The effect of crown ethers, tetraalkylammonium salts, and polyoxyethylene amphiphiles on pirarubicin incorporation in K562 resistant cells

Interaction of crown ethers with the ABCG2 transporter and their implication for multidrug resistance reversal

Overexpression of ABC transporters, such as ABCB1 and ABCG2, plays an important role in mediating multidrug resistance (MDR) in cancer. This feature is also attributed to a subpopulation of cancer stem cells (CSCs), having enhanced tumourigenic potential. ABCG2 is specifically associated with the CSC phenotype, making it a valuable target for eliminating aggressive and resistant cells. Several natural and synthetic ionophores have been discovered as CSC-selective drugs that may also have MDR-reversing ability, whereas their interaction with ABCG2 has not yet been explored. We previously reported the biological activities, including ABCB1 inhibition, of a group of adamantane-substituted diaza-18-crown-6 (DAC) compounds that possess ionophore capabilities. In this study, we investigated the mechanism of ABCG2-inhibitory activity of DAC compounds and the natural ionophores salinomycin, monensin and nigericin. We used a series of functional assays, including real-time microscopic analysis of ABCG2-mediated fluorescent substrate transport in cells, and docking studies to provide comparative aspects for the transporter-compound interactions and their role in restoring chemosensitivity. We found that natural ionophores did not inhibit ABCG2, suggesting that their CSC selectivity is likely mediated by other mechanisms. In contrast, DACs with amide linkage in the side arms demonstrated noteworthy ABCG2-inhibitory activity, with DAC-3Amide proving to be the most potent. This compound induced conformational changes of the transporter and likely binds to both Cavity 1 and the NBD-TMD interface. DAC-3Amide reversed ABCG2-mediated MDR in model cells, without affecting ABCG2 expression or localization. These results pave the way for the development of new crown ether compounds with improved ABCG2-inhibitory properties.

Recent Updates on Supramolecular-Based Drug Delivery - Macrocycles and Supramolecular Gels

Supramolecules-based drug delivery has attracted significant recent research attention as it could enhance drug solubility, retention time, targeting, and stimuli responsiveness. Among the different supramolecules and assemblies, the macrocycles and the supramolecular hydrogels are the two important categories investigated to a greater extent. Here, we provide the most recent advancements in these categories. Under macrocycles, reports on drug delivery by cyclodextrins, cucurbiturils, calixarenes/pillararenes, crown ethers and porphyrins are detailed. The second category discusses the supramolecular hydrogels of macrocycles/polymers and low molecular weight gelators. The updated information provided could be helpful to advance R & D in this vital area.

Medicinal chemistry strategies to discover P-glycoprotein inhibitors: An update

The presence of multidrug resistance (MDR) in malignant tumors is one of the primary causes of treatment failure in cancer chemotherapy. The overexpression of the ATP binding cassette (ABC) transporter, P-glycoprotein (P-gp), which significantly increases the efflux of certain anticancer drugs from tumor cells, produces MDR. Therefore, inhibition of P-gp may represent a viable therapeutic strategy to overcome cancer MDR. Over the past 4 decades, many compounds with P-gp inhibitory efficacy (referred to as first- and second-generation P-gp inhibitors) have been identified or synthesized. However, these compounds were not successful in clinical trials due to a lack of efficacy and/or untoward toxicity. Subsequently, third- and fourth-generation P-gp inhibitors were developed but dedicated clinical trials did not indicate a significant therapeutic effect. In recent years, an extraordinary array of highly potent, selective, and low-toxicity P-gp inhibitors have been reported. Herein, we provide a comprehensive review of the synthetic and natural products that have specific inhibitory activity on P-gp drug efflux as well as promising chemosensitizing efficacy in MDR cancer cells. The present review focuses primarily on the structural features, design strategies, and structure-activity relationships (SAR) of these compounds.

