Partitioning of triphenylalkylphosphonium homologues in gel bead-immobilized liposomes: chromatographic measurement of their membrane partition coefficients

Uncoupling and Toxic Action of Alkyltriphenylphosphonium Cations on Mitochondria and the Bacterium Bacillus subtilis as a Function of Alkyl Chain Length

A series of permeating cations based on alkyl derivatives of triphenylphosphonium (C(n)-TPP(+)) containing linear hydrocarbon chains (butyl, octyl, decyl, and dodecyl) was investigated in systems of isolated mitochondria, bacteria, and liposomes. In contrast to some derivatives (esters) of rhodamine-19, wherein butyl rhodamine possessed the maximum activity, in the case of C(n)-TPP a stimulatory effect on mitochondrial respiration steadily increased with growing length of the alkyl radical. Tetraphenylphosphonium and butyl-TPP(+) at a dose of several hundred micromoles exhibited an uncoupling effect, which might be related to interaction between C(n)-TPP(+) and endogenous fatty acids and induction of their own cyclic transfer, resulting in transport of protons across the mitochondrial membrane. Such a mechanism was investigated by measuring efflux of carboxyfluorescein from liposomes influenced by C(n)-TPP(+). Experiments with bacteria demonstrated that dodecyl-TPP(+), decyl-TPP(+), and octyl-TPP(+) similarly to quinone-containing analog (SkQ1) inhibited growth of the Gram-positive bacterium Bacillus subtilis, wherein the inhibitory effect was upregulated with growing lipophilicity. These cations did not display toxic effect on growth of the Gram-negative bacterium Escherichia coli. It is assumed that the difference in toxic action on various bacterial species might be related to different permeability of bacterial coats for the examined triphenylphosphonium cations.

Mitochondria-targeted penetrating cations as carriers of hydrophobic anions through lipid membranes

High negative electric potential inside mitochondria provides a driving force for mitochondria-targeted delivery of cargo molecules linked to hydrophobic penetrating cations. This principle is utilized in construction of mitochondria-targeted antioxidants (MTA) carrying quinone moieties which produce a number of health benefitting effects by protecting cells and organisms from oxidative stress. Here, a series of penetrating cations including MTA were shown to induce the release of the liposome-entrapped carboxyfluorescein anion (CF), but not of glucose or ATP. The ability to induce the leakage of CF from liposomes strongly depended on the number of carbon atoms in alkyl chain (n) of alkyltriphenylphosphonium and alkylrhodamine derivatives. In particular, the leakage of CF was maximal at n about 10-12 and substantially decreased at n=16. Organic anions (palmitate, oleate, laurylsulfate) competed with CF for the penetrating cation-induced efflux. The reduced activity of alkylrhodamines with n=16 or n=18 as compared to that with n=12 was ascribed to a lower rate of partitioning of the former into liposomal membranes, because electrical current relaxation studies on planar bilayer lipid membranes showed rather close translocation rate constants for alkylrhodamines with n=18 and n=12. Changes in the alkylrhodamine absorption spectra upon anion addition confirmed direct interaction between alkylrhodamines and the anion. Thus, mitochondria-targeted penetrating cations can serve as carriers of hydrophobic anions across bilayer lipid membranes.

Evaluation of bilayer disks as plant cell membrane models in partition studies

We have studied the partitioning of a set of phenolic compounds used as lignin precursor models into lipid bilayer disks and liposomes. The bilayer disks are open bilayer structures stabilized by polyethylene glycol-conjugated lipids. Our results indicate that disks generate more accurate partition data than do liposomes. Furthermore, we show that the partitioning into the membrane phase is reduced slightly if disks composed of 1,2-distearoyl-sn-glycero-3-phosphocholine and cholesterol are exchanged for disks with a lipid composition mimicking that of the root tissue of Zea mays L.

