A model for computer simulation of P-glycoprotein and transmembrane delta pH-mediated anthracycline transport in multidrug-resistant tumor cells

Current advances of tubulin inhibitors as dual acting small molecules for cancer therapy

Microtubule (MT)-targeting agents are highly successful drugs as chemotherapeutic agents, and this is attributed to their ability to target MT dynamics and interfere with critical cellular functions, including, mitosis, cell signaling, intracellular trafficking, and angiogenesis. Because MT dynamics vary in the different stages of the cell cycle, these drugs tend to be the most effective against mitotic cells. While this class of drug has proven to be effective against many cancer types, significant hurdles still exist and include overcoming aspects such as dose limited toxicities and the development of resistance. Newer generations of developed drugs attack these problems and alternative approaches such as the development of dual tubulin and kinase inhibitors are being investigated. This approach offers the potential to show increased efficacy and lower toxicities. This review covers different categories of MT-targeting agents, recent advances in dual inhibitors, and current challenges for this drug target.

Understanding cancer and the anticancer activities of naphthoquinones – a review

Naphthoquinone moieties are present in drugs such as doxorubicin which are used clinically to treat solid cancers.

ABC transporters in multidrug resistance and pharmacokinetics, and strategies for drug development

Multidrug resistance (MDR) is a serious problem that hampers the success of cancer pharmacotherapy. A common mechanism is the overexpression of ATP-binding cassette (ABC) efflux transporters in cancer cells such as P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1) and breast cancer resistance protein (BCRP/ABCG2) that limit the exposure to anticancer drugs. One way to overcome MDR is to develop ABC efflux transporter inhibitors to sensitize cancer cells to chemotherapeutic drugs. The complete clinical trials thus far have showen that those tested chemosensitizers only add limited or no benefits to cancer patients. Some MDR modulators are merely toxic, and others induce unwanted drug-drug interactions. Actually, many ABC transporters are also expressed abundantly in the gastrointestinal tract, liver, kidney, brain and other normal tissues, and they largely determine drug absorption, distribution and excretion, and affect the overall pharmacokinetic properties of drugs in humans. In addition, ABC transporters such as P-gp, MRP1 and BCRP co-expressed in tumors show a broad and overlapped specificity for substrates and MDR modulators. Thus reliable preclinical assays and models are required for the assessment of transporter-mediated flux and potential effects on pharmacokinetics in drug development. In this review, we provide an overview of the role of ABC efflux transporters in MDR and pharmacokinetics. Preclinical assays for the assessment of drug transport and development of MDR modulators are also discussed.

The dynamics of drug resistance: a mathematical perspective

Resistance to chemotherapy is a key impediment to successful cancer treatment that has been intensively studied for the last three decades. Several central mechanisms have been identified as contributing to the resistance. In the case of multidrug resistance (MDR), the cell becomes resistant to a variety of structurally and mechanistically unrelated drugs in addition to the drug initially administered. Mathematical models of drug resistance have dealt with many of the known aspects of this field, such as pharmacologic sanctuary and location/diffusion resistance, intrinsic resistance, induced resistance and acquired resistance. In addition, there are mathematical models that take into account the kinetic/phase resistance, and models that investigate intracellular mechanisms based on specific biological functions (such as ABC transporters, apoptosis and repair mechanisms). This review covers aspects of MDR that have been mathematically studied, and explains how, from a methodological perspective, mathematics can be used to study drug resistance. We discuss quantitative approaches of mathematical analysis, and demonstrate how mathematics can be used in combination with other experimental and clinical tools. We emphasize the potential benefits of integrating analytical and mathematical methods into future clinical and experimental studies of drug resistance.

Metabolomic studies of human lung carcinoma cell lines using in vitro (1)H NMR of whole cells and cellular extracts

We report principal component analysis (PCA) of (1)H NMR spectra recorded for a group of human lung carcinoma cell lines in culture and (1)H NMR analysis of extracts from the same samples. The samples studied were cells of lung tumour origin with different chemotherapy drug resistance patterns. For whole cells, it was found that the statistically significant causes of spectral variation were an increase in the choline and a decrease in the methylene mobile lipid (1)H resonance intensities, which correlate with our knowledge of the level of resistance displayed by the different cells. Similarly, in the (1)H NMR spectra of the aqueous and lipophilic extracts, significant quantitative differences in the metabolite distributions were apparent, which are consistent with the PCA results.

Predicting drug pharmacokinetics and effect in vascularized tumors using computer simulation