Interactions between artemisinin derivatives and P-glycoprotein

BACKGROUND

Artemisinin was isolated and identified in 1972, which was the starting point for a new era in antimalarial drug therapy. Furthermore, numerous studies have demonstrated that artemisinin and its derivatives exhibit considerable anticancer activity both in vitro, in vivo, and even in clinical Phase I/II trials. P-glycoprotein (P-gp) mediated multi-drug resistance (MDR) is one of the most serious causes of chemotherapy failure in cancer treatment. Interestingly, many artemisinin derivatives exhibit excellent ability to overcome P-gp mediated MDR and even show collateral sensitivity against MDR cancer cells. Furthermore, some artemisinin derivatives show P-gp-mediated MDR reversal activity. Therefore, the interaction between P-gp and artemisinin derivatives is important to develop novel combination treatment protocols with artemisinin derivatives and established anticancer drugs that are P-gp substrates.

PURPOSE

This systematic review provides an updated overview on the interaction between artemisinin derivatives and P-gp and the effect of artemisinin derivatives on the P-gp expression level.

RESULTS

Artemisinin derivatives exhibit multi-specific interactions with P-gp. The currently used artemisinin derivatives are not transported by P-gp. However, some of novel synthetized artemisinin derivatives exhibit P-gp substrate properties. Furthermore, many artemisinin derivatives act as P-gp inhibitors, which exhibit the potential to reverse MDR towards clinically used anticancer drugs.

CONCLUSION

Therefore, studies on the interaction between artemisinin derivatives and P-gp provide important information for the development of novel anti-cancer artemisinin derivatives to reverse P-gp mediated MDR and for the design of rational artemisinin-based combination therapies against cancer.

Crown ethers reverse P-glycoprotein-mediated multidrug resistance in cancer cells

Multidrug resistance (MDR) is a widespread phenomenon exhibited by many cancers and represents a fundamental obstacle for successful cancer treatments. Tumour cells commonly achieve MDR phenotype through overexpression and/or increased activity of ABC transporters. P-glycoprotein transporter (P-gp, ABCB1) is a major cause of MDR and therefore represents a valuable target for MDR reversal. Several naturally occurring potassium ionophores (e.g. salinomycin) were shown to inhibit P-gp effectively. We have previously shown antitumour activity of a number of 18-crown-6 ether compounds that transport potassium ions across membranes. Here we present data on P-gp inhibitory activity of 16 adamantane-substituted monoaza- and diaza-18-crown-6 ether compounds, and their effect on MDR reversal in model cell lines. We show that crown ether activity depends on their lipophilicity as well as on the linker to adamantane moiety. The most active crown ethers were shown to be more effective in sensitising MDR cells to paclitaxel and adriamycin than verapamil, a well-known P-gp inhibitor. Altogether our data demonstrate a novel use of crown ethers for inhibition of P-gp and reversal of MDR phenotype.

Transport of organic cationic drugs: effect of ion-pair formation with bile salts on the biliary excretion and pharmacokinetics

More than 40% of clinically used drugs are organic cations (OCs), which are positively charged at a physiologic pH, and recent reports have established that these drugs are substrates of membrane transporters. The transport of OCs via membrane transporters may play important roles in gastrointestinal absorption, distribution to target sites, and biliary and/or renal elimination of various OC drugs. Almost 40 years ago, a molecular weight (Mw) threshold of 200 was reported to exist in rats for monoquaternary ammonium (mono QA) compounds to be substantially (e.g., >10% of iv dose) excreted to bile. It is well known that some OCs interact with appropriate endogenous organic anions in the body (e.g., bile salts) to form lipophilic ion-pair complexes. The ion-pair formation may influence the affinity or binding of OCs to membrane transporters that are relevant to biliary excretion. In that sense, the association of the ion-pair formation with the existence of the Mw threshold appears to be worthy of examination. It assumes the ion-pair formation of high Mw mono QA compounds (i.e., >200) in the presence of bile salts in the liver, followed by accelerated transport of the ion-pair complexes via relevant bile canalicular transporter(s). In this article, therefore, the transport of OC drugs will be reviewed with a special focus on the ion-pair formation hypothesis. Such information will deepen the understanding of the pharmacokinetics of OC drugs as well as the physiological roles of endogenous bile salts in the detoxification or phase II metabolism of high Mw QA drugs.