High sensitivity detection of bisphenol A using liposome chromatography

An antibody column in tandem with a fluorescent dye entrapped liposome column was developed for highly sensitive detection of an endocrine disruptor, bisphenol A (BPA). Anti-BPA antibody was immobilized in a protein G column with orientation control. A derivative of BPA was conjugated to phospholipase A2 (PLA2). BPA sample solutions mixed with the BPA-PLA2 conjugates were injected on to the anti-BPA antibody column and competitive binding occurred in the antibody column. The amount of the free conjugate was proportional to the concentration of the BPA sample. The eluted conjugates were injected on to the second column gel on which calcein-entrapped liposomes were immobilized and the PLA2-catalyzed hydrolysis of liposomal phospholipids causing fluorescent dye leakage as a signal amplification. In this system, the mixture of BPA and BPA-PLA2 conjugate were incubated for 60 min in the anti-BPA column, and then the collected solution was applied to the liposome column. The BPA detection range of 0.02-140 ng mL(-1) was wider than 0.03-6.6 ng mL(-1) obtained by the method of competitive ELISA using the same antibody. Moreover, this system could be adapted to an HPLC system resulting in almost the same detection limit in online detection. The method could be applied to environmental samples, river water and soil extracts. The BPA concentration of 0.1 ng mL(-1) and 10 ng g(-1) was detectable in water and soil extract, respectively.

Interactions of drugs and an oligonucleotide with charged membranes analyzed by immobilized liposome chromatography

We studied the effect of charged lipids or detergent on the retention of drugs and an oligonucleotide by immobilized liposome chromatography to characterize solute-membrane interactions. This is a novel approach in analysis of oligonucleotide-liposome interactions. The charged lipids (phosphatidylserine or distearoyltrimethylammoniumpropane) or detergent (sodium dodecylsulfate) interacted electrostatically in a concentration-dependent matter with the solutes. The oligonucleotide ions presumably bound to the liposomes by multipoint interactions, which was saturable. Sodium dodecylsulfate seemed to affect the drug-membrane interactions more strongly than phosphatidylserine did, probably due to different positioning in the bilayer.

Detection of polychlorinated biphenyls using an antibody column in tandem with a fluorescent liposome column

Phospholipase A2 (PLA2)-catalyzed membrane leakage can be detected by immobilized liposomes containing a self-quenching fluorescent dye, 3,3-bis[N,N-di(carboxymethyl)aminomethyl]fluorescein (calcein). This enzymatic reaction was applied as signal amplification for biosensor detection of low concentrations of polychlorinated biphenyls (PCBs). In order to increase the fluorescent signal for improvement of PCBs detection, the effect of BSA on optimal lipid composition for PLA2-catalyzed membrane leakage from fluorescent liposomes has been investigated in this report. Various kinds of calcein-entrapped liposomes were immobilized in Sephacryl S1000 gel beads using avidin-biotin binding. In a contrast, free calcein was removed by size exclusion chromatography on Sephacryl S300 for free liposome suspensions. The PLA2-catalyzed membrane leakage was detected both in these gel-bead-immobilized liposomes and in free liposome suspensions. In both systems, the fluorescent release from the liposomes by PLA2 hydrolytic action significantly increased with increasing albumin concentration. The most rapid and greatest membrane leakage by PLA2 hydrolysis was found in anionic liposomes in the presence of albumin, both in free liposome suspensions and gel-bead-immobilized liposomes. Finally, the stabilities of various free liposomes and gel-bead-immobilized liposomes were monitored. Immobilized 1-palmitoy-2-oleoylphosphatidylcholine (POPC)/1-palmitoy-2-oleoylphosphatidylglycerol (POPG) liposome gel was chosen due to its excellent stability and large dye leakage by PLA2. A concentration of PCBs as low as 0.1 ng/mL was detectable using this tandem column system.