In this paper, we investigate the pharmacokinetics and effect of doxorubicin and cisplatin in vascularized tumors through two-dimensional simulations. We take into account especially vascular and morphological heterogeneity as well as cellular and lesion-level pharmacokinetic determinants like P-glycoprotein (Pgp) efflux and cell density. To do this we construct a multi-compartment PKPD model calibrated from published experimental data and simulate 2-h bolus administrations followed by 18-h drug washout. Our results show that lesion-scale drug and nutrient distribution may significantly impact therapeutic efficacy and should be considered as carefully as genetic determinants modulating, for example, the production of multidrug-resistance protein or topoisomerase II. We visualize and rigorously quantify distributions of nutrient, drug, and resulting cell inhibition. A main result is the existence of significant heterogeneity in all three, yielding poor inhibition in a large fraction of the lesion, and commensurately increased serum drug concentration necessary for an average 50% inhibition throughout the lesion (the IC(50) concentration). For doxorubicin the effect of hypoxia and hypoglycemia ("nutrient effect") is isolated and shown to further increase cell inhibition heterogeneity and double the IC(50), both undesirable. We also show how the therapeutic effectiveness of doxorubicin penetration therapy depends upon other determinants affecting drug distribution, such as cellular efflux and density, offering some insight into the conditions under which otherwise promising therapies may fail and, more importantly, when they will succeed. Cisplatin is used as a contrast to doxorubicin since both published experimental data and our simulations indicate its lesion distribution is more uniform than that of doxorubicin. Because of this some of the complexity in predicting its therapeutic efficacy is mitigated. Using this advantage, we show results suggesting that in vitro monolayer assays using this drug may more accurately predict in vivo performance than for drugs like doxorubicin. The nonlinear interaction among various determinants representing cell and lesion phenotype as well as therapeutic strategies is a unifying theme of our results. Throughout it can be appreciated that macroscopic environmental conditions, notably drug and nutrient distributions, give rise to considerable variation in lesion response, hence clinical resistance. Moreover, the synergy or antagonism of combined therapeutic strategies depends heavily upon this environment.

Enhanced intercellular retention activity of novel pH-sensitive polymeric micelles in wild and multidrug resistant MCF-7 cells

PURPOSE

The purpose of this work was to demonstrate the advantage of using pH-sensitive polymeric mixed micelles (PHSM) composed of poly(L: -histidine) (polyHis)/poly(ethylene glycol) (PEG) and poly(L: -lactic acid) (pLLA)/PEG block copolymers with folate conjugation to increase drug retention in wild-type and MDR tumor cells.

MATERIALS AND METHODS

Both wild-type and multidrug resistant (MDR) human breast adenocarcinoma (MCF-7) cell lines were used to investigate the accumulation and elimination of doxorubicin (DOX), PHSM with folate (PHSM/f), and pH-insensitive micelles composed of pLLA/PEG block copolymer with folate (PHIM/f).

RESULTS

Cells treated with PHSM/f showed decelerated elimination kinetics compared to cells treated with PHIM/f. MDR cells treated with drug-containing PHSM/f for 30 min retained 80% of doxorubicin (DOX) even after incubation for 24 h in the absence of drug. On the other hand, cells treated with drug-containing PHIM/f retained only 40% of DOX within the same period of time. Flow cytometry and confocal microscopy confirmed these results.

CONCLUSIONS

Cellular entry of the micelles occurred via receptor-mediated endocytosis using folate receptors. The pH-induced destabilization of PHSM/f led to rapid distribution of drug and polymer throughout the cells, most likely due to polyHis-mediated endosomal disruption. This reduced the likelihood of drug efflux via exocytosis from resistant tumor cells.

The role of pH dynamics and the Na+/H+ antiporter in the etiopathogenesis and treatment of cancer. Two faces of the same coin--one single nature

Looked at from the genetic point-of-view cancer represents a daunting and, frankly, confusing multiplicity of diseases (at least 100) that require an equally large variety of therapeutic strategies and substances designed to treat the particular tumor. However, when analyzed phenotypically cancer is a relatively uniform disease of very conserved 'hallmark' behaviors across the entire spectrum of tissue and genetic differences [D. Hanahan, R.A. Weinberg, Hallmarks of cancer, Cell 100 (2000) 57-70]. This suggests that cancers do, indeed, share common biochemical and physiological characteristics that are independent of the varied genetic backgrounds, and that there may be a common mechanism underlying both the neoplastic transformation/progression side and the antineoplastic/therapy side of oncology. The challenge of modern oncology is to integrate all the diverse experimental data to create a physiological/metabolic/energetic paradigm that can unite our thinking in order to understand how both neoplastic progression and therapies function. This reductionist view gives the hope that, as in chemistry and physics, it will possible to identify common underlying driving forces that define a tumor and will permit, for the first time, the actual calculated manipulation of their state. That is, a rational therapeutic design. In the present review, we present evidence, obtained from a great number of studies, for a fundamental, underlying mechanism involved in the initiation and evolution of the neoplastic process. There is an ever growing body of evidence that all the important neoplastic phenotypes are driven by an alkalization of the transformed cell, a process which seems specific for transformed cells since the same alkalinization has no effect in cells that have not been transformed. Seen in that light, different fields of cancer research, from etiopathogenesis, cancer cell metabolism and neovascularization, to multiple drug resistance (MDR), selective apoptosis, modern cancer chemotherapy and the spontaneous regression of cancer (SRC) all appear to have in common a pivotal characteristic, the aberrant regulation of hydrogen ion dynamics [S. Harguindey, J.L. Pedraz, R. García Cañero, J. Pérez de Diego, E.J. Cragoe Jr., Hydrogen ion-dependent oncogenesis and parallel new avenues to cancer prevention and treatment using a H+-mediated unifying approach: pH-related and pH-unrelated mechanisms, Crit. Rev. Oncog. 6 (1) (1995) 1-33]. Cancer cells have an acid-base disturbance that is completely different than observed in normal tissues and that increases in correspondence with increasing neoplastic state: an interstitial acid microenvironment linked to an intracellular alkalosis.