Biomedical potentials of crown ethers: prospective antitumor agents

Crown ethers are of enormous interest and importance in chemistry, biochemistry, materials science, catalysis, separation, transport and encapsulated processes, as well as in the design and synthesis of various synthetic systems with specific properties, diverse capabilities, and programmable functions. Classical crown ethers are macrocyclic polyethers that contain 3-20 oxygen atoms separated from each other by two or more carbon atoms. They are exceptionally versatile in selectively binding a range of metal ions and a variety of organic neutral and ionic species. Crown ethers are currently being studied and used in a variety of applications beyond their traditional place in chemistry. This review presents additional applications and the ever-increasing biomedical potentials of these intriguing compounds, with particular emphasis on the prospects of their relevance as anticancer agents. We believe that further research in this direction should be encouraged, as crown compounds could either induce toxicities that are different from those of conventional antitumor drugs, or complement drugs in current use, thereby providing a valuable adjunct to therapy.

Perturbation of membrane microdomains in GLC4 multidrug-resistant lung cancer cells − modification of ABCC1 (MRP1) localization and functionality

The multidrug resistance-associated protein transporter ABCC1 (MRP1) is an integral plasma membrane protein involved in the multidrug resistance phenotype. It actively expels a number of cytotoxic molecules from cells. To gain insight into the modulation of the functional properties of this integral membrane protein by cholesterol, a main component of the lipid bilayer, we used multidrug-resistant GLC4/ADR cells, which overexpress MRP1. Upon altering the plasma membrane cholesterol content of these cells, membrane localization and the activity of MRP1 were analyzed. A detergent-free methodology was used to separate "light" and "heavy" plasma membrane fractions. Our data show that MRP1 was exclusively found in "light" fractions known as L0 phase membrane microdomains, together with 23% of gangliosides GM1 and 40% of caveolin-1. Depletion of the membrane cholesterol level to 40% by treatment with the cholesterol-chelating agent methyl-beta-cyclodextrin did not modify MRP1 activity, as evidenced either by the rate of efflux of pirarubicin or that of glutathione. Further cholesterol depletion below 40% yielded both a partial shift of MRP1 to the high-density fraction and a decrease of its functionality. Taken together, these data suggest that MRP1 functionality depends on its localization in cholesterol-rich membrane microdomains.

Enhancement of drug absorption by noncharged detergents through membrane and P-glycoprotein binding

Noncharged detergents are used as excipients in drug formulations. Until recently, they were considered as inert compounds, enhancing drug absorption essentially by improving drug solubility. However, many detergents insert into lipid membranes, although to different extents, and change the lateral packing density of membranes at high concentrations. Moreover, they bind to the efflux transporter P-glycoprotein (P-gp) and most likely to related transporters and metabolising enzymes with overlapping substrate specificities. If their affinity to P-gp is higher than that of the coadministered drug they act as modulators or inhibitors of P-gp and enhance drug absorption. Inhibition of P-gp and related proteins can, however, cause severe side effects. This paper first reviews the membrane binding propensity of different noncharged detergents (including poloxamers) and discusses their ability to bind to P-gp. Second, literature data on drug uptake enhancement by noncharged detergents, obtained in vivo and in vitro, are analysed at the molecular level. The present analysis provides the tools for an approximate and simple prior estimate of the membrane and P-gp binding ability of noncharged detergents based on a modular binding approach.

Field-dependent effect of crown ether (18-crown-6) on ionic conductance of alpha-hemolysin channels

Closing linear poly(ethylene glycol) (PEG) into a circular "crown" dramatically changes its dynamics in the alpha-hemolysin channel. In the electrically neutral crown ether (C2H4O)6, six ethylene oxide monomers are linked into a circle that gives the molecule ion-complexing capacity and increases its rigidity. As with linear PEG, addition of the crown to the membrane-bathing solution decreases the ionic conductance of the channel and generates additional conductance noise. However, in contrast to linear PEG, both the conductance reduction (reporting on crown partitioning into the channel pore) and the noise (reporting on crown dynamics in the pore) now depend on voltage strongly and nonmonotonically. Within the whole frequency range accessible in channel reconstitution experiments, the noise power spectrum is "white", showing that crown exchange between the channel and the bulk solution is fast. Analyzing these data in the framework of a Markovian two-state model, we are able to characterize the process quantitatively. We show that the lifetime of the crown in the channel reaches its maximum (a few microseconds) at about the same voltage (approximately 100 mV, negative from the side of protein addition) where the crown's reduction of the channel conductance is most pronounced. Our interpretation is that, because of its rigidity, the crown feels an effective steric barrier in the narrowest part of the channel pore. This barrier together with crown-ion complexing and resultant interaction with the applied field leads to behavior usually associated with voltage-dependent binding in the channel pore.