Development and initial evaluation of PEG-stabilized bilayer disks as novel model membranes

We show in this study that stable dispersions dominated by flat bilayer disks may be prepared from a carefully optimized mixture of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), cholesterol, and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-5000] [PEG-DSPE(5000)]. By varying the content of the latter component, the average diameter of the disks can be changed in the interval from about 15 to 60 nm. The disks show excellent long-term stability, and their size and structure remain unaltered in the temperature range between 25 and 37 degrees C. The utility of the disks as artificial model membranes was confirmed and compared to uni- and multilamellar liposomes in a series of drug partition studies. Data obtained by isothermal titration calorimetry and drug partition chromatography (also referred to as immobilized liposome chromatography) indicate that the bilayer disks may serve as an attractive and sometimes superior alternative to liposomes in studies aiming at the investigation of drug-membrane interactions. The disks may, in addition, hold great potential for structure/function studies of membrane-bound proteins. Furthermore, we suggest that the sterically stabilized bilayer disks may prove interesting as carriers for in vivo delivery of protein/peptide, as well as conventional amphiphilic and/or hydrophobic, drugs.

The elution profile of immobilized liposome chromatography: determination of association and dissociation rate constants

The interaction of lipophilic cations, tetraphenylphosphonium and triphenylphosphonium homologues with liposomes was investigated using immobilized liposome chromatography (ILC). Large unilamellar liposomes with a mean diameter of 100 nm were stably immobilized in chromatographic gel beads by avidin-biotin. The distribution coefficient calculated from (Ve-V0)/Vs (Ve, retention volume; V0, the void volume; Vs, the stationary phase volume) was found to be independent of flow rate, injection amount and gel bed volume, which is consistent with chromatograph theory. The relationship between the bandwidth and solvent flow rate did not follow band-broadening theories reported thus far. We hypothesized that the solvent might be forced to produce large eddies, spirals or turbulent flow due to the presence of liposomes fixed in the gel. Therefore, we developed a new theory for ILC elution: The column is composed of a number of thin disks containing liposomes and solution, and within each disk the solution is well mixed. This theory accounts for our results, and we were able to use it to estimate the rate constants of association and dissociation of the phosphonium to/from liposomes.

Drug partition chromatography on immobilized porcine intestinal brush border membranes

We immobilized porcine intestinal brush border membrane vesicles (BBMVs) for chromatographic analyses of drug partitioning into the membranes determined as Ks, the drug retention per phospholipid amount. For positive and neutral drugs Ks decreased day by day, whereas Ks for negative drugs increased marginally. Similar results on vesicle-lipid liposomes indicated a gradual loss of negative charge from the columns. The Ks values for positive drugs were higher than those for negative drugs with the same octanol/water partitioning or the same Ks on egg yolk phospholipid bilayers. Electrostatic interactions seem to be important for the partitioning of charged drugs into brush border membranes.

Effects of ions and detergents in drug partition chromatography on liposomes

We have determined drug partitioning into phospholipid bilayers by immobilized-liposome chromatography (ILC). Electrostatic effects on the drug partitioning were observed on neutral bilayers at low ionic strength. The size of the counterions affected the partitioning. When liposomes were supplemented with ionic detergents the partitioning of charged drugs was strongly affected, allowing complete separation of drugs of different charges which showed similar retention on neutral bilayers. Partial separation was obtained on bilayers containing fatty acid. Detergent ions or fatty acid inserted into phospholipid bilayers affected the partitioning of drugs much more than did free ions or phospholipid head group charges.

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

Chromatographic approach for determining the relative membrane permeability of drugs

With the aid of the experimental dependence of the theoretical plate height (H) on the flow-rate (U), values of diffusion coefficients as the permeation rate, of the compounds in a polymeric stationary phase were calculated from solute mass transfer. This approach is proposed for modeling the relative diffusion rate of a drug within the membrane. After the successful separation of opioid compounds using a C(18) derivatized polystyrene-divinylbenzene polymer HPLC column, the slopes of H-U plots increase quantitatively in the order of meperidine<alfentanil<fentanyl<sufentanil, indicating that the large mass transfer resistance slows down the penetration of molecules. A constant intercept for the experimental plate height supports the proposal interpretation. A good correlation between the diffusion coefficients and hydrophobicity (log P(oct)) from the traditional shake-flask method was obtained for the opioid compounds, demonstrating that the more lipophilic molecules penetrate deeper into the stationary phase leading to a lower migration rate under the specified conditions. Plot of the diffusion coefficients versus potency ratio for different opioids after intravenous administration reflect the values of the dynamic process in drug studies. The work herein differs from existing studies by measuring the permeability of drugs into the stationary phase rather than providing membrane partition coefficients for a series of analogues. Thus, the study of drug permeability combined with other physico-chemical properties, such as hydrophobicity, may provide additional information on drug-membrane interactions.