The elementary mass action rate constants of P-gp transport for a confluent monolayer of MDCKII-hMDR1 cells

The human multi-drug resistance membrane transporter, P-glycoprotein, or P-gp, has been extensively studied due to its importance to human health and disease. Thus far, the kinetic analysis of P-gp transport has been limited to steady-state Michaelis-Menten approaches or to compartmental models, neither of which can prove molecular mechanisms. Determination of the elementary kinetic rate constants of transport will be essential to understanding how P-gp works. The experimental system we use is a confluent monolayer of MDCKII-hMDR1 cells that overexpress P-gp. It is a physiologically relevant model system, and transport is measured without biochemical manipulations of P-gp. The Michaelis-Menten mass action reaction is used to model P-gp transport. Without imposing the steady-state assumptions, this reaction depends upon several parameters that must be simultaneously fitted. An exhaustive fitting of transport data to find all possible parameter vectors that best fit the data was accomplished with a reasonable computation time using a hierarchical algorithm. For three P-gp substrates (amprenavir, loperamide, and quinidine), we have successfully fitted the elementary rate constants, i.e., drug association to P-gp from the apical membrane inner monolayer, drug dissociation back into the apical membrane inner monolayer, and drug efflux from P-gp into the apical chamber, as well as the density of efflux active P-gp. All three drugs had overlapping ranges for the efflux active P-gp, which was a benchmark for the validity of the fitting process. One novel finding was that the association to P-gp appears to be rate-limited solely by drug lateral diffusion within the inner monolayer of the plasma membrane for all three drugs. This would be expected if P-gp structure were open to the lipids of the apical membrane inner monolayer, as has been suggested by recent structural studies. The fitted kinetic parameters show how P-gp efflux of a wide range of xenobiotics has been maximized.

Drug sequestration in cytoplasmic organelles does not contribute to the diminished sensitivity of anthracyclines in multidrug resistant K562 cells

Cells that acquire multidrug resistance (MDR) are characterized by a decreased accumulation of a variety of drugs. In addition, sequestration of drugs in intracellular vesicles has often been associated with MDR. However, the nature and role of intracellular vesicles in MDR are unclear. We addressed the relationship between MDR and vesicular anthracycline accumulation in the erythroleukemia cell line K562 and a drug-resistant counterpart K562/ADR that overexpresses P-glycoprotein. We used four anthracyclines (all of which are P-glycoprotein substrates): daunorubicin and idarubicin, which have good affinity for DNA and as weak bases can accumulate inside acidic compartments; hydroxyrubicin, which binds to DNA but is uncharged at physiological or acidic pH and thus cannot accumulate in acidic compartments; and WP900, an enantiomer of daunorubicin, which is a weak DNA binder but has the same pKa and lipophilicity as daunorubicin. The intrinsic fluorescence of anthracyclines allowed us to use macro- and micro-spectrofluorescence, flow cytometry, and confocal microscopy to characterize their nuclear or intravesicular accumulation in living cells. We found that vesicular accumulation of daunorubicin, WP900 and idarubicin, containing a basic 3'-amine was predominantly restricted to lysosomes in both cell lines, that pH regulation of acidic compartments was not defective in human K562 cells, and that vesicular drug accumulation was much more pronounced in the parental tumor cell line than in the multidrug-resistant cells. These results indicate that vesicular anthracycline sequestration does not contribute to the diminished sensitivity to anthracyclines in multidrug-resistant K562 cells.

Multidrug resistance (MDR) in cancer. Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition, its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated in various studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR), reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokinetic interactions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first and second generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible in eliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDR modulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution, clearance, and interactions) will also be discussed.

Relating multidrug resistance phenotypes to the kinetic properties of their drug-efflux pumps

The simplest model for pump-mediated multidrug resistance is elaborated quantitatively. The way in which toxicity data should be evaluated to characterize most effectively the drug-efflux pump is then examined. The isotoxic drug dose (D10) depends on too many unrelated properties. The D10 of a cell line taken relative to that of the parental (nonresistant) cell line has been called the relative resistance (RR). This is inappropriate for characterizing the drug pump, as it depends on the extent of amplification of the latter. The reduced RR (RRR) is newly defined as the ratio of the (RR - 1) for one drug to the (RR - 1) for a different drug. This RRR should be independent of both the drug-target affinity and the extent of amplification of the drug pump in cell lines belonging to a family. The RRR depends on the avidities with which the pump extrudes the drugs relative to the passive membrane permeabilities of the latter. In plots of RRR for one drug combination vs. that for a second drug combination, cell lines that have the same pump amplified should cluster, whereas those with amplification of (functionally) different drug-efflux pumps should segregate. Both a set of new experimental data and literature results are discussed in terms of RRR. RRRs discriminate between human MDR1 and mouse mdr1a and mdr1b, between hamster pgp1 and a mutant thereof, as well as between human MDR1 and a mutant thereof. RRRs are not affected by changes in membrane surface area. Our results indicate that RRR may be used to (a) characterize drug-resistance mechanisms and (b) determine which drug-resistance mechanism is operative. Moreover, our analysis suggests that some of the reported phenotypic diversity among multidrug-resistant cell lines may not be due to diversity in the resistance mechanism.

Evaluating limited specificity of drug pumps reduced relative resistance in human MDR phenotypes

In the parallel paper, we developed a property to characterize drug efflux pumps, i.e. the reduced relative resistance (RRR). Using this RRR, we here investigate whether the observed diversity in human multidrug resistance (MDR) phenotypes might be due to variable levels of P-glycoprotein encoded by MDR1. We analyzed resistance phenotypes of various human cell lines in which either one, or both, classical human multidrug resistance genes, MDR1 and MDR3, are overexpressed. In addition, RRR values were calculated for MDR phenotypes presented in the literature. The results suggest that more than a single mechanism is required to account for the observed phenotypic diversity of classical multidrug resistance. This diversity is only partly due to differences in plasma membrane permeabilities between cell line families. It is discussed whether the alternative MDR phenotypes might be MDR1 phenotypes modified by other factors that do not themselves cause MDR. The method we here apply may also be useful for other nonspecific enzymes or pumps.

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).

pH and drug resistance. II. Turnover of acidic vesicles and resistance to weakly basic chemotherapeutic drugs

Resistance to chemotherapeutic agents is a major cause of treatment failure in patients with cancer. The primary mechanism leading to a multidrug-resistant phenotype is assumed to be plasma-membrane localized overexpression of drug efflux transporters, such as P-glycoprotein (P-gp). However, acidic intracellular organelles can also participate in resistance to chemotherapeutic drugs. In this study, we investigated, both experimentally and theoretically, the effect of acidic vesicle turnover on drug resistance. We have developed a general model to account for multiple mechanisms of resistance to weakly basic organic cations, e.g. anthracyclines and Vinca alkaloids. The model predicts that lower cytosolic concentrations of drugs can be achieved through a combination of high endosomal turnover rates, a low endosomal pH, and an alkaline-inside pH gradient between cytosol and the extracellular fluid. Measured values for these parameters have been inserted into the model. Computations using conservative values of all parameters indicate that turnover of acidic vesicles can be an important contributor to the drug-resistant phenotype, especially if vesicles contain an active uptake system, such as H+/cation exchange. Even conservative estimates of organic cation-proton antiport activity would be sufficient to make endosomal drug extrusion a potent mechanism of resistance to weakly basic drugs. The effectiveness of such a drug export mechanism would be comparable to drug extrusion via drug pumps such as P-gp. Thus, turnover of acidic vesicles can be an important factor in chemoresistance, especially in cells that do not overexpress plasma membrane-bound drug pumps like P-glycoprotein.

Kinetics of anthracycline accumulation in multidrug-resistant tumor cells: relationship to drug lipophilicity and serum albumin binding

A multidrug-resistant Ehrlich ascites tumor cell line (EHR2/DNR+) was used to examine the membrane transport kinetics of lipophilic anthracycline derivatives in the presence of serum albumin. We present a model for theoretical data analysis with consideration of drug-albumin complex formation. For a set of five derivatives (doxorubicin, daunorubicin, 4-demethoxydaunorubicin, 4'-deoxy-4'-iododoxorubicin, and 13-dihydro-4'-deoxy-4'-iododoxorubicin), data were given on the rates of diffusional drug uptake, and membrane permeability coefficients of the noncharged molecules were estimated. Both the initial rates and the steady-state levels of drug uptake were found to decrease by addition of BSA at concentrations ranging from 5 to 75 mg/mL. For each drug, this effect of serum albumin could be accounted for by the altered distribution between free and protein-bound drug molecules in the bulk aqueous medium. A good fit of theoretical accumulation curves to the experimental data was obtained. It was concluded that a mathematical simulation method makes it possible to predict the uptake characteristics of lipophilic anthracycline compounds into tumor cells under serum conditions.

Equilibrium binding of anthracycline cytostatics to serum albumin and small unilamellar phospholipid vesicles as measured by gel filtration

A Sephadex G-200 gel filtration method was used to measure directly the equilibrium binding of five important anthracycline analogs to serum albumin. The order of the overall binding constant (K) in a 150 mM NaCl, 20 mM Hepes buffer (pH 7.45) was doxorubicin < daunorubicin < 4-demethoxydaunorubicin approximately 13-dihydro-4'-deoxy-4'-iododoxorubicin < 4'-deoxy-4'-iododoxorubicin for human serum albumin (K = 2.67 +/- 0.07 mM(-1) to 24.5 +/- 3.1 mM[-1]) and bovine serum albumin (K = 1.36 +/- 0.25 mM(-1) to 48.4 +/- 5.2 mM[-1]). Data were given on the pH-dependence of K. The anthracycline-albumin association reaction was compared with measurements of drug partitioning into unilamellar phospholipid membranes and octanol. The results provide important new data required for a systematic kinetic analysis of anthracycline transport in tumor cells under serum conditions in a biological system.

Identification of novel drug resistance-associated proteins by a panel of rat monoclonal antibodies

Since some multidrug-resistant (MDR) tumor cell lines show drug accumulation defects but do not over-express Pgp or MDR protein (MRP), a search was made for novel MDR-related transporter proteins by immunizing rats with non-small cell lung cancer SW- 1573/2R120 cells to produce monoclonal antibodies (MAbs). Five rat MAbs (LMR-4, -12, -42, -44 and -94) were generated, showing strong membranous staining of non-Pgp MDR SW- 1573/2R120 tumor cells and minimal reactivity to the corresponding parental and revertant cell lines. In addition, a 6th MAb (LMR-5) was isolated, recognizing the MDR-related lung resistance protein (LRP), previously identified as the major vault protein. The first 5 LMR MAbs show predominantly membranous staining of several non-Pgp MDR tumor cell lines of different histogenetic origins, except for LMR-4, which recognizes only MDR sublines of the SW- 1573 cell line. Flow-cytometric analysis revealed that all MAbs, except LMR-4 and -5, detect outside epitopes. Functional studies showed that these MAbs did not restore the daunorubicin accumulation defect. All but one of the MAbs (LMR-42) showed staining of distinct normal human tissues, notably epithelial cells lining the airways and digestive tract. In addition, staining of vascular endothelial cells was found with all MAbs except LMR-4. Three MAbs (LMR-12, -44 and -94) showed remarkable immunoreactivity with vincristine-selected SW- 1573 sublines. By immunoblotting and precipitation, the LMR antigens were found to be in the 42-69 kDa range.

Altered drug translocation mediated by the MDR protein: direct, indirect, or both?

Overexpression of the MDR protein, or p-glycoprotein (p-GP), in cells leads to decreased initial rates of accumulation and altered intracellular retention of chemotherapeutic drugs and a variety of other compounds. Thus, increased expression of the protein is related to increased drug resistance. Since several homologues of the MDR protein (CRP, ItpGPA, PDR5, sapABCDF) are also involved in conferring drug resistance phenomena in microorganisms, elucidating the function of the MDR protein at a molecular level will have important general applications. Although MDR protein function has been studied for nearly 20 years, interpretation of most data is complicated by the drug-selection conditions used to create model MDR cell lines. Precisely what level of resistance to particular drugs is conferred by a given amount of MDR protein, as well as a variety of other critical issues, are not yet resolved. Data from a number of laboratories has been gathered in support of at least four different models for the MDR protein. One model is that the protein uses the energy released from ATP hydrolysis to directly translocate drugs out of cells in some fashion. Another is that MDR protein overexpression perturbs electrical membrane potential (delta psi) and/or intracellular pH (pHi) and thereby indirectly alters translocation and intracellular retention of hydrophobic drugs that are cationic, weakly basic, and/or that react with intracellular targets in a pHi or delta psi-dependent manner. A third model proposes that the protein alternates between drug pump and Cl- channel (or channel regulator) conformations, implying that both direct and indirect mechanisms of altered drug translocation may be catalyzed by MDR protein. A fourth is that the protein acts as an ATP channel. Our recent work has tested predictions of these models via kinetic analysis of drug transport and single-cell photometry analysis of pHi, delta psi, and volume regulation in novel MDR and CFTR transfectants that have not been exposed to chemotherapeutic drugs prior to analysis. This paper reviews these data and previous work from other laboratories, as well as relevant transport physiology concepts, and summarizes how they either support or contradict the different models for MDR protein function.

Are altered pHi and membrane potential in hu MDR 1 transfectants sufficient to cause MDR protein-mediated multidrug resistance?

Multidrug resistance (MDR) mediated by overexpression of the MDR protein (P-glycoprotein) has been associated with intracellular alkalinization, membrane depolarization, and other cellular alterations. However, virtually all MDR cell lines studied in detail have been created via protocols that involve growth on chemotherapeutic drugs, which can alter cells in many ways. Thus it is not clear which phenotypic alterations are explicitly due to MDR protein overexpression alone. To more precisely define the MDR phenotype mediated by hu MDR 1 protein, we co-transfected hu MDR 1 cDNA and a neomycin resistance marker into LR73 Chinese hamster ovary fibroblasts and selected stable G418 (geneticin) resistant transfectants. Several clones expressing different levels of hu MDR 1 protein were isolated. Unlike previous work with hu MDR 1 transfectants, the clones were not further selected with, or maintained on, chemotherapeutic drugs. These clones were analyzed for chemotherapeutic drug resistance, intracellular pH (pHi), membrane electrical potential (Vm), and stability of MDR 1 protein overexpression. LR73/hu MDR 1 clones exhibit elevated pHi and are depolarized, consistent with previous work with LR73/mu MDR 1 transfectants (Luz, J.G. L.Y. Wei, S. Basu, and P.D. Roepe. 1994. Biochemistry. 33:7239-7249). The extent of these perturbations is related to the level of hu MDR 1 protein that is expressed. Cytotoxicity experiments with untransfected LR73 cells with elevated pHi due to manipulating percent CO2 show that the pHi perturbations in the MDR 1 clones can account for much of the measured drug resistance. Membrane depolarization in the absence of MDR protein expression is also found to confer mild drug resistance, and we find that the pHi and Vm changes can conceivably account for the altered drug accumulation measured for representative clones. These data indicate that the MDR phenotype unequivocally mediated by MDR 1 protein overexpression alone can be fully explained by the perturbations in Vm and pHi that accompany this overexpression. In addition, MDR mediated by MDR protein overexpression alone differs significantly from that observed for MDR cell lines expressing similar levels of MDR protein but also exposed to chemotherapeutic drugs.

Ormaplatin resistance is associated with decreased accumulation of its platinum (II) analogue, dichloro(D,L-trans)1,2-diaminocyclohexaneplatinum (II)

Ormaplatin (also known as tetraplatin) is a platinum-containing analogue which has recently undergone clinical trials. Ormaplatin may undergo conversion to dichloro(D,L-trans)-1,2-diaminocyclohexaneplatinum(II) [P1Cl2(trans-dach)]. The cisplatin-resistant murine lymphoma cell lines E8 and E5, were found to be cross-resistant to ormaplatin and PtCl2(trans-dach). We found an inverse rank correlation between drug resistance and drug accumulation for PtCl2(trans-dach) similar to our previous findings with cisplatin; however, accumulation of ormaplatin in the resistant cells was increased. Ormaplatin cytotoxicity appears to result primarily from extracellular conversion to PtCl2(trans-dach), since ormaplatin cytotoxicity was decreased under conditions where extracellular conversion to PtCl2(trans-dach) was minimised. Co-incubation with different inhibitors of energy metabolism resulted in a 65-70% increase in PtCl2(trans-dach) accumulation in the parental cell line R1.1 and a 113-307% increase in the resistant cell line E5 which suggests that the decrease in accumulation in E5 may be at least partly energy dependent. We conclude from these findings that cross-resistance to ormaplatin is associated with an energy-dependent decreased accumulation of PtCl2(trans-dach) in these cisplatin-resistant cell lines.

Intracellular pH does not affect drug extrusion by P-glycoprotein

The intracellular pH (pH(i)) of cells exhibiting multidrug resistance (MDR) related to the expression of the P-glycoprotein (Pgp) is often more alkaline than that of the parental cells, as also observed for the KB-V1/KB-3-1 system in this paper. The possible role of an elevated pH(i) in Pgp-related MDR has been investigated by shifting back the pH(i) of the MDR+ cells to a more acidic value using the mobile proton ionophore carbonylcyanide m-chlorophenylhydrazone (CCCP). The influence of CCCP-evoked delta pH(i) on relative daunorubicin (DNR) accumulation was similar in the case of several Pgp positive and negative cell lines, in view of flow cytometric and radioactive drug accumulation studies and measuring DNR levels in the medium in a flow-through system. Our data argue against a significant effect of pH(i) on Pgp pumping efficiency. However, an indirect connection between pH(i) regulation and the MDR phenotype is suggested by the fact that acidification of the external medium in the presence of verpamil could be observed exclusively in MDR+ cells.

Saturable P-glycoprotein kinetics assayed by fluorescence studies of drug efflux from suspended human KB8-5 cells

This article describes a new and rapid method to determine the pumping rate of P-glycoprotein (P-gp) in intact cells. Multidrug resistant (MDR) human epidermoid carcinoma KB8-5 cells (containing P-gp) were loaded with daunorubicin (DNR) in the absence or in the presence of verapamil, sufficient to inhibit DNR pumping by P-gp. In either case, the cells were resuspended in medium devoid of DNR and the subsequent increase of the DNR fluorescence intensity was measured as a function of time. For cells loaded with the same amount of drug, the free cytosolic drug concentration (Ci(t)) was a unique function of the DNR medium concentration (Co(t)). The cellular drug content in the presence of verapamil decreased nonlinearly with decreasing extracellular drug concentration, indicating that the intracellular drug apparent distribution volume increased with decreasing cellular drug content. At each fluorescence intensity, we calculated the P-gp mediated (verapamil-inhibitable) DNR transport rate from the rate of increase of the DNR fluorescence intensity in the absence of verapamil minus the rate of increase of the DNR fluorescence intensity in the presence of verapamil. When plotted against the intracellular free drug concentration (as calculated from the total cellular drug content and a separately determined relation between the total cellular drug content and the intracellular free drug concentration: the apparent distribution volume), this P-gp mediated DNR transport rate showed saturation of P-gp at higher DNR concentrations. The results imply that P-gp mediated DNR transport is saturable (the value of Km is in the order of 1 microM).

Overexpression of the cystic fibrosis transmembrane conductance regulator in NIH 3T3 cells lowers membrane potential and intracellular pH and confers a multidrug resistance phenotype

Because of the similarities between the cystic fibrosis transmembrane conductance regulator (CFTR) and multidrug resistance (MDR) proteins, recent observations of decreased plasma membrane electrical potential (delta psi) in cells overexpressing either MDR protein or the CFTR, and the effects of delta psi on passive diffusion of chemotherapeutic drugs, we have analyzed chemotherapeutic drug resistance for NIH 3T3 cells overexpressing different levels of functional CFTR. Three separate clones not previously exposed to chemotherapeutic drugs exhibit resistance to doxorubicin, vincristine, and colchicine that is similar to MDR transfectants not previously exposed to chemotherapeutic drugs. Two other clones expressing lower levels of CFTR are less resistant. As shown previously these clones exhibit decreased plasma membrane delta psi similar to MDR transfectants, but four of five exhibit mildly acidified intracellular pH in contrast to MDR transfectants, which are in general alkaline. Thus the MDR protein and CFTR-mediated MDR phenotypes are distinctly different. Selection of two separate CFTR clones on either doxorubicin or vincristine substantially increases the observed MDR and leads to increased CFTR (but not measurable MDR or MRP) mRNA expression. CFTR overexpressors also exhibit a decreased rate of 3H -vinblastine uptake. These data reveal a new and previously unrecognized consequence of CFTR expression, and are consistent with the hypothesis that membrane depolarization is an important determinant of tumor cell MDR.

Kinetics of daunorubicin transport in Ehrlich ascites tumor cells with different expression of P-glycoprotein

The classical multidrug resistance (MDR) phenotype is characterized by a decrease in the intracellular drug concentration in resistant cells as compared to sensitive cells. P-glycoprotein (P-gp) is thought to be responsible for an active efflux of lipophilic drugs. Four Ehrlich ascites tumor cell lines selected in vivo for resistance to daunorubicin (DNR) and their sensitive counterpart were investigated. The resistant sublines EHR2/0.1, EHR2/0.2, EHR2/0.4, and EHR2/0.8 were developed by treatment of tumor bearing mice with DNR 0.1, 0.2, 0.4, and 0.8 mg/kg x 4 weekly, respectively. One passage from EHR2/0.1, EHR2/0.2, and EHR2/0.4 and two passages from EHR2/0.8 were investigated. Western blot analysis showed significantly different amounts of P-gp (a 6-fold variation). Efflux of DNR in a drug free medium was investigated with and without presence of verapamil (VER). Efflux from sensitive and resistant cells was described by mono- and bi-exponential kinetics, respectively. In all cases but one, a correlation between resistance, expression of P-gp, P-gp mediated efflux capacity and effect of VER was established. In passage No. 12 of EHR2/0.8, however, a high expression of P-gp was found in spite of a low degree of resistance and a low efflux capacity. In this subline the effect of VER did not correlate to the expression of P-gp. Active efflux seemed to be saturable and was suggested to constitute the major route of efflux in MDR cells. A dose-response relationship was established for the effect of VER on efflux. In conclusion, the results support that P-gp acts as a drug efflux pump. No simple correlation, however, could be established between P-gp and drug transport in all the investigated cell lines. Other factors which might influence transmembranous transportation of DNR are suggested. The active efflux capacity of the cell lines seemed to determine the degree of resistance and the sensitivity to circumvention by VER.

A plasma membrane 'vacuum cleaner' for daunorubicin in non-P-glycoprotein multidrug-resistant SW-1573 human non-small cell lung carcinoma cells. A study using fluorescence resonance energy transfer

A multidrug resistant (MDR) human non-small cell lung carcinoma cell line, SW-1573/2R120 (2R120), not containing the drug-efflux pump P-glycoprotein (PgP), has been studied for the transport of daunorubicin (DN) across the cellular plasma membrane. Earlier, reduced initial DN-uptake rates and lower cellular DN steady-state concentrations were found for this cell line, when it was compared to the SW-1573 wild-type cell line. This finding was an indication for the presence of another cellular drug-efflux pump. However, we found similar DN-efflux rates in drug-free medium for the two cell lines, while for Pgp-containing MDR SW-1573/2R160 (2R160) cells the efflux rate was increased compared to wild-type cells. In order to elucidate differences in DN transport across the cellular plasma membrane, the association of DN with plasma membranes of intact cells was investigated, using fluorescence-resonance-energy transfer. For this purpose, the plasma-membrane probe 1-(4-trimethyl-ammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) was chosen since, because of the overlap between the emission spectrum of TMA-DPH and the excitation spectrum of DN, transfer of energy can be achieved from TMA-DPH to DN. Cells were loaded with TMA-DPH and, after addition of 10 microM DN, the TMA-DPH fluorescence was quenched. Rapid initial quenching proved to be similar in the MDR 2R160 (Pgp-containing) cells and in the SW-1573 wild-type cells (21 +/- 1% and 20 +/- 2%, respectively), but was less in the MDR 2R120 cells not containing Pgp (14 +/- 1%). This finding correlated with a lowered amount of DN dissolved in the plasma membrane of 2R120 cells. We interpret these data to be the result of a 'vacuum-cleaner' pumping system other than Pgp which removes DN from a plasma membrane compartment and equilibrates relatively slowly with the interior of the cell.

Changes in intra- or extracellular pH do not mediate P-glycoprotein-dependent multidrug resistance

P-glycoprotein (Pgp)-mediated multidrug resistance (MDR) is thought to result from active extrusion of lipid-soluble, titratable chemotherapeutic agents. Given the lack of demonstration of coupling between ATP hydrolysis and drug transport, the resistance to chemically unrelated compounds, and findings of elevated intracellular pH (pHi), it has been proposed that reduced intracellular accumulation of drugs in MDR is due to changes in the pH difference across the plasma membrane. Elevation of pHi or decrease in local extracellular pH (pHo) could reduce the intracellular accumulation of the protonated chemotherapeutic drugs and account for Pgp-mediated MDR. Alternatively, changes in pHi or pHo could increase drug efflux by other mechanisms, such as coupled transport involving H+ or OH-, or allosteric effects on Pgp or other proteins. Both mechanisms could operate independently of the charge of the substrate. The possibility of a role of pHi in drug efflux is important to test because of the clinical significance of the phenomenon of MDR of tumors. We tested this hypothesis and found that MDR can occur in cells with low, normal, or high pHi. Further, resistant cells exhibited reduced steady-state drug accumulation and increased efflux without changes in local pHo. Finally, acute changes in pHi had no appreciable effect on Pgp-mediated drug efflux. We conclude that Pgp-mediated MDR is not a consequence of changes in pHi or pHo.

Mathematical models for multidrug resistance and its reversal

Mathematical models describing drug resistance are briefly reviewed. One model which describes the molecular function of the P-glycoprotein pump in multidrug resistant (MDR) cell lines has been developed and is presented in detail. The pump is modeled as an energy dependent facilitated diffusion process. A partial differential equation linked to a pair of ordinary differential equations forms the core of the model. To describe MDR reversal, the model is extended to add an inhibitor. Equations for competitive, one-site noncompetitive, and two-site noncompetitive inhibition are derived. Numerical simulations have been run to describe P-glycoprotein dynamics both in the presence and absence of these kinds of inhibition. These results are briefly reviewed. The character of the pump and its response to inhibition are discussed within the context of the models. All discussions, descriptions, and conclusions are presented in nonmathematical terms. The paper is aimed at a scientifically sophisticated but mathematically innocent audience.

Possible coexistence of two independent mechanisms contributing to anthracycline resistance in leukaemia P388 cells

Murine leukaemia P388 and L1210 cell sublines with varying degrees of resistance to the anthracycline daunomycin (DNM) have been used to monitor (i) intracellular accumulation of DNM, (ii) expression of the drug efflux pump P-glycoprotein (pgp) and (iii) cytoplasmic pH changes. Drug-resistant L1210/65 cells (65-fold resistance), overexpress pgp, and display decreased intracellular accumulation of DNM and identical intracellular pH as compared to the parental drug-sensitive L1210 cell line. On the other hand, moderately drug-resistant P388/20 cells (20-fold resistance), which also exhibit a decreased intracellular drug accumulation with respect to drug-sensitive P388/S cells, display only moderate pgp-encoding mdr1 gene transcription without detectable levels of pgp protein, and undergo cytoplasmic alkalinisation (up to approximately 0.2 pH units). A further increase in the level of drug resistance (P388/100 cells, 100-fold resistance), results in a more pronounced decrease in drug accumulation, significant pgp expression and slightly higher intracellular alkalinisation. Alterations in the degree of protonation of DNM have been shown previously to influence processes such as the rate of uptake and the intracellular accumulation of the drug. On this basis, we propose that the changes in intracellular pH, observed at low levels of drug resistance (P388/20 cells), could constitute an early cellular response aimed at decreasing the intracellular accumulation of ionisable anti-neoplastics. As the level of resistance increases (P388/100), the cells seem to require more efficient mechanisms of defense against the drug, such as that represented by the expression of pgp. Since there is no apparent correlation between the extent of the changes in intracellular pH and the level of pgp expression in DNM-resistant P388 cell sublines, it is suggested that these two cellular responses contributing to drug resistance could operate independently.

Kinetics of daunorubicin transport by P-glycoprotein of intact cancer cells

Drug permeation across the plasma membrane of multidrug-resistant cells depends on the kinetics of the P-glycoprotein-mediated pump activity as well as on the passive permeation of the drug. We here demonstrate a method to characterize kinetically the pump in intact cells. To this purpose, we examined the membrane-transport properties of daunorubicin in various sensitive cancer cell lines and in their multidrug resistant (MDR) counterparts. First, we determined the passive permeability coefficient for daunorubicin. Then, using a flow-through system, the drug flux into the cell was measured after inhibition of the P-glycoprotein-mediated efflux pump. Combining the two results allowed us to calculate the intracellular free concentration of the drug. In the steady-state, the pump rate must equal the net rate of passive diffusion of the drug and, therefore, the same experiments gave us the pumping rate of daunorubicin. These experiments were then repeated at various extracellular drug concentrations. By plotting the pumping rate versus the intracellular drug concentration, we then characterized the P-glycoprotein kinetically. Four independent methods were used to measure the passive permeability coefficient for the cell line A2780. Similar values were obtained. Maximal pump rates (Vmax) showed a good correlation with the amount of P-glycoprotein in the cell lines used. We obtained saturation curves for the variation of the pump rates with the intracellular daunorubicin concentrations. These curves were typical for positive cooperativity, which provides evidence that at least two binding sites for daunorubicin are present on the active transport system of daunorubicin. The apparent Km values for P-glycoprotein-mediated transport, the intracellular free cytosolic daunorubicin concentrations at half-maximal velocity for the cell lines used, were approximately 1.5 microM. Except for the cell lines with the highest amount of P-glycoprotein, the passive efflux rate of daunorubicin proved to be a substantial part of the total daunorubicin efflux rate for the cell lines used. In cell lines with relatively low levels of P-glycoprotein, passive daunorubicin efflux was even the main route of daunorubicin transport from the cells, determining the intracellular steady-state concentrations of daunorubicin.

Probing daunorubicin accumulation defects in non-P-glycoprotein expressing multidrug-resistant cell lines using digitonin

Multidrug resistance (MDR) in tumor cells is frequently associated with reduced cellular cytostatic drug accumulation, caused by the drug efflux protein, P-glycoprotein (Pgp). The action of Pgp in tumor cells can be detected by measuring the increase of daunorubicin accumulation upon blocking Pgp with drugs such as verapamil. A number of MDR cell lines have been described, characterized by decreased drug accumulation without Pgp being present. For such non-Pgp MDR cells no gene probes or functional assays are available to study this phenotype in clinical tumor specimens. We have worked out a method which enables the detection of drug-transport-related decreases in cellular daunorubicin accumulations without the need for the use of specific Pgp blockers. The cells used were SW-1573-, GLC4- and HT1080-sensitive cell lines, which accumulated (corrected for DNA content) 272%, 1,288% and 203% more daunorubicin than the non-Pgp MDR sublines SW-1573/2R120, GLC4/ADR and HT1080/DR4. When the plasma membranes of these MDR lines were permeabilized with 20 microM digitonin an increase to 282%, 1,260% and 239% of 14C-daunorubicin control accumulation was measured (at pH = 7.35). The intracellular pH measured with BCECF was the same in parent and corresponding MDR cells, excluding the role of pH differences in the measured effects. This method provides a tool allowing the detection of cellular mechanisms (including Pgp) which are related to active outward transport of daunorubicin.