Decrease of P-Glycoprotein Activity in K562/ADR Cells by M?CD and Filipin and Lack of Effect Induced by Cholesterol Oxidase Indicate That This Transporter Is Not Located in Rafts

The effect of low-density membrane domains on function of the plasma membrane transporter P-glycoprotéine (P-gp), involved in multidrug resistance (MDR) phenotype, has been investigated in K562/ADR cells. To this end we reversibly altered the cholesterol content of K562/ADR cells by using methyl-beta-cyclodextrin as a cholesterol chelator and conversely we repleted them through incubation with cholesterol in culture medium. We also used the cholesterol-binding fluorochrome filipin and cholesterol oxidase. Our data show that either cholesterol depletion or complex formation with filipin resulted in a strong decrease of P-gp activity. However, when cells were incubated with cholesterol oxidase that are known to disrupt rafts, no modification of the P-gp activity was observed. In addition, using a free-detergent methodology to separate by ultracentrifugation, "light," "heavy," and "extra heavy" fractions we show that no P-gp is found in the "light" fraction where rafts are usually detected. Altogether, our data strongly suggest that, in this cell line, P-gp is not localized in rafts.

Preferential efflux by P-glycoprotein, but not MRP1, of compounds containing a free electron donor amine

Multidrug resistance (MDR) in model systems is known to be conferred by two different integral proteins, the 170-kDa P-glycoprotein (P-gp) and the 190-kDa multidrug resistance-associated protein (MRP1), both of which pump drugs out of MDR cells. The presence of a nitrogen atom, charged at physiological pH, has frequently been considered to be a hallmark of P-gp substrates and inhibitors. The present study was aimed at investigating the role of nitrogen in the ability of the pump to recognise substrate. We measured the kinetics of active efflux of seven new anthracycline derivatives in P-gp-expressing K562/ADR cells and in MRP1-expressing GLC4/ADR cells. Six of these compounds represent analogues of daunorubicin in which the amino sugar nitrogen is bound to an amino- or a nitro-substituted benzyl moiety, the seventh is a doxorubicin derivative in which benzyl group is bound with 4'-oxygen. We found that the compounds with a nitro group on the benzyl ring were poor substrates for P-gp despite the presence of a secondary amine that can be protonated. In contrast, compounds that have a free amino group were very good substrates even though this amine is not protonated in the pH range studied (pK approximately 3). These results show that the nitrogen atom does not interact with P-gp in a charged form but rather as an electron donating group.

Multidrug resistance protein functionality: no effect of intracellular or extracellular pH changes

A major problem in the treatment of cancer is cellular resistance to cytotoxic drugs. In tumor cells in vitro, the development of multidrug resistance is usually accompanied by increased expression of drug transporters, either P-glycoprotein (P-gp) or multidrug resistance-associated protein (MRP(1)). Both proteins belong to the superfamily of ATP-binding cassette (ABC) transporter proteins and mediate the transport of a broad range of drugs. Altenberg et al. (Proc Natl Acad Sci USA90: 9735-9738, 1993) have shown that changes in intra- or extracellular pH do not mediate P-gp-dependent multidrug resistance. Therefore, we similarly studied whether changes in intra- or extracellular pH could mediate MRP(1)-dependent multidrug resistance. In particular, we measured the MRP(1)-mediated efflux of hydroxyrubicin from GLC4/ADR cells. Since hydroxyrubicin is a fully neutral anthracycline derivative that has no deprotonable function at pH lower than 10 and so cannot accumulate in non-nuclear compartments under the influence of pH or transmembrane gradients, we hypothesized that any modifications of its kinetics of efflux as a function of pH can be assigned to a modification of the transporter efficiency. However, as our data show, modifications of extra- and/or intracellular pH yielded no modification of the MRP(1)-mediated efflux of hydroxyrubicin.

Analysis of the tangled relationships between P-glycoprotein-mediated multidrug resistance and the lipid phase of the cell membrane

P-glycoprotein (Pgp), the so-called multidrug transporter, is a plasma membrane glycoprotein often involved in the resistance of cancer cells towards multiple anticancer agents in the multidrug-resistant (MDR) phenotype. It has long been recognized that the lipid phase of the plasma membrane plays an important role with respect to multidrug resistance and Pgp because: the compounds involved in the MDR phenotype are hydrophobic and diffuse passively through the membrane; Pgp domains involved in drug binding are located within the putative transmembrane segments; Pgp activity is highly sensitive to its lipid environment; and Pgp may be involved in lipid trafficking and metabolism. Unraveling the different roles played by the membrane lipid phase in MDR is relevant, not only to the evaluation of the precise role of Pgp, but also to the understanding of the mechanism of action and function of Pgp. With this aim, I review the data from different fields (cancer research, medicinal chemistry, membrane biophysics, pharmaceutical research) concerning drug-membrane, as well as Pgp-membrane, interactions. It is emphasized that the lipid phase of the membrane cannot be overlooked while investigating the MDR phenotype. Taking into account these aspects should be useful in the search of ways to obviate MDR and could also be relevant to the study of other multidrug transporters.

Energy-dependent efflux from Leishmania promastigotes of substrates of the mammalian multidrug resistance pumps

We demonstrated the existence of three transport activities in promastigotes of Leishmania braziliensis, Leishmania guyanensis, and Leishmania mexicana. The first activity, an energy-dependent efflux of pirarubicin, was observed in all Leishmania species and inhibited by verapamil, by 2-[4-(diphenylmethyl)-1-piperazinyl]ethyl-5-(trans-4,6-dimethyl-1, 3,2-dioxaphosphorinan-2-yl)-2,6-dimethyl-4-(3-nitrophenyl)-3-py ridinecarboxylate P oxide (PAK104P) and by the phenothiazine derivatives: thioridazine, prochlorperazine, trifluoperazine, chlorpromazine and trifluoropromazine. The second activity, an energy-dependent efflux of calcein acetoxymethylester, was observed in all Leishmania species and inhibited by PAK104P and the same phenothiazine derivatives, but not by verapamil. The third activity, an energy-dependent efflux of calcein, was clearly detected in L. braziliensis and guyanensis and inhibited only by prochlorperazine and trifluoperazine. The fact that prochlorperazine and trifluoperazine inhibited the energy-dependent efflux of the three substrates suggests that these activities are mediated by the same transport system. It is noteworthy that the transport system identified in this study shares several properties with the mammalian multidrug resistance pump, MRP1. Pirarubicin, calcein acetoxymethylester and calcein are well known substrates of the MRP. Furthermore, the three types of inhibitors are also inhibitors of the MRP function.

The ability of verapamil to restore intracellular accumulation of anthracyclines in multidrug resistant cells depends on the kinetics of their uptake

The basic distinguishing feature of all cells expressing functional P-glycoprotein-multidrug resistance is a decrease of steady state drug levels as compared to those in drug-sensitive controls. A variety of small molecules, such as verapamil and cyclosporin A, bind to P-glycoprotein and inhibit its ability to pump out antitumor drugs. The kinetics of P-glycoprotein-mediated efflux of various anthracycline derivatives was measured in multidrug-resistant (MDR) K562 cells in the presence of verapamil. Used for the purpose were daunorubicin, idarubicin and 8-S-fluoro-idarubicin which have the same pKa of deprotonation equal to 8.4, but different lipophilicity, 4'-epi-2'-bromo-daunorubicin which has a lipophilicity which is comparable to that of daunorubicin but a pKa equal to 6.3, pirarubicin with pKa equal to 7.7 and lipophilicity different from that of these derivatives were used. Our data show (1) that verapamil is unable to completely block the P-glycoprotein-mediated efflux of anthracyclines and that 10% of its functionality remains even with high verapamil concentrations, (2) that the ability of verapamil to restore intracellular accumulation of anthracyclines in MDR cells depends on the kinetics of their uptake. With fast kinetics uptake, as is the case for idarubicin, 8-S-fluoro-idarubicin, 4'-epi-2'-bromo-daunorubicin and pirarubicin (which have either a low pKa and/or high lipophilicity), verapamil can restore in multidrug resistant cells an intracellular drug level which is comparable to that observed in sensitive cells. This is not possible when the kinetics of uptake is low as is the case for daunorubicin. Cyclosporin A is a more potent modulator and is able to fully restore daunorubicin accumulation in multidrug resistant cells.

Kinetics of anthracycline efflux from multidrug resistance protein-expressing cancer cells compared with P-glycoprotein-expressing cancer cells

The multidrug resistance protein (MRP) has been shown to mediate ATP-dependent efflux of anticancer agents of diverse structure, such as daunorubicin (DNR), vincristine and etoposide. Thus, this protein does confer a multidrug resistant phenotype to cancer cells, similar to P-glycoprotein (Pgp). The substrate specificity of both transporter proteins is partly overlapping but is otherwise very distinct; because MRP is a multiple organic anion transporter, it transports certain glutathione conjugates and may be partly dependent on intracellular glutathione levels for the transport of anthracyclines. We have studied the transport kinetics of a series of anthracyclines in MRP and Pgp that overexpress tumor cell lines to obtain information on the substrate specificity of these proteins. The anthracyclines have modifications in the sugar moiety. The mean active efflux coefficient Ka, used to characterize the efficiency of the active efflux, was very similar for DNR and one of its 4'-deoxy-derivatives (eso-DNR) for MRP and Pgp [10-20 x 10(-10)/sec/(cells/ml)]. The permanently neutral derivatives 3'-deamino-3'-hydroxy-doxorubicin (OH-DOX) and 3'-deamino-3'-hydroxy-daunorubicin (OH-DNR) were effluxed by both proteins but had a lower Ka [2 x 10(-10) and 6 x 10(-10)/sec/(cells/ml) (OH-DOX)] and 2 x 10(-10) and 5 x 10(-10)/sec/(cells/ml) (OH-DNR)] for MRP and Pgp. Two anthracyclines, the doxorubicin derivative pirarubicin and 2'-bromo-4'-epi-DNR seemed to have a slightly higher Ka value for Pgp than for MRP. The apparent Michaelis-Menten constants (K(m)) and maximal efflux rates (VM) for the active transport were within a narrow range for both transporters, except for OH-DOX and OH-DNR, which had a lower VM in the case of MRP-mediated transport, suggesting a role of the amino group in the interaction with glutathione. Determination of the Hill coefficient (nH) of the MRP-mediated efflux gave most values close to 2, which suggests cooperativity of the transport of anthracyclines as reported before for Pgp. In conclusion, the transport kinetics of anthracyclines by MRP and Pgp are very similar.

Study of P-glycoprotein functionality in living resistant K562 cells after photolabeling with a verapamil analogue

To our knowledge, this is the first study to investigate the modification of P-glycoprotein functionality in living resistant cells after photolabeling. For this purpose, four new photoactive verapamil analogues were synthesized. These compounds have the same efficacy as verapamil to increase pirarubicin (pira) incorporation into living multidrug resistant (MDR) K562 cells and to sensitize them to the cytotoxic effect of this anthracycline derivative, indicating that they act as typical MDR modifiers in MDR cells. These compounds were used to photolabel P-glycoprotein (P-gp) in living resistant cells. Irradiation did not result in photodamage to cells, and P-gp functionality was verified by the ability of living cells to incorporate pira. The irradiation of resistant cells, 10(6)/mL, in the presence of a verapamil analogue at concentrations equal to or higher than 3 microM yielded 70% inhibition of P-gp functionality. Our data provide the first evidence that the binding of a verapamil analogue to P-gp is not sufficient to completely inhibit the efflux of this anthracycline. The cells were, subsequently, cultured for several days. Resistance was progressively recovered with time, with the treated cells being just as resistant as before photolabeling after 6 days.