Immobilized liposome chromatography to study drug-membrane interactions. Correlation with drug absorption in humans

For rapid screening of drug-membrane interactions and predicting drug absorption in vivo, unilamellar liposomes were stably immobilized in the pores of gel beads by avidin-biotin binding. Interactions of a diverse set of well-described drugs with the immobilized liposomal membranes were reflected by their elution profiles. The membrane partitioning coefficients (KLM) of the drugs were determined from the retention volumes. The drug retentions on egg phosphatidylcholine (EPC)-phosphatidylserine (PS)-cholesterol (chol) and EPC-PS-phosphatidylethanolamine (PE)-chol columns intended to mimic small intestine membranes were similar, although the positively-charged drugs were more strongly retarded on the negatively-charged liposomes than the negatively-charged drugs. The relationship between log KLM with the drug fraction absorbed in humans showed that the log KLM values obtained with unilamellar liposomes can be used to predict drug passive transcellular absorption, similarly to that previously shown for entrapped multilamellar liposomes. The immobilized liposome chromatography method should be useful for screening compounds at an early stage of the drug discovery process. The avidin-biotin immobilization of the liposomes prolongs the lifetime of the columns.

Immobilized-biomembrane affinity chromatography for binding studies of membrane proteins

Analyses of specific interactions between solutes and a membrane protein can serve to characterize the protein. Frontal affinity chromatography of an interactant on a column containing the membrane protein immobilized in a lipid environment is a simple and robust approach for series of experiments with particular protein molecules. Regression analysis of the retention volumes at a series of interactant concentrations shows the affinity of the protein for the interactant and the amount of active binding sites. The higher the affinity, the fewer sites are required to give sufficient retention. Competition experiments provide the affinities of even weakly binding solutes and the non-specific retention of the primary interactant. Hummel and Dreyer size-exclusion chromatography allows complementary analyses of non-immobilized membrane materials. Analyses of the human facilitative glucose transporter GLUT1 by use of the inhibitor cytochalasin B (radioactively labeled) and the competitive substrate D-glucose (non-labeled) showed that GLUT1 interconverted between two states, exhibiting one or two cytochalasin B-binding sites per two GLUTI monomers, dependent on the membrane composition and environment. Similar analyses of a nucleoside transporter, a photosynthetic reaction center, nicotinic acetylcholine receptors and a P-glycoprotein, alternative techniques, and immobilized-liposome chromatographic approaches are presented briefly.

Effects of cholesterol and model transmembrane proteins on drug partitioning into lipid bilayers as analysed by immobilized-liposome chromatography

We have analysed how cholesterol and transmembrane proteins in phospholipid bilayers modulate drug partitioning into the bilayers. For this purpose we determined the chromatographic retention of drugs on liposomes or proteoliposomes entrapped in gel beads. The drug retention per phospholipid amount (the capacity factor Ks) reflects the drug partitioning. Cholesterol in the bilayers decreased the Ks value and hence the partitioning into the membrane in proportion to the cholesterol fraction. On average this cholesterol effect decreased with increasing temperature. Model transmembrane proteins, the glucose transporter GLUT1 and bacteriorhodopsin, interacted electrostatically with charged drugs to increase or decrease the drug partitioning into the bilayers. Bacteriorhodopsin proteoliposomes containing cholesterol combined the effects of the protein and the cholesterol and approached the partitioning properties of red blood cell membranes. For positively charged drugs the correlation between calculated intestinal permeability and log Ks was fair for both liposomes and bacteriorhodopsin-cholesterol proteoliposomes. Detailed modeling of solute partitioning into biological membranes may require an extensive knowledge of their structures.

Phospholipase A(2)-catalyzed membrane leakage studied by immobilized liposome chromatography with online fluorescent detection

Unilamellar liposomes composed of phosphatidylcholine with an entrapped self-quenching fluorescent dye, calcein, were immobilized in chromatographic gel beads by avidin-biotin binding. Bee venom phospholipase A(2) (PLA(2)) was applied in a small amount onto the immobilized liposome column. The release of calcein from the immobilized liposomes resulting from the catalyzed hydrolysis of the phospholipids was detected online by immobilized liposome chromatography (ILC) using a flow fluorescent detector. The PLA(2)-catalyzed membrane leakage of the immobilized liposomes as studied with ILC was found to be affected by the gel pore size used for immobilization, by liposome size, and as expected by the concentration of calcium, but was unaffected by the flow rate of ILC. The largest PLA(2)-induced calcein release from the liposome column was detected on large unilamellar liposomes immobilized on TSK G6000PW or Sephacryl S-1000 gel in the presence of 1 mM Ca(2+) in the aqueous mobile phase. Comparison with the PLA(2)-catalyzed membrane leakage in free liposome suspensions, we conclude that the fluorescent leakage from liposomes hydrolyzed by PLA(2) can be rapidly and sensitively detected by ILC runs using large amount of immobilized liposomes with entrapped fluorescent dye.

Effect of liposome type and membrane fluidity on drug–membrane partitioning analyzed by immobilized liposome chromatography

Immobilized liposome chromatography (ILC) has been proven to be a useful method for the study or rapid screening of drug-membrane interactions. To obtain an adequate liposomal membrane phase for ILC, unilamellar liposomes were immobilized in gel beads by avidin-biotin binding. The retardation of 15 basic drugs on the liposome column could be converted to membrane partitioning coefficients, K(LM). The effects of small or large unilamellar liposomes and multilamellar liposomes on the drug-membrane partitioning were compared. The K(LM) values for both small and large liposomes were similar, but higher than those for the multilamellar liposomes. The basic drugs showed stronger partitioning into negatively charged liposomes than into either neutral liposomes or positively charged liposomes. The membrane fluidity of the immobilized liposomes was modulated by incorporating cholesterol into the liposomal membranes, by changing the acyl chain length and degree of unsaturation of the phospholipids, and by changing the temperature for ILC runs. Our data show that K(LM) obtained using ILC correlated well with those reported by batch studies using free liposomes. It is concluded that negatively charged or cholesterol-containing large unilamellar liposomes are suitable models for the ILC analysis of drug-membrane interactions.

Chromatographic retention of drug molecules on immobilised liposomes prepared from egg phospholipids and from chemically pure phospholipids

The partitioning of a chemically diverse set of drugs into liposomes was studied by immobilised liposome chromatography (ILC). For this purpose liposomes composed of (i) purified egg phospholipids (EPL), (ii) synthetic phosphatidylcholine (PC), (iii) PC--synthetic phosphatidylethanolamine (PE) 80:20 (mol/mol) and (iv) PC--synthetic phosphatidylserine (PS) 80:20 (mol/mol) were immobilised in gel beads by freeze-thawing. The drug partitioning was assessed from the retention volume, which was expressed as a capacity factor, K(s), normalised with respect to the amount of immobilised phospholipid. The drug retention on EPL, PC and PC--PE liposomes was very similar, whereas the negatively charged PC--PS liposomes increased the retention of positively charged and decreased retention of negatively charged drugs. The partitioning of drugs on liposome columns (log K(s)) versus their octanol--water partitioning (log P(oct)) showed three separate rectilinear relationships, depending on the charge of the compound (neutral, positive, or negative). Statistical analysis (ANCOVA) proved that the lines had similar slopes. Repeated analysis of four reference compounds showed a low variation (<0.12 log units) over time (about 250 days). A close relationship was observed between the drug retention in short EPL columns with a low content of phospholipids and the retention in longer standard EPL columns. The short 'quick screen bilayer columns' permit analysis of highly lipophilic compounds within 30 min and are thus applicable for medium-throughput screening in drug discovery settings. A very strong rectilinear relationship (r(2)=0.95, n=13) between log K(s) (EPL) and published liposome partitioning data (log D(mem)) confirmed that the ILC drug retention reflects the drug partitioning into the lipid bilayers. A moderate to fair rectilinear relationship was observed between the normalised retention on PC, PC-PE and EPL liposomes (r(2)=0.79, 0.86 and 0.85, respectively, n=24) and corresponding published log k'(IAM) data obtained on immobilised artificial membrane (IAM) columns. Transport across Caco-2 cell monolayers (log P(c)) showed curvilinear relationships with log K(s), log k'(IAM), log P(oct) and log D(oct). The drug fraction absorbed in humans showed a similar relationship to log K(s) values as to surface plasmon resonance signals representing drug-liposome interaction (Danelian et al., 2000 J Med Chem, 43, 2083--2086).

Avidin-biotin-immobilized liposome column for chromatographic fluorescence on-line analysis of solute-membrane interactions

Unilamellar liposomes with entrapped fluorescent dye calcein were stably immobilized in gel beads by avidin-biotin-binding. The immobilized liposomes remained extremely stable upon storage and chromatographic runs. The immobilized calcein-entrapped liposomes were utilized for fluorescent analysis of solute-membrane interactions, which in some cases are too weak to be detected by chromatographic retardation. A liposome column was used as a sensitive probe to detect the interactions of membranes with pharmaceutical drugs, peptides and proteins. Retardation of the solutes was monitored using a UV detector. Perturbation of the membranes, reflected as leakage of the entrapped calcein by some of the solutes, can thus be detected on-line using a flow-fluorescent detector. For the amphiphilic drugs or synthetic peptides, perturbation of membranes became more pronounced when the retardation (hydrophobicity) of the molecules increased. On the other hand, in the case of positively-charged peptides, polylysine, or partially denatured bovine carbonic anhydrase, significant dye leakage from the liposomes was observed although the retardation was hardly to be measured. Weak protein-membrane interactions can thus be assumed from the large leakage of calcein from the liposomes. This provides additional useful information for solute-membrane interactions, as perturbation of the membranes was also indicated by avidin-biotin-immobilized liposome chromatography (ILC).

Applications of immobilized stationary-phase liquid chromatography: a potential in vitro technique

Immobilized artificial-membrane chromatography is a potential in vitro technique for determining lipophilicity and studying drug transport and membrane interactions. It is reproducible, efficient and simple. Several other and newer applications of immobilized stationary-phase liquid chromatography have been reported, including the purification of membrane proteins, the synthesis of biomolecules and the simultaneous determination of enzyme activity and enantioselectivity. This article describes the immobilized artificial-membrane concept and provides an overview of the applications, advantages and limitations, in general, of immobilized stationary-phase chromatography.

Immobilized liposome chromatography for refolding and purification of protein

Small unilamellar liposomes were utilized as a kind of aqueous two-phase system and artificial chaperone which specifically recognize protein conformation with fluctuated structure. Liposomes showed highly selective binding ability to conformationally changed proteins treated with various concentrations of guanidinium hydrochloride, as evaluated by immobilized liposome chromatography (ILC). In refolding of proteins, liposomes bound to refolding intermediate of proteins and prevented them from forming intermolecular aggregates. Refolding of bovine carbonic anhydrase, lysozyme and ribonuclease A was significantly improved in the presence of liposomes. Furthermore, by utilizing ILC, refolding of proteins was also successfully and simply carried out with considerable high reactivation yield.

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.