Anthracycline incorporation in human lymphocytes. Kinetics of uptake and nuclear concentration

DNA damage induced by NADPH cytochrome P450 reductase-activated idarubicin in sensitive and multidrug resistant MCF7 breast cancer cells

BACKGROUND

Idarubicin (IDA) is one of clinically important anticancer drugs belonging to the anthracycline antibiotic family. The aim of this study was to examine DNA damage induced by NADPH cytochrome P450 reductase (CPR)-activated IDA in human sensitive MCF7 and multidrug resistant MCF7/DOX₅₀₀ (overexpressing P-gp) breast adenocarcinoma cells.

METHODS

The evaluation of DNA fragmentation caused by single strand breaks (SSB) and double strand breaks (DSB) was performed using terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) test. Additionally, DSB formation was examined using H2AX histone phosphorylation assays.

RESULTS

It was found that IDA alone and CPR-activated used at IC₉₀ caused a higher level of DNA strand breaks in sensitive MCF7 cells detected by TUNEL assessments (p=0.0011 for IDA alone and p=0.0109 for IDA reductively activated, Kruskal-Wallis test) and γ-H2AX-positive staining (p=0.0003 for IDA alone and p=0.0193 for IDA reductively activated, Kruskal-Wallis test) than in multidrug resistant MCF7/DOX₅₀₀ cells. However, no changes were observed in the percentage of TUNEL-positive and DSB-positive cells for MCF7 as well as MCF7/DOX₅₀₀ cells in the case of IDA alone and the drug pretreated in the presence of the activating system.

CONCLUSIONS

The obtained results suggest that CPR-activation of IDA does not significantly change the cellular DNA damage response of studied sensitive MCF7 and multidrug resistant MCF7/DOX₅₀₀ breast cancer cells, even if the results concerning the interaction of IDA undergoing CPR activation with naked DNA showed the important differences in comparison with the drug alone (non-activated).

A Drosera-bioinspired hydrogel for catching and killing cancer cells

A variety of bioinspired materials have been successfully synthesized to mimic the sophisticated structures or functions of biological systems. However, it is still challenging to develop materials with multiple functions that can be performed synergistically or sequentially. The purpose of this work was to demonstrate a novel bioinspired hydrogel that can interact with cancer cells, functionally similar to Drosera in catching and killing prey. This hydrogel had two layers with the top one functionalized with oligonucleotide aptamers and the bottom one functionalized with double-stranded DNA. The results show that the top hydrogel layer was able to catch target cells with high efficiency and specificity, and that the bottom hydrogel layer could sequester doxorubicin (Dox) for sustained drug release. Importantly, the released Dox could kill 90% of the cells after 1-h residence of the cells on the hydrogel. After the cell release, this bifunctional hydrogel could be regenerated for continuous cell catching and killing. Therefore, the data presented in this study has successfully demonstrated the potential of developing a material system with the functions of attracting, catching and killing diseased cells (e.g., circulating tumor cells) or even invading microorganisms (e.g., bacteria).

A Flow Cytometric Clonogenic Assay Reveals the Single-Cell Potency of Doxorubicin

Standard cell proliferation assays use bulk media drug concentration to ascertain the potency of chemotherapeutic drugs; however, the relevant quantity is clearly the amount of drug actually taken up by the cell. To address this discrepancy, we have developed a flow cytometric clonogenic assay to correlate the amount of drug in a single cell with the cell's ability to proliferate using a cell tracing dye and doxorubicin, a naturally fluorescent chemotherapeutic drug. By varying doxorubicin concentration in the media, length of treatment time, and treatment with verapamil, an efflux pump inhibitor, we introduced 10(5) -10(10) doxorubicin molecules per cell; then used a dye-dilution assay to simultaneously assess the number of cell divisions. We find that a cell's ability to proliferate is a surprisingly conserved function of the number of intracellular doxorubicin molecules, resulting in single-cell IC50 values of 4-12 million intracellular doxorubicin molecules. The developed assay is a straightforward method for understanding a drug's single-cell potency and can be used for any fluorescent or fluorescently labeled drug, including nanoparticles or antibody-drug conjugates.

Bioinspired affinity DNA polymers on nanoparticles for drug sequestration and detoxification

Nanomaterials with the ability of sequestering target molecules hold great potential for a variety of applications. To ensure the stable sequestration, most of these nanomaterials have been traditionally designed with a clear boundary or compact structures and behave as closed systems. While this feature is beneficial to applications such as drug delivery, it may pose a challenge to applications where fast molecular transport from the environment to nanomaterials is critical. Thus, this study was aimed at exploring a nanomaterial with affinity DNA polymers and nanoparticles as an open system with function similar to jellyfish tentacles in sequestering target molecules from surroundings. The results show that this nanomaterial can effectively and rapidly sequester both small molecule drugs and large molecule biologics and resultantly mitigate their biological effects. Thus, this nanomaterial holds potential as a universal nanoscale antidote for drug removal and detoxification. While this nanomaterial was evaluated by using drug removal and detoxification as a model, the synthesis of periodically oriented affinity polymers on a nanoparticle with the capability of sequestering target molecules may be tuned for broad applications such as separation, sensing, imaging and drug delivery.

Multiscale kinetic modeling of liposomal Doxorubicin delivery quantifies the role of tumor and drug-specific parameters in local delivery to tumors

Nanoparticle encapsulation has been used as a means to manipulate the pharmacokinetic (PK) and safety profile of drugs in oncology. Using pegylated liposomal doxorubicin (PLD) vs. conventional doxorubicin as a model system, we developed and experimentally validated a multiscale computational model of liposomal drug delivery. We demonstrated that, for varying tumor transport properties, there is a regimen where liposomal and conventional doxorubicin deliver identical amounts of doxorubicin to tumor cell nuclei. In mice, typical tumor properties consistently favor improved delivery via liposomes relative to free drug. However, in humans, we predict that some tumors will have properties wherein liposomal delivery delivers the identical amount of drug to its target relative to dosing with free drug. The ability to identify tumor types and/or individual patient tumors with high degree of liposome deposition may be critical for optimizing the success of nanoparticle and liposomal anticancer therapeutics.

Reduction of doxorubicin resistance in P-glycoprotein overexpressing cells by hybrid cell-penetrating and drug-binding peptide

Drug efflux by the membrane transporter P-glycoprotein (P-gp) plays a key role in multidrug resistance (MDR). In order to bypass P-gp, thus overcoming MDR, a hybrid peptide comprising a cell penetrating peptide (Tat) and a drug binding motif (DBM) has been developed to noncovalently bind and deliver doxorubicin (Dox) into MDR cells. The uptake of Dox into the leukemia cell line K562 and its P-gp overexpressing subline KD30 increased in the presence of DBM-Tat peptide. Confocal microscopy indicated that DBM-Tat associated Dox was directed to a perinuclear area of KD30 cells, while this was not observed in parent K562 cells. When KD30 cells were pretreated with the endosomotropic agent chloroquine (CLQ), peptide associated Dox redistributed into the cytosol, indicating that endocytosis was the predominant uptake route. Altered drug uptake kinetics observed by cellular accumulation assay also supported an endocytic uptake. In the presence of CLQ, DBM-Tat was able to enhance the cytotoxicity of Dox by 68.4% at 5 microM peptide concentration in KD30 cells but there were only minor effects on Dox cytotoxicity in K562 cells even in the presence of CLQ. Thus, combining Dox with DBM-Tat reduces P-gp mediated drug efflux, without a requirement for drug modification or inhibiting P-gp function.

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.

N-(2-Hydroxylpropyl)-Methacrylamide-Attached Doxorubicin Induces Cytotoxicity to Prostate Cancer Cell Line DU145

ABSTRACT Successful use of doxorubicin as an antitumor agent is limited by its cardiotoxicity, which takes different forms and results from multiple biochemical alterations in the cell. This study addresses the possibility to overcome these adverse effects by studying the effects of N-(2-hydroxypropyl)methacrylamide (HPMA) polymer, which contains doxorubicin attaching to it. Both time-and dose-dependent studies were conducted using DU145 cell lines. The results of this study reveal that doxorubicin attached to HPMA can be used successfully in treating cancer instead of doxorubicin alone, which may open a new gate for clinicians and scientists, leading to overcoming the resistance and side effects of the currently used antitumor agents.

The role of bioreductive activation of doxorubicin in cytotoxic activity against leukaemia HL60-sensitive cell line and its multidrug-resistant sublines

Clinical usefulness of doxorubicin (DOX) is limited by the occurrence of multidrug resistance (MDR) associated with the presence of membrane transporters (e.g. P-glycoprotein, MRP1) responsible for the active efflux of drugs out of resistant cells. Doxorubicin is a well-known bioreductive antitumour drug. Its ability to undergo a one-electron reduction by cellular oxidoreductases is related to the formation of an unstable semiquionone radical and followed by the production of reactive oxygen species. There is an increasing body of evidence that the activation of bioreductive drugs could result in the alkylation or crosslinking binding of DNA and lead to the significant increase in the cytotoxic activity against tumour cells. The aim of this study was to examine the role of reductive activation of DOX by the human liver NADPH cytochrome P450 reductase (CPR) in increasing its cytotoxic activity especially in regard to MDR tumour cells. It has been evidenced that, upon CPR catalysis, DOX underwent only the redox cycling (at low NADPH concentration) or a multistage chemical transformation (at high NADPH concentration). It was also found, using superoxide dismutase (SOD), that the first stage undergoing reductive activation according to the mechanism of the redox cycling had the key importance for the metabolic conversion of DOX. In the second part of this work, the ability of DOX to inhibit the growth of human promyelocytic-sensitive leukaemia HL60 cell line as well as its MDR sublines exhibiting two different phenotypes of MDR related to the overexpression of P-glycoprotein (HL60/VINC) or MRP1 (HL60/DOX) was studied in the presence of exogenously added CPR. Our assays showed that the presence of CPR catalysing only the redox cycling of DOX had no effect in increasing its cytotoxicity against sensitive and MDR tumour cells. In contrast, an important increase in cytotoxic activity of DOX after its reductive conversion by CPR was observed against HL60 as well as HL60/VINC and HL60/DOX cells.

The role of structural factors in the kinetics of cellular uptake of pyrazoloacridines and pyrazolopyrimidoacridines: implications for overcoming multidrug resistance towards leukaemia K562/DOX cells

The appearance of multidrug resistance (MDR) of tumour cells to a wide array of antitumour drugs, structurally diverse and having different mechanisms of action, constitutes the major obstacle to the successful treatment of cancer. Our approach to search for non-cross resistant antitumour agents is based on the rational design of derivatives, which have a high kinetics of passive cellular uptake rendering their active efflux by MDR exporting pumps inefficient. Recently, two families of acridine cytotoxic agents were obtained, pyrazoloacridines (PACs) and pyrazolopyrimidoacridines (PPACs). The aim of this study was to examine molecular basis of the reported differences in retaining cytotoxic activity of these derivatives at cellular level against resistant erythroleukaemia K562/DOX (overexpressing P-glycoprotein) cell line. The study was performed using a spectrofluorometric method, which allows continuous monitoring of the uptake and efflux of fluorescent molecules by living cells. It was demonstrated that the presence of two additional rings, pyrazole and pyrimidine, fused to the acridine chromophore structure (PPAC) favoured more rapid cellular diffusion than the presence of only one additional pyrazole ring (PAC). The presence of hydrophobic substituent OCH3 markedly favoured the cellular uptake of pyrazoloacridines and pyrazolopyrimidoacridines while compounds having hydrophilic substituent OH exhibited very low kinetics of cellular uptake. In contrast, it was found that neither structure of the ring system nor the hydrophobic/hydrophilic character of examined substituents determined the rate of active efflux of these compounds by P-glycoprotein. Our data showed that a nearly linear relation exists between the resistance factor (RF) and lnV+ reflecting the impact of the cellular uptake rate (V+) on the ability of these compounds to overcome MDR.

2-Pyrrolinodoxorubicin and its peptide-vectorized form bypass multidrug resistance

A well-known mechanism leading to the emergence of multidrug-resistant tumor cells is the overexpression of P-glycoprotein, which is capable of lowering intracellular drug concentrations. In the present study, we tested the capability of 2-pyrrolinodoxorubicin (p-DOX), a highly potent derivative of DOX, to bypass multidrug resistance. The accumulation, intracellular distribution and cytotoxicity of p-DOX were tested in two cell lines (K562 and A2780) and their DOX-resistant counterparts (K562/ADR and A2780/ADR). Cellular accumulation and cytotoxicity were dramatically lowered for DOX in resistant cell lines, in comparison with non-resistant cells. In contrast, cellular accumulation, intracellular distribution and cytotoxicity of p-DOX were independent of the nature of the cell lines. The p-DOX showed potent dose-dependent inhibition of cell growth against resistant cells as compared with DOX. After treatment of resistant cells with verapamil, the intracellular levels of DOX were markedly increased and consequent cytotoxicity improved. In contrast, treatment of resistant cells with verapamil did not cause any further enhancement of cell uptake or an increase in the cytotoxic effect of the derivative p-DOX, indicating that the compound bypasses the P-glycoprotein. Finally, we show that vectorization of p-DOX by a peptide vector (SynB3) which has been shown to enhance the brain uptake of DOX and to decrease its heart accumulation does not affect this property. These results indicate that p-DOX and its vectorized form are potent and effective in overcoming multidrug resistance.

The activity of latent benzoperimidine esters to inhibit P-glycoprotein and multidrug resistance-associated protein 1 dependent efflux of pirarubicin from several lines of multidrug resistant tumor cells

Multidrug resistance of tumor cells is associated with the presence of membrane proteins responsible for the cytostatics export. Recently, we have synthesized a new family of benzoperimidines causing the futile cycle of MDR pumps. In this study, biological data for benzoperimidine esters are presented for selected cell lines: sensitive (HL-60, GLC4, K562), P-gp resistant (HL-60/VINC, K562/DX), MRP1 resistant (HL-60/DX) and MRP1/LRP resistant (GLC4/DX). Their ability to inhibit the efflux of anthracycline antitumor drug, pirarubicin and to restore its accumulation in MDR cells was studied using a spectrofluorometric method which allows to follow the uptake and efflux of fluorescent molecules by living cells. Benzoperimidine esters had high effectiveness in inhibiting pirarubicin efflux and in restoring its accumulation in resistant cells. In contrast, examined esters were less active in vitro in restoration of pirarubicin cytotoxicity towards resistant cells because an enzymatic cleavage of esters occurs in presence of serum esterases.

Functional study of intracellular P-gp- and MRP1-mediated pumping of free cytosolic pirarubicin into acidic organelles in intrinsic resistant SiHa cells

We sought to determine the efficiency of the intracellular functional P-gp- and MRP1-mediated pumping of THP into acidic organelles in SiHa cells and etoposide-resistant SiHa/VP16 cells. The expression of both MDR1 and MRP1 genes of SiHa and SiHa/VP16 cells was clearly shown by using RT-PCR. The functional studies of both intracellular functional P-gp- and MRP1-mediated pumping were performed by using THP in a conventional spectrofluorometer, and they demonstrated that SiHa and SiHa/VP16 cells are good models to illustrate the functional role of intracellular P-gp and MRP1 in the transport of free cytosolic drug into acidic organelles. The functional P-gp and MRP1 proteins were identified both on plasma membranes and on intracellular vesicle membranes. Within the limit of experimental error, similar efficiencies in THP transport were observed in the two proteins at both locations in SiHa and SiHa/VP16 cells. The P-gp- and MRP1-mediated pump coefficient (k v a), Michealis-Menten's constant (K V m), and maximal pumping rate (V V max) values of those located on vesicular membranes were 1.87 +/- 0.30 pL x cell-1 x s-1, 1.63 +/- 0.21 microM, and 4.95 +/- 0.45 nM x s-1</sup>, respectively. Drug retention inside acidic organelles (C mon V) of SiHa cells was significantly higher than that of SiHa/VP16 cells, perhaps a consequence of slower movement of recycling endosomes and (or) lysosomes to the cell membrane of SiHa cells, leading to distended organelles and cell death. Our results suggest that intracellular P-gp and MRP1 proteins play an important role in the transport of free drug from cytosol to cytoplasmic acidic organelles.

Relation between MDR1 mRNA levels, resistance factor, and the efficiency of P-glycoprotein-mediated efflux of pirarubicin in multidrug-resistant K562 sublines

In this work, we sought to investigate the relation existing between MDR1 mRNA levels, the resistance factor (RF), and the efficiency of efflux of pirarubicin (THP) mediated by P-glycoprotein (P-gp) in multidrug-resistant (MDR) K562 sublines. The MDR K562 sublines were selected from K562/adr cells by exposure to different adriamycin concentrations: 300 nM (K562/300), 1,000 nM (K562/1,000), and 10,000 nM (K562/10,000), yielding RF values of 23.2, 26.5, and 39.6, respectively. The analysis of the P-gp encoding MDR1 gene overexpression by reverse transcriptase - polymerase chain reaction provided evidence of increased MDR1 mRNA levels when the adriamycin concentration used for the MDR cell selection increased. We used spectrofluorometric methods to determine the kinetics of the uptake and P-gp-mediated efflux of THP in the different selected MDR K562 sublines. Our data showed that (i) the maximal rate of P-gp-mediated efflux of THP, Vmax, increased with increasing RF; (ii) the observed Michaelis constant, Km, had the same value for all selected sublines, thus leading to an overall increase in the ratio Vmax/Km (5.1 x 10(-3), 6.2 x 10(-3), 6.8 x 10(-3), and 9.3 x 10(-3) s(-1) for K562/adr, K562/300, K562/1,000, and K562/10,000 cells, respectively), and (iii) the determination of the Hill coefficient (nH) gave values close to 2, which suggested a positive cooperative transport of THP with the expelling of two molecules of THP per turnover of P-gp. This study demonstrated that, in the K562/adr sublines used in our experiments, P-gp played a major role in conferring the MDR phenotype. Moreover, under our experimental conditions, intracellular acidic organelles were shown to contribute to decreased drug-target interaction and, thereby, decreased cytotoxicity. The variation of the concentrations of THP accumulated in the acidic organelles as a function of the total TFP concentration added to the cells was the same, within the limits of experimental errors, whatever the degree of resistance of the studied MDR K562 sublines. Finally, this study suggested that, in the selected MDR K562 sublines, the K+/H+ antiporter exchanger could be activated by the pirarubicin transport, leading to a probable acidification of intracellular pH. The P-gp-mediated efflux of THP and an accumulation of THP in acidic organelles confer an advantage for MDR cells in surviving prolonged exposure to cytotoxic agents and giving rise to high degrees of resistance.

Anthrapyridones, a novel group of antitumour non-cross resistant anthraquinone analogues. Synthesis and molecular basis of the cytotoxic activity towards K562/DOX cells

1. Multidrug resistance (MDR) to antitumour agents, structurally dissimilar and having different intracellular targets, is the major problem in cancer therapy. MDR phenomenon is associated with the presence of membrane proteins which belong to the ATP-binding cassette family transporters responsible for the active drug efflux leading to the decreased intracellular accumulation. 2. The search of new compounds able to overcome MDR is of prime importance. 3. Recently we have synthesized a new family of anthrapyridone compounds. The series contained derivatives modified with appropriate hydrophobic or hydrophylic substituents at the side chain. 4. The interaction of these derivatives with erythroleukemia K562 sensitive and K562/DOX resistant (overexpressing P-glycoprotein) cell lines has been examined. The study was performed using a spectrofluorometric method which allows to continuously follow the uptake and efflux of fluorescent molecules by living cells. 5. It was demonstrated that the increase in the lipophilicity of anthrapyridones favoured the very fast cellular uptake exceeding the rate of P-gp dependent efflux out of the cell. For these derivatives, very high accumulation (the same for sensitive and resistant cells) was observed and the in vitro biological data confirmed that these compounds exhibited comparable cytotoxic activity towards sensitive and P-gp resistant cell line. In contrast, anthrapyridones modified with hydrophylic substituents exhibited relatively low kinetics of cellular uptake. 6. For these derivatives decreased accumulation in resistant cells was observed and the in vitro biological data demonstrated that they were much less active against P-gp resistant cells in comparison to sensitive cells. 7. We also studied, using confocal microscopy, the intracellular distribution of anthrapyridones in NIH-3T3 cells. Our data showed that these compounds were strongly accumulated in the nucleus and lysosomes.

Transport of new non-cross-resistant antitumor compounds of the benzoperimidine family in multidrug resistant cells

Multidrug resistance (MDR) phenotype in mammalian cells is often correlated with overexpression of P-glycoprotein or multidrug resistance-associated protein (MRP1). Both proteins are energy-dependent drug efflux pumps that efficiently reduce the intracellular accumulation and hence the cytotoxicity of many natural cytotoxins. The influx and efflux of drugs across the cell membrane are in large part responsible for their intracellular concentrations, and in the search for new compounds able to overcome MDR, it is of prime importance to determine the molecular parameters whose modification would lead to an increase in the kinetics of uptake and/or to a decrease in the pump-mediated efflux. Here, we studied three members of a new family of benzoperimidine antitumor compounds which exhibit comparable cytotoxicity towards resistant cells expressing P-glycoprotein, or MRP1, and sensitive cells. We used spectrofluorometric methods to determine the kinetics of the uptake and release of these three drugs in different cell lines: the erythroleukemia cell line K562 and the resistant K562/Adr expressing P-glycoprotein, the small-cell lung cancer cell line GLC4 and resistant GLC4/Adr expressing MRP1. We also studied, using confocal microscopy, the intracellular distribution of these drugs in NIH/3T3 cells. Our data show that (i) the kinetics for the uptake of these drugs is very rapid, higher than 2 x 10(-17) mole cell(-1) s(-1), (ii) the drugs are strongly accumulated in the nucleus and lysosomes, (iii) the three drugs are recognized and pumped out by both transporters, as shown by the inhibition of P-glycoprotein- and MRP1-mediated efflux of pirarubicin by benzoperimidine, with inhibitory constants of 1.5 and 2.1 microM for P-glycoprotein and MRP1, respectively, suggesting that benzoperimidine is transported by the two transporters with K(m) approximately 2 microM. In conclusion, the fast uptake kinetics of the benzoperimidines counterbalance their efflux by P-glycoprotein and MRP1.

Doxorubicin-peptide conjugates overcome multidrug resistance

A well-known mechanism leading to the emergence of multidrug-resistant tumor cells is the overexpression of P-glycoprotein (P-gp), which is capable of lowering intracellular drug concentrations. To overcome this problem, we tested the capability of two peptide vectors that are able to cross cellular membranes to deliver doxorubicin in P-gp-expressing cells. The antitumor effect of peptide-conjugated doxorubicin was tested in human erythroleukemic (K562/ ADR) resistant cells. The conjugate showed potent dose-dependent inhibition of cell growth against K562/ADR cells as compared with doxorubicin alone. Doxorubicin exhibited IC50 concentrations of 65 microM in the resistant cells, whereas vectorized doxorubicin was more effective with IC50 concentrations of 3 microM. After treatment of the resistant cells with verapamil, the intracellular levels of doxorubicin were markedly increased and consequent cytotoxicity was improved. In contrast, treatment of resistant cells with verapamil did not cause any further enhancement in the cell uptake nor in the cytotoxic effect of the conjugated doxorubicin, indicating that the conjugate bypasses the P-gp. Finally, we show by the in situ brain perfusion method in P-gp-deficient and competent mice that vectorized doxorubicin bypasses the P-gp present at the luminal site of the blood-brain barrier. These results indicate that vectorization of doxorubicin with peptide vectors is effective in overcoming multidrug resistance.

Modulation of anthracycline activity in canine mammary tumour cells in vitro by medroxyprogesterone acetate

Failure of chemotherapy with anthracyclines as a result of drug resistance and toxicity is a major problem in the clinical management of neoplasia. The aim of the present study was to evaluate the activity of medroxyprogesterone acetate (MPA) as a chemosensitiser on anthracycline cytotoxicity. The study investigated whether such an effect could be related to an increase in lipid peroxidation, nitric oxide production, membrane fluidity and intracellular anthracycline concentration. The results showed that anthracyclines decreased nitric oxide production but increased membrane viscosity (polarisation constant) and lipid hydroperoxide formation in canine mammary tumour cells. Moreover, it was found that both drug-induced cytotoxicity and membrane viscosity increased in the presence of MPA. Conversely, lipid hydroperoxides decreased in MPA-supplemented cells. Medroxyprogesterone acetate did not show any effect on nitric oxide production. The two anthracyclines used (doxorubicin and idarubicin) showed differential intranuclear accumulation in canine mammary tumour cells, and MPA significantly modified intracellular concentration of anthracyclines.

Regulation of cellular glutathione modulates nuclear accumulation of daunorubicin in human MCF7 cells overexpressing multidrug resistance associated protein

Multidrug resistance (MDR) is frequently associated with the overexpression of P-glycoprotein (Pgp) and/or multidrug resistance associated protein (MRP1), both members of the ABC superfamily of transporters. Pgp and MRP1 function as ATP-dependent efflux pumps that extrude cytotoxic drugs from tumour cells. Glutathione (GSH) has been considered to play an important role in the MRP1-mediated MDR. In our study, we examined the effects of buthionine sulphoximine (BSO), an inhibitor of GSH biosynthesis, on the nuclear accumulation of daunorubicin (DNR), in etoposide (VP16) and doxorubicin (ADR) resistant MCF7 cell lines, overexpressing respectively MRP1 (MCF7/VP) and Pgp (MCF7/ADR). The study of DNR transport was carried out using scanning confocal microspectrofluorometry. This technique allows the determination of the nuclear accumulation of anthracyclines in single living tumour cells. Treatment of MCF7/VP cells with BSO increased the sensitivity of these cells to DNR whilst the cytotoxicity of the drug in MCF7/ADR cells remained unchanged. In MCF7 resistant cells treated with BSO, their GSH level decreased as observed by confocal microscopy. DNR nuclear accumulation in MCF7/VP cells was increased by BSO whereas in MCF7/ADR cells BSO was unable to significantly increase the DNR nuclear accumulation. These data suggest a requirement for GSH in MRP1-mediated resistance whilst the nuclear efflux of GSH conjugates is probably not the primary mechanism of Pgp-mediated MDR. Finally, BSO might be a useful agent in clinical assays for facilitating detection of MRP1 expression.

A mathematical model of drug transport in human breast cancer

A mathematical model of drug transport in tissue has been developed on the basis of a clinical study of patients with breast cancer, treated with the drug doxorubicin and of drug transport experiments using cultured human breast cancer cells. The clinical study revealed doxorubicin gradients in tumor islets of densely packed cancer cells. The mathematical model allows simultaneous drug transport through the cellular network (transcellular pathway), through the intercellular interstitium (paracellular pathway), and across the boundary between the two networks. The effective diffusion coefficient of the interstitial network is found to be much higher than that of the cellular network, in spite of the fact that the interstitium thickness is only 20-40 nm. The model simulations can be made to fit the results of the clinical study. A long-continued simulation (40 days) of drug transport into a spherical islet with a radius of 150 microm, after a bolus injection of doxorubicin, reveals that the maximum average drug concentration at the islet centre is only reached after 224 h, while it decreases by a factor 15 from the boundary to the centre of the islet. The area under the curve in a plot of the average drug concentration versus time only decreases by 10% from the boundary to the centre of the islet.

Correlation between the kinetics of anthracycline uptake and the resistance factor in cancer cells expressing the multidrug resistance protein or the P-glycoprotein

Multidrug resistance (MDR) in model systems is known to be conferred by two different integral proteins, the 170-kDa P-glycoprotein (Pgp) and the 190-kDa multidrug resistance-associated protein (MRP1). One possible pharmacological approach to overcome drug resistance is the use of specific inhibitors, which enhance the cytotoxicity of known antineoplastic agents. However, while many compounds have been proven to be very efficient in inhibiting Pgp activity only some of them are able to inhibit MRP1. The other likely approach is based on the design and synthesis of new non-cross-resistant drugs with physicochemical properties favoring the uptake of the drug by the resistant cells. The intracellular drug retention influences its cytotoxic effect. The level of the intracellular drug content is a function of the amount of drug transported inside the cell (influx) and the amount of drug expelled from the cell (efflux). In this work, the kinetics of drug uptake and the kinetics of active efflux of several anthracycline derivatives in both Pgp expressing K562/Adr cells and MRP1 expressing GLC4/Adr cells was determined. Our data have shown that in both cell lines there is no correlation between the resistance factor and the kinetics of drug efflux by these pumping systems. However, a very good correlation between the resistance factor and the kinetics of drug uptake has been established in both cell lines: the resistance factor decreases when the kinetics of drug uptake increases. This work has clearly shown that when the rate of transmembrane transport of anthracycline is high enough, the efflux mediated by the protein transporter is not able to pace with it. The protein transporter essentially operates in a futile cycle and the resistance factor is tending to one. It does not mean, however, that when the resistance factor is close to one the anthracycline is not transported by the pump.

Idarubicin overcomes P-glycoprotein-related multidrug resistance: comparison with doxorubicin and daunorubicin in human multiple myeloma cell lines

The clinical utility of anthracyclines like doxorubicin (DOX) and daunorubicin (DNR) for treatment of multiple myeloma (MM) is limited by the occurrence of multidrug resistance (MDR). Highly lipophilic anthracyclines like idarubicin (IDA) might circumvent MDR and thereby enhance chemotherapeutic efficacy. To determine the efficacy of IDA in myeloma cells, the pharmacokinetics and cytotoxicity of IDA and its major metabolite idarubicinol (IDAol) were compared with those of DNR, DOX, and doxorubicinol (DOXol) in the cell line RPMI 8226-S and two MDR sublines (8226-R7 and 8226-Dox40) that overexpress the drug transporter P-glycoprotein (Pgp). Cytotoxicity assays using MTT (viability) or annexin V (apoptosis) showed a 10-50-fold higher potency of IDA compared with DNR or DOX in the MDR variant cell lines. The difference in cytotoxicity was lower in the sensitive parental cell line (3-fold). These results are explained by a better intracellular uptake of IDA compared to DNR in resistant 8226 cell lines. The Pgp-inhibitor verapamil affected IDA uptake only in the most resistant cell line 8226-Dox40. This indicates that IDA is less sensitive than DNR to transport-mediated MDR. IDAol was at least 32-fold more cytotoxic than DOXol, and more susceptible to Pgp transport than IDA. These studies demonstrate that the efficacy of IDA in MDR MM cell lines is superior to that of DOX or DNR, and that IDA may become an important drug in the treatment of MM, especially in refractory disease.

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).

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.

A method for studying plasma membrane transport with intact cells using computerized fluorometry

A new method is presented for measuring rapid efflux of fluorescent compounds from monolayer cells. Cells grown on a glass coverslip were loaded with a fluorescent substrate. Thereafter, the coverslip was installed outside the light path in a stirred and thermostated cuvette of a fluorometer. The efflux was recorded by measuring the changes of fluorescence in the extracellular medium. The method was used to study the kinetics of active and passive plasma membrane transport of the P-glycoprotein substrates rhodamine 123 and daunorubicin. The method has advantages over other methods: (1) no radioactively labeled substrate is needed, (2) fluorescence of the transported substrate is not compromised by the cells, (3) changes in the extracellular concentration of the substrate can be monitored continuously and therefore a substantial improvement of the kinetic resolution is obtained, and (4) the measurement setup is relatively simple and a standard fluorometer can be used. From the efflux data, cellular transport parameters could be calculated, such as passive permeation coefficients and active transport rates.

Interaction of doxorubicin with ATP: quantification of complexes and effect on its diffusion into DNA-loaded liposomes--implication for ATP-driven transport studies

Doxorubicin, a drug largely used in chemotherapy, is transported by P-glycoprotein, a protein involved in the multidrug-resistance phenotype. Taking advantage of the doxorubicin fluorescence quenching upon interaction with DNA, a sensitive assay of this active transport can be carried out: quantitative in vitro studies could be achieved with DNA-loaded proteoliposomes, after correction for the doxorubicin passive diffusion through phospholipids. In this paper, we describe experimental conditions that will be relevant to P-glycoprotein studies. Efficient DNA entrapment in preformed liposomes was obtained using the freeze/thawing procedure, and the doxorubicin passive diffusion was quantified in the presence of ATP/Mg2+, the second substrate of P-glycoprotein. The doxorubicin diffusion rate decreases in the presence of ATP, indicating an interaction between doxorubicin and ATP that will hinder any measurement of ATP-driven transport. The interaction between doxorubicin and ATP was studied by fluorescence quenching, octanol/buffer partition coefficient, and diffusion rate into DNA-loaded liposomes. The results give evidence for complex interactions. However, under our experimental conditions, these interactions are only slightly modified in the presence of Mg2+. Since this cation is essential for P-glycoprotein activity, it can be concluded that in these conditions the accurate evaluation of P-glycoprotein-catalyzed doxorubicin transport will be obtained from the Mg2+-sensitive transport into DNA-loaded proteoliposomes.

Kinetic analysis in living cells of the inhibition of the P-glycoprotein-mediated efflux of anthracyclines by vinca alkaloids

Cells that overexpress the mdr 1 gene have decreased steady-state accumulation and increased efflux of many anticancer drugs including anthracyclines and vinca alkaloids. The mechanism(s) of P-glycoprotein-mediated efflux of drugs is (are) still poorly understood. In an attempt to identify mechanism(s) by which multidrug resistance can be circumvented, the cellular accumulation has been examined of pirarubicin, doxorubicin and idarubicin alone and in conjunction with four vinca alkaloid derivatives--vinblastine, navelbine, vindesine and vincristine. The present study was performed using a spectrofluorometric method with which it is possible to follow continuously the uptake and release of fluorescent molecules by living cells, as the incubation of the cells with the drug proceeds. Erythroleukemia K562 cell lines were used. It has been shown that the P-glycoprotein-mediated efflux of these three anthracyclines can be inhibited by vinca alkaloids derivatives. At pH 7.2, 50% of the P-glycoprotein-mediated efflux of daunorubicin and idarubicin was inhibited by about 40 +/- 10 microM vinblastine and that of pirarubicin by 10 +/- 2 microM vinblastine. The vinblastine concentration required to inhibit 50% of the active efflux of these anthracyclines did not depend on the anthracycline concentrations used, indicating that the inhibition was non competitive. The ability of navelbine, vincristine and vindesine to inhibit the active efflux of pirarubicin was also checked; 15 +/- 3 microM navelbine are required to inhibit 50% of the active efflux but at concentrations lower than 100 microM, neither vincristine nor vindesine were able to inhibit this efflux, indicating that the vinca alkaloids compounds which are the most efficient are the most lipophilic. For the four vinca alkaloids, the concentration required to inhibit 50% of the efflux was lower as the pH was higher. A detailed kinetics analysis of the P-glycoprotein-mediated efflux of pirarubicin in the presence of vinblastine indicates a non competitive inhibition with K(I) = 12 +/- 2 microM.

Cytotoxicity and DNA binding characteristics of dextran-conjugated doxorubicins

The antitumor antibiotic doxorubicin was conjugated with polymeric dextrans of various molecular weights and the cytotoxicity of the conjugates against human carcinoma KB-3-1 cells and its multidrug-resistant subclone KB-V-1 cells was measured by tetrazolium salt MTT assay. The conjugates were much less toxic to the KB-3-1 cells than the free doxorubicin but exhibited similar toxicity to the KB-V-1 cells. The conjugate-DNA interactions were monitored in real-time using an optical biosensor based on evanescent wave detection to obtain the association (ka) and dissociation (kd) rate constants as well as the equilibrium binding constants (KA) of the bindings. Both ka and kd values for the conjugates are more than three magnitudes smaller than those for free doxorubicin, while the KA values of the conjugate-DNA complexes are only about 10 times smaller than that of the free doxorubicin-DNA complex. The results indicate that the cytotoxicity and the DNA-binding kinetics of doxorubicin may be modified with dextran conjugation. The KA values obtained from the biosensor measurements were in close agreement with those determined in solution by fluorescent titration method, verifying the utility of the label-free biosensing measurements as an efficient method for studying ligand-DNA interactions.

Permeability of lipid bilayer to anthracycline derivatives. Role of the bilayer composition and of the temperature

The uptake of three anthracycline derivatives: doxorubicin, daunorubicin and pirarubicin, into large unilamellar vesicles (LUV) in response to a driving force provided by DNA encapsulated inside the LUV has been investigated as a function of the temperature and of the bilayers lipid composition. The kinetics of the decay of the anthracycline fluorescence in the presence of DNA-containing liposome was used to follow the diffusion of the drug through the membrane. For the three drugs, the permeability coefficient of the neutral form of the drug (P0) decreases as the amount of negatively charged phospholipid in the bilayers increases. This can be explained by the fact that the kinetics of passive diffusion of the drugs depends on the amount of neutral form embedded in the polar head group region, which decreases as the quantity of negatively charged phospholipids increases. P0 also decreases as the amount of cholesterol, that makes the bilayer more rigid, increases. The activation energies, