine, whereas polymorphic Pgh-1 variants, associated with altered drug responsivenesses, transport halofantrine but not quinine and chloroquine. Our data further suggest that quinine acts as an inhibitor of Pgh-1. Our data are discussed in terms of the model that Pgh-1-mediates, in a variant-specific manner, import of certain drugs into the P. falciparum digestive vacuole, and that this contributes to accumulation of, and susceptibility to, the drug in question.

Schistosomicidal activity of the antimalarial drug, mefloquine, in Schistosoma mansoni-infected mice

Therapeutic effects of racemic mefloquine were assessed in Schistosoma mansoni-infected mice, and evaluated by recording worm burden, the status of egg maturation and viability, and intestinal mast cell recruitment. Age-matched mice were divided into four groups, of which two were infected. At 8 weeks postinfection, one group of infected and one group of uninfected mice were treated with a single dose of mefloquine (150 mg/kg). Ten days after treatment, all animals were killed. Mefloquine at 150 mg/kg had no effect on worm burden, but significantly reduced the number of eggs in the first three developmental egg stages. Analysis of intestinal mast cell numbers showed that mefloquine induced mastocytosis both in infected and control animals. In conclusion, mefloquine significantly reduces egg production in S. mansoni-infected mice, suggesting a therapeutic potency in schistosomiasis therapy. Mefloquine also exerts a significant proinflammatory effect on the intestine. Through its effect on egg production, mefloquine may be a cause of silent schistosomiasis in travelers using mefloquine for malaria chemoprophylaxis. Further study of the anti-schistosomal activity of mefloquine is warranted, as its activity against other helminths.

New functional assay of P-glycoprotein activity using Hoechst 33342

In this study we describe a simplified, HTS-capable functional assay for the multidrug resistance (MDR) transporter P-glycoprotein (P-gp) based on its substrate Hoechst 33342. The physicochemical properties of Hoechst 33342 and the enormous milieu dependency of its fluorescence intensity allowed performing the assay in a homogeneous manner. This new assay served as an effective tool to estimate the potency of 10 well recognized P-gp substrates and modulators. Further, the potency of these compounds was also estimated in the calcein AM assay. The Hoechst 33342 and calcein AM assays yielded significantly comparable results for all compounds tested. Principal component analysis (PCA) applied to literature data on inhibition of P-gp activity and our results obtained in the Hoechst 33342 and calcein AM assay indicated similarity of compared functional transport assays. However, no correlation could be detected between these functional assays and the ATPase activity assay.

Future perspectives for the development of P-glycoprotein modulators

Resistance to chemotherapeutic agents constitutes one of the major obstacles to the successful treatment of cancer. While several mechanisms underlying drug resistance have been elucidated, the most widely studied mechanism involves the efflux of antineoplastic drugs from cancer cells by P-glycoprotein, the 170 kD glycoprotein product of the MDR-I gene. The observation that several compounds are able to inhibit P-glycoprotein in vitro created optimism that the problem of multidrug resistance in cancer could be quickly resolved by moving these compounds into the clinic. However, despite a large number of clinical trials with several different putative Pgp modulators, the value of Pgp modulation in clinical oncologic practice remains unresolved. While these initial trials have not answered the question of whether Pgp is an important mechanism of resistance in human cancers, or whether modulation of Pgp is likely to positively impact on the treatment of cancer, they have provided insights regarding the problems inherent in conducting trials of this nature. These clinical insights, along with knowledge gained from continued basic research on drug resistance mediated by Pgp and related transporters, will form a strong foundation for future research into the role of Pgp and Pgp modulation in the treatment of cancer. The ubiquitous nature of transporters and the high prevalence of transporter substrates among antineoplastic drugs, compel the development of modulators that can be used to prevent or reverse drug resistance.

Pesticides and their binary combinations as P-glycoprotein inhibitors in NIH 3T3/MDR1 cells

The purpose of the present study was to assess do selected pesticides as well as their binary combinations act as inhibitors of P-glycoprotein (P-gp) activity of NIH 3T3 mouse fibroblasts stably transfected with human MDR1 gene (NIH 3T3/MDR1). As a result of P-gp inhibition, the increase of intracellular accumulation of a model P-gp substrate fluorescent calcein acetoxymethyl ester was measured. Pesticide and verapamil individual dose-response data were scaled and expressed as percent of maximum effect. Results showed that out of 14 pure pesticides tested, endosulfan, phosalone and propiconazole were nearly as potent as model inhibitor verapamil (EC(50)=1.5μM), while diazinon showed a lower potency of inhibiting P-gp transport activity (EC(50)=58.4μM). Concentrations of pesticides that produced the same inhibiting effect (isoboles) were combined binary. Results calculated using the isobole method revealed that diazinon caused synergistic effect in inhibiting P-gp transport activity in all combinations.

Insights into the molecular mechanism of action of Celastraceae sesquiterpenes as specific, non-transported inhibitors of human P-glycoprotein

Dihydro-beta-agarofuran sesquiterpenes from Celastraceae have been recently shown to bind to human P-glycoprotein (Pgp), functioning as specific, mixed-type inhibitors of its drug transport activity, as well as multidrug resistance (MDR) modulators in vitro. However, nothing is known about whether such compounds are themselves transported by Pgp, or whether they affect Pgp expression as well as its activity, or about the location of their binding site within the protein. We performed transport experiments with a newly synthesized fluorescent sesquiterpene derivative, which retains the anti-Pgp activity of its natural precursor. This probe was poorly transported by Pgp, MRP1, MRP2 and BCRP transporters, compared with classical MDR substrates. Moreover, Pgp did not confer cross-resistance to the most potent dihydro-beta-agarofurans, which did not affect Pgp expression levels in several MDR cell lines. Finally, we observed competitive and non-competitive interactions between one of such dihydro-beta-agarofurans (Mama12) and classical Pgp modulators such as cyclosporin A, verapamil, progesterone, vinblastine and GF120918. These findings suggest that multidrug ABC transporters do not confer resistance to dihydro-beta-agarofurans and could not affect their absorption and biodistribution in the body. Moreover, we mapped their binding site(s) within Pgp, which may prove useful for the rational design of improved modulators based on the structure of dihydro-beta-agarofurans.

Interactions of mefloquine with ABC proteins, MRP1 (ABCC1) and MRP4 (ABCC4) that are present in human red cell membranes

Human erythrocyte membranes express the multidrug resistance-associated proteins, MRP1, MRP4 and 5, that collectively can efflux oxidised glutathione, glutathione conjugates and cyclic nucleotides. It is already known that the quinoline derivative, MK-571, is a potent inhibitor of MRP-mediated transport. We here examine whether the quinoline-based antimalarial drugs, amodiaquine, chloroquine, mefloquine, primaquine, quinidine and quinine, also interact with erythrocyte MRPs with consequences for their access to the intracellular parasites or for efflux of oxidised glutathione from infected cells. Using inside-out vesicles prepared from human erythrocytes we have shown that mefloquine and MK-571 inhibit transport of 3 microM [(3)H]DNP-SG known to be mediated by MRP1 (IC(50) 127 and 1.1 microM, respectively) and of 3.3 microM [(3)H]cGMP thought but not proven to be mediated primarily by MRP4 (IC(50) 21 and 0.41 microM). They also inhibited transport in membrane vesicles prepared from tumour cells expressing MRP1 or MRP4 and blocked calcein efflux from MRP1-overexpressing cells and BCECF efflux from MRP4-overexpressing cells. Both stimulated ATPase activity in membranes prepared from MRP1 and MRP4-overexpressing cells and inhibited activity stimulated by quercetin or PGE(1), respectively. Neither inhibited [alpha-(32)P]8-azidoATP binding confirming that the interactions are not at the ATP binding site. These results demonstrate that mefloquine and MK-571 both inhibit transport of other substrates and stimulate ATPase activity and thus may themselves be substrates for transport. But at concentrations achieved clinically mefloquine is unlikely to affect the MRP1-mediated transport of GSSG across the erythrocyte membrane.

Structural mechanism of the simultaneous binding of two drugs to a multidrug-binding protein

The structural basis of simultaneous binding of two or more different drugs by any multidrug-binding protein is unknown and also how this can lead to a noncompetitive, uncompetitive or cooperative binding mechanism. Here, we describe the crystal structure of the Staphylococcus aureus multidrug-binding transcription repressor, QacR, bound simultaneously to ethidium (Et) and proflavin (Pf). The structure underscores the plasticity of the multidrug-binding pocket and reveals an alternative, Pf-induced binding mode for Et. To monitor the simultaneous binding of Pf and Et to QacR, as well as to determine the effects on the binding affinity of one drug when the other drug is prebound, a novel application of near-ultraviolet circular dichroism (UVCD) was developed. The UVCD equilibrium-binding studies revealed identical affinities of Pf for QacR in the presence or absence of Et, but significantly diminished affinity of Et for QacR when Pf is prebound, findings that are readily explicable by their structures. The principles for simultaneous binding of two different drugs discerned here are likely employed by the multidrug efflux transporters.

Cerebral uptake of mefloquine enantiomers with and without the P-gp inhibitor elacridar (GF1210918) in mice

1. Mefloquine is a chiral neurotoxic antimalarial agent showing stereoselective brain uptake in humans and rats. It is a substrate and an inhibitor of the efflux protein P-glycoprotein. 2. We investigated the stereoselective uptake and efflux of mefloquine in mice, and the consequences of the combination with an efflux protein inhibitor, elacridar (GF120918) on its brain transport. 3. Racemic mefloquine (25 mg kg(-1)) was administered intraperitoneally with or without elacridar (10 mg kg(-1)). Six to seven mice were killed at each of 11 time-points between 30 min and 168 h after administration. Blood and brain concentrations of mefloquine enantiomers were determined using liquid chromatography. 4. A three-compartment model with zero-order absorption from the injection site was found to best represent the pharmacokinetics of both enantiomers in blood and brain. (-)Mefloquine had a lower blood and brain apparent volume of distribution and a lower efflux clearance from the brain, resulting in a larger brain/blood ratio compared to (+)mefloquine. Elacridar did not modify blood concentrations or the elimination rate from blood for either enantiomers. However, cerebral AUC(inf) of both enantiomers were increased, with a stronger effect on (+)mefloquine. The efflux clearance from the brain decreased for both enantiomers, with a larger decrease for (+)mefloquine. 5. After administration of racemic mefloquine in mice, blood and brain pharmacokinetics are stereoselective, (+)mefloquine being excreted from brain more rapidly than its antipode, showing that mefloquine is a substrate of efflux proteins and that mefloquine enantiomers undergo efflux in a stereoselective manner. Moreover, pretreatment with elacridar reduced the brain efflux clearances with a more pronounced effect on (+)mefloquine.

Role of chiral chromatography in therapeutic drug monitoring and in clinical and forensic toxicology

Advances in chiral chromatographic separations have given pharmacologists and toxicologists the tools to examine unexpected clinical results involving chiral drugs. The ability to unravel complex phenomena associated with drug transport and drug metabolism is presented in this manuscript. The relation between the chirality of the drug mefloquine and the intracellular concentrations of the drug cyclosporine is illustrated by examining the effect of the enantiomers of mefloquine on the transport activity of P-glycoprotein (Pgp). These studies were conducted using a liquid chromatographic column containing immobilized Pgp. The results demonstrated that (+)-mefloquine competitively displaced the Pgp substrate cyclosporine whereas (-)-mefloquine had no effect on cyclosporine-Pgp binding. The data suggest that cyclosporine cellular and CNS concentrations can be increased through the concomitant administration of (+)-mefloquine. The use of chirality in clinical and forensic situations is also illustrated by the metabolism of the enantiomers of ketamine (KET). The plasma concentrations of (+)-KET and (-)-KET and the norketamine metabolites (+)-NK and (-)-NK were measured in rat plasma using enantioselective gas chromatography. The separations were accomplished using a gas chromatography chiral stationary phase based on beta-cyclodextrin. The pharmacokinetic profiles of (+)-, (-)-KET and (+)-, (-)-NK were determined in control and protein-calorie malnourished (PCM) rats to determine the effect of PCM on ketamine metabolism and clearance. The results indicate that PCM produced a significant and stereoselective decrease in KET and NK metabolism. The data suggest that the effects of environmental factors (smoking, alcohol use, diet) and drug interactions (coadministered agents) can be measured using the changes in stereochemical metabolic and pharmacokinetic patterns of KET and similar drugs.

The absence of stereoselective P-glycoprotein- and multidrug resistance-associated protein-mediated transport of daunorubicin

Multidrug resistance phenotype in mammalian cells is often correlated with overexpression of P-glycoprotein (P-gp) or multidrug resistance-associated protein (MRP1). Both proteins are energy-dependent drug efflux pumps that efficiently reduce the intracellular accumulation and hence the cytotoxicity of many natural cytotoxins. Thus, both P-gp and MRP1 proteins are able to transport anthracycline but the role of chirality has not, up to now, been addressed. In this study, we compared the P-gp- and MRP1-mediated efflux of daunorubicin and its enantiomer WP900 in multidrug-resistant cells overexpressing either P-gp (K562/ADR cells) or MRP1 (GLC4/ADR cells). Using fluorescence techniques, we showed that in both cell lines the presence of the pump yielded a gradient of drug concentration: the intracellular free drug concentration in the cytosol was lower than the extracellular free drug concentration. Our data showed that the gradient of concentration generated by the pump was the same whether DNR or WP900 was used. This means that P-gp on the one hand and MRP1 on the other recognise WP900 as well as DNR and that the chirality of the molecule plays no role.

Tamoxifen and colchicine-modulated vinblastine followed by 5-fluorouracil in advanced renal cell carcinoma: a phase II study

OBJECTIVES

Chemotherapy resistance of renal cell carcinoma (RCC) has been attributed in large part to multidrug resistance (MDR). Reported MDR-modulated chemotherapy for RCC, however, has resulted in only marginal response benefits. In this study, the MDR-modulated effect of paired tamoxifen and colchicine on vinblastine and the possible additive effect of 5-fluorouracil (5-FU) were investigated in the treatment of advanced RCC.

METHODS

Chemotherapy was administered every 4 weeks with biweekly vinblastine (4 mg/m(2)/day, intravenously on days 1 and 15) modulated by oral tamoxifen (100 mg/day) and colchicine (1 mg/day) from days -1 to 2 and from days 13 to 16. 5-FU (800 mg/m(2)/day from days 2 to 5) was administered after vinblastine administration as a continuous infusion.

RESULTS

Of 17 eligible patients with advanced RCC available for evaluation, 1 achieved a complete response (CR) and 3 a partial response (PR), with an overall response (CR plus PR) rate of 23.5%. The median overall survival time of all patients was 10 months (95% confidence interval [CI] 3.5 to 16.5); that of our patients with poor, intermediate, and favorable risks as stratified by Motzer's model was 6 (95% CI 1.7 to 10.3), 10 (95% CI 7.9 to 12.2), and 26 (95% CI 24.4 to 27.6) months, respectively. These results are encouraging in view of the poor efficacy of chemotherapy in RCC observed previously. Additionally, the treatment toxicity was limited: toxicity of grade 3 or greater occurred in only 1 patient with leukopenia, and no treatment-related mortality was found.

CONCLUSIONS

The encouraging response rates and overall survival with limited toxicity warrant further investigation of this combination therapy as an integrated part of immunochemotherapy for RCC.

Interactions of racemic mefloquine and its enantiomers with P-glycoprotein in an immortalised rat brain capillary endothelial cell line, GPNT

The brain distribution of the enantiomers of the antimalarial drug mefloquine is stereoselective according to the species. This stereoselectivity may be related to species-specific differences in the properties of some membrane-bound transport proteins, such as P-glycoprotein (P-gp). The interactions of racemic mefloquine and its individual enantiomers with the P-glycoprotein efflux transport system have been analysed in immortalised rat brain capillary endothelial GPNT cells. Parallel studies were carried out for comparison in human colon carcinoma Caco-2 cells. The cellular accumulation of the P-glycoprotein substrate, [(3)H]vinblastine, was significantly increased both in GPNT cells and in Caco-2 cells when treated with racemic mefloquine and the individual enantiomers. In GPNT cells, the (+)-stereoisomer of mefloquine was up to 8-fold more effective than its antipode in increasing cellular accumulation of [(3)H]vinblastine, while in Caco-2 cells, both enantiomers were equally effective. These results suggest that racemic mefloquine and its enantiomers are effective inhibitors of P-gp. Furthermore, a stereoselective P-glycoprotein inhibition is observed in rat cells but not in human cells. The efflux of [(14)C]mefloquine from GPNT cells was decreased when the cells were incubated with the P-gp modulators, verapamil, cyclosporin A or chlorpromazine, suggesting that MQ could be a P-gp substrate.

Cooperativity in the inhibition of P-glycoprotein-mediated daunorubicin transport: evidence for half-of-the-sites reactivity

P-Glycoprotein (Pgp) is an important transport enzyme composed of two homologous domains and transports a wide range of structurally diverse xenobiotics from the cell. Recent studies have indicated that allosteric interactions occur between the nucleotide binding domains and between the substrate binding domains of the two halves, but the extent of this interaction as well as the means by which the enzyme can transport such a wide variety of substrates has not been elucidated. Herein, the Pgp-mediated transport of a marker substrate, daunorubicin (DNR), out of viable cells was examined in the presence of a variety of other known substrates of Pgp. For most of the typical Pgp substrates examined, the relationship between inhibition of DNR efflux and competing substrate concentration was sigmoidal and therefore not a simple mutually exclusive competitive inhibition of transport. The Hill coefficient ranged from about 3 to 5 for the inhibition of transport of DNR. This negative cooperativity in combination with recent evidence, including several examples of noncompetitive inhibition between the homologous halves of Pgp, indicates a "half-of-the-sites" reactivity. Our data support the mechanistic proposal that substrate binding at one putative transport binding site precludes activity at another unequal site; many of the substrates examined exert a negative allosteric effect on the other transport site (and vice versa). A half-of-the-sites reactivity model would account for many of these observations and may be critical to the efficiency of Pgp substrate transport of a broad spectrum of compounds.

Two transport binding sites of P-glycoprotein are unequal yet contingent: initial rate kinetic analysis by ATP hydrolysis demonstrates intersite dependence

The ATP-dependent transport enzyme known as P-glycoprotein (P-gp) confers multidrug resistance (MDR) against many unrelated drugs and xenobiotics. To understand better the broad substrate specificity of the enzyme as well as the mechanism of substrate transport out of the cell, it is critical to characterize the substrate binding sites. Since approximately 1 ATP is hydrolyzed per transport event, phosphate release rate provides a steady-state kinetics assay. Notably, the substrate H33342 causes a decrease in the baseline hydrolysis of ATP (probably due to competition for transport with an endogenous membrane lipid substrate) providing an excellent tool for a comprehensive graphical kinetic analysis of the interaction of substrate pairs at the transport site(s) allowing the determination of inhibition type and hence characterization of transport binding sites. The substrate H33342 interacted with quinidine, progesterone, and propranolol in a non-competitive manner, indicating that binding of H33342 precludes active transport of these other substrates at a distinct site. Compounds such as TPP+ and verapamil, and perhaps also nicardipine, interacted with H33342 as mixed-type inhibitors. This type of interaction results from a reduced affinity at the opposing active site by a factor of alpha and sometimes a partial activity of a fraction beta. Indeed, H33342 binding caused a roughly four-fold reduced affinity for TPP+. Using this definitive approach to inhibition kinetics, we were able to establish traits of a second transport site in P-gp. Therefore, the sites are unequal; however, the performance at one site is contingent on the other being unoccupied, and transport is also sometimes mitigated when the other site is occupied.

Photoaffinity analogs for multidrug resistance-related transporters and their use in identifying chemosensitizers

A major obstacle in cancer treatment is the development of resistance to multiple chemotherapeutic agents in tumor cells. The hallmark of this multidrug resistance (MDR) is overexpression of the MDR 1 P-glycoprotein or the multidrug resistance protein MRP1. It is well documented that these proteins confer MDR in cancer cells. Much evidence indicates that control of intracellular drug levels in MDR cells is determined by P-glycoprotein or MRP, and therefore these proteins are suitable targets for identifying MDR-reversing agents (MDR modulators). We originally explored the drug-binding ability of P-glycoprotein by synthesizing and using radioactive photoaffinity analogs of vinblastine. Since our initial discovery that P-glycoprotein binds to vinblastine photoaffinity analogs, many P-glycoprotein- and MRP-specific photoaffinity analogs have been developed. In this review, photoaffinity analogs which specifically bind to P-glycoprotein or MRP are discussed. Moreover, utilizing these photoprobes to identify, characterize and localize the drug binding sites of P-glycoprotein and MRP is described. Using P-glycoprotein-specific photoaffinity analogs in combination with site-directed antibodies to several domains of this protein has allowed the localization of the general binding domains of some of the cytotoxic agents an MDR modulators on P-glycoprotein. However, the molecular architecture of the drug binding sites, their exact location on the P-glycoprotein molecule, and the total number of the drug binding sites remain to be determined. This review discusses recent advances in delineating the structure of the drug-binding sites of P-glycoprotein. Moreover, novel MRP1 photoaffinity analogs are reviewed. Copyright 1999 Harcourt Publishers Ltd.

The molecular interaction of the high affinity reversal agent XR9576 with P-glycoprotein

1 The kinetics and nature of equilibrium binding were used to characterize the molecular interaction of the anthranilic acid derivative [3H]-XR9576 with the multidrug resistance P-glycoprotein (P-gp). XR9576 displayed specific high-affinity binding to P-gp (Bmax = 275 pmol mg-1, Kd = 5.1 nM). The transport substrates [3H]-vinblastine and [3H]-paclitaxel displayed 4 fold and 20 fold lower affinity respectively for P-gp. The duration of action of XR9576 with P-gp was increased in comparison to that of vinblastine which displayed a slower rate of association and a faster dissociation rate. 2 The relative affinities of several modulators and transport substrates to interact with P-gp were determined from displacement drug equilibrium binding assays. Vinblastine and paclitaxel could only fractionally displace [3H]-XR9576 binding, displaying Ki values significantly different from their measured Kd values. This suggests a non-competitive interaction between XR9576 and the P-gp substrates vinblastine and paclitaxel. 3 XR9576 was shown to be a potent modulator of P-gp mediated [3H]-vinblastine and [3H]-paclitaxel transport as it increased the steady-state accumulation of these cytotoxics in CHrB30 cells to levels observed in non-P-gp-expressing AuxB1 cells (EC50 = 487+/-50 nM). This inhibition of drug transport is not mediated through competition for transport since [3H]-XR9576 accumulation was not influenced by P-gp expression or function. 4 These results demonstrate that the P-gp modulator XR9576 exhibits greater selectivity, duration of inhibition and potency of interaction with this transporter than any other reported modulators. Several lines of evidence suggest that XR9576 inhibits P-gp function by binding at a site which is distinct from the site of interaction of transport substrates. The two sites may be classified as serving modulatory or transport functions.

Mechanism of action of P-glycoprotein in relation to passive membrane permeation

This review presents a survey of studies of the movement of chemotherapeutic drugs into cells, their extrusion from multidrug-resistant (MDR) cells overexpressing P-glycoprotein (Pgp), and the mode of sensitization of MDR cells to anticancer drugs by Pgp modulators. The consistent features of the kinetics from studies of the operation of Pgp in cells were combined in a computer model that enables the simulation of experimental scenarios. MDR-type drugs are hydrophobic and positively charged and as such bind readily to negatively charged phospholipid head groups of the membrane. Transmembrane movement of MDR-type drugs, such as doxorubicin, occurs by a flip-flop mechanism with a lifetime of about 1 min rather than by diffusion down a gradient present in the lipid core. A long residence time of a drug in the membrane leaflet increases the probability that P-glycoprotein will remove it from the cell. In a manner similar to ion-transporting ATPases, such as Na+,K(+)-ATPase, Pgp transports close to one drug molecule per ATP molecule hydrolyzed. Computer simulation of cellular pharmacokinetics, based on partial reactions measured in vitro, show that the efficiency of Pgp, in conferring MDR on cells, depends on the pumping capacity of Pgp and its affinity toward the specific drug, the transmembrane movement rate of the drug, the affinity of the drug toward its pharmacological cellular target, and the affinity of the drug toward intracellular trapping sites. Pgp activities present in MDR cells allow for the efficient removal of drugs, whether directly from the cytoplasm or from the inner leaflet of the plasma membrane. A prerequisite for a successful modulator, capable of overcoming cellular Pgp, is the rapid passive transbilayer movement, allowing it to reenter the cell immediately and thus successfully occupy the Pgp active site(s).

Efflux of Rhodamine From CD56+ Cells as a Surrogate Marker for Reversal of P-Glycoprotein–Mediated Drug Efflux by PSC 833

The expression of high levels of P-glycoprotein (Pgp) in circulating mononuclear cells allowed us to use an ex vivo assay as a surrogate measure of Pgp antagonism. Efflux of rhodamine from CD56+cells was measured before the start of PSC 833 and at varying times thereafter. Patients receiving PSC 833 had decreased rhodamine efflux from their circulating CD56+ cells. Time course studies showed that following a single oral dose of PSC 833, decreased rhodamine efflux was found in some patients within 15 minutes of treatment. Maximal inhibition was observed at times ranging from 45 minutes to 60 minutes. A dose-response relationship was shown between the concentration of PSC 833 in the blood and the inhibition of rhodamine efflux, with an apparent plateau of the inhibition of rhodamine efflux at approximately 1,000 ng/mL. The Ki, defined as the concentration required for half-maximal inhibition of Pgp-mediated rhodamine efflux, was determined to be in the range of 29 to 181 ng/mL; although results in two patients were distinctly different, with Ki values of 914 and 916 ng/mL. MRK-16 staining was similar among all patients. We conclude that measurement of rhodamine efflux from CD56+ cells provides a surrogate assay with the potential for monitoring Pgp antagonism in clinical trials.

Efflux of Rhodamine From CD56+ Cells as a Surrogate Marker for Reversal of P-Glycoprotein–Mediated Drug Efflux by PSC 833

The expression of high levels of P-glycoprotein (Pgp) in circulating mononuclear cells allowed us to use an ex vivo assay as a surrogate measure of Pgp antagonism. Efflux of rhodamine from CD56+cells was measured before the start of PSC 833 and at varying times thereafter. Patients receiving PSC 833 had decreased rhodamine efflux from their circulating CD56+ cells. Time course studies showed that following a single oral dose of PSC 833, decreased rhodamine efflux was found in some patients within 15 minutes of treatment. Maximal inhibition was observed at times ranging from 45 minutes to 60 minutes. A dose-response relationship was shown between the concentration of PSC 833 in the blood and the inhibition of rhodamine efflux, with an apparent plateau of the inhibition of rhodamine efflux at approximately 1,000 ng/mL. The Ki, defined as the concentration required for half-maximal inhibition of Pgp-mediated rhodamine efflux, was determined to be in the range of 29 to 181 ng/mL; although results in two patients were distinctly different, with Ki values of 914 and 916 ng/mL. MRK-16 staining was similar among all patients. We conclude that measurement of rhodamine efflux from CD56+ cells provides a surrogate assay with the potential for monitoring Pgp antagonism in clinical trials.

Activation of nitric oxide synthase is involved in tamoxifen-induced apoptosis of human erythroleukemia K562 cells

Tamoxifen induces apoptosis (programmed cell death) in human erythroleukemia K562 cells. Nitric oxide synthase activity and expression increased in apoptotic cells by 315% and 280%, respectively, compared to controls. The specific inhibitor of nitric oxide synthase, L-NAME, protected K562 cells from tamoxifen-induced apoptosis, whereas the nitric oxide donor, sodium nitroprusside (SNP), potentiated the apoptotic effect of the drug. In addition, 5-lipoxygenase was activated by tamoxifen and the specific enzyme inhibitor, MK886, protected K562 cells against the drug. Conversely, the 5-lipoxygenase product, 5-hydroperoxyeicosatetraenoic acid, enhanced the tamoxifen-induced apoptosis. Finally, tamoxifen altered also membrane properties of K562 cells.

A bacterial antibiotic-resistance gene that complements the human multidrug-resistance P-glycoprotein gene

Bacteria have developed many fascinating antibiotic-resistance mechanisms. A protein in Lactococcus lactis, LmrA, mediates antibiotic resistance by extruding amphiphilic compounds from the inner leaflet of the cytoplasmic membrane. Unlike other known bacterial multidrug-resistance proteins, LmrA is an ATP-binding cassette (ABC) transporter. The human multidrug-resistance P-glycoprotein, encoded by the MDR1 gene, is also an ABC transporter, overexpression of which is one of the principal causes of resistance of human cancers to chemotherapy. We expressed lmrA in human lung fibroblast cells. Surprisingly, LmrA was targeted to the plasma membrane and conferred typical multidrug resistance on these human cells. The pharmacological characteristics of LmrA and P-glycoprotein-expressing lung fibroblasts were very similar, and the affinities of both proteins for vinblastine and magnesium-ATP were indistinguishable. Blockers of P-glycoprotein-mediated multidrug resistance also inhibited LmrA-dependent drug resistance. Kinetic analysis of drug dissociation from LmrA expressed in plasma membranes of insect cells revealed the presence of two allosterically linked drug-binding sites indistinguishable from those of P-glycoprotein. These findings have implications for the reversal of antibiotic resistance in pathogenic microorganisms. Taken together, they demonstrate that bacterial LmrA and human P-glycoprotein are functionally interchangeable and that this type of multidrug-resistance efflux pump is conserved from bacteria to man.

Positively cooperative sites for drug transport by P-glycoprotein with distinct drug specificities

In this paper, we show that P-glycoprotein contains two distinct sites for drug binding and transport, and that, unexpectedly, these sites interact in a positively cooperative manner. The kinetics of transport of rhodamine 123 and Hoechst 33342 in isolated P-glycoprotein-rich plasma membrane vesicles from Chinese hamster ovary CH(R)B30 cells were followed by continuous fluorescence monitoring. Each substrate stimulated P-glycoprotein-mediated transport of the other. Colchicine and quercetin stimulated rhodamine 123 transport and inhibited Hoechst 33342 transport. In contrast, anthracyclines such as daunorubicin and doxorubicin stimulated Hoechst 33342 transport and inhibited rhodamine 123 transport. Vinblastine, actinomycin D, and etoposide inhibited transport of both dyes. The results are consistent with a functional model of P-glycoprotein containing at least two positively cooperative sites (H site and R site) for drug binding and transport. This model is consistent with earlier observations of competitive and non-competitive effects of P-glycoprotein substrates and chemosensitizers. Such a two-site model may be fundamental to multidrug transport by P-glycoprotein, and it may be a feature common to other ATP-dependent transporters belonging to the ATP-binding cassette superfamily.

Extraction of Hoechst 33342 from the cytoplasmic leaflet of the plasma membrane by P-glycoprotein

P-glycoprotein is an ATP-dependent plasma membrane multidrug transporter of broad specificity. A common chemical property of its substrates is that all are lipophilic. Using Hoechst 33342 as the substrate, we have previously shown that P-glycoprotein extracts the substrate directly from the lipid bilayer [Shapiro, A. B., Corder, A. B. & Ling, V. (1997) Eur. J. Biochem. 250, 115-121]. In this paper, we determined the leaflet of the plasma membrane from which P-glycoprotein extracts Hoechst 33342. The initial rate of Hoechst 33342 transport upon ATP addition to P-glycoprotein-rich inside-out plasma membrane vesicles decreased slightly with the amount of time previously elapsed for slow diffusion of Hoechst 33342 to the extracellular leaflet. This result is consistent with transport from the cytoplasmic leaflet. Fluorescence resonance energy transfer from donor Hoechst 33342 to acceptor 2-[6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoyl-sn-glycero- 3-phosphocholine (Nbd-C6-HPC) in the cytoplasmic leaflet was used to monitor the amount of Hoechst 33342 in the cytoplasmic leaflet versus time. The initial rate of decrease of the energy-transfer-related Nbd-C6-HPC fluorescence after ATP addition exceeded that of the Hoechst 33342 fluorescence and continued to decrease after decrease of the Hoechst 33342 fluorescence had ceased. These effects were consistent with transport of Hoechst 33342 from the cytoplasmic leaflet to the aqueous interior of the vesicles, followed by rebinding to the extracellular leaflet. This demonstrates that P-glycoprotein transports drugs from the cytoplasmic leaflet of the plasma membrane directly to the aqueous extracellular medium. This finding has implications for efforts to localize the drug-binding site(s) within P-glycoprotein.

Localization of the iodomycin binding site in hamster P-glycoprotein

P-glycoprotein, the overexpression of which is a major cause for the failure of cancer chemotherapy in man, recognizes and transports a broad range of structurally unrelated amphiphilic compounds. This study reports on the localization of the binding site of P-glycoprotein for iodomycin, the Bolton-Hunter derivative of the anthracycline daunomycin. Plasma membrane vesicles isolated from multidrug-resistant Chinese hamster ovary B30 cells were photolabeled with [125I]iodomycin. After chemical cleavage behind the tryptophan residues, 125I-labeled peptides were separated by electrophoresis and high performance liquid chromatography. Edman sequencing revealed that [125I]iodomycin had been predominantly incorporated into the fragment 230-312 of isoform I of hamster P-glycoprotein. According to models based on hydropathy plots, the amino acid sequence 230-312 forms the distal part of transmembrane segment 4, the second cytoplasmic loop, and the proximal part of transmembrane segment 5 in the N-terminal half of P-glycoprotein. The binding site for iodomycin is recognized with high affinity by vinblastine and cyclosporin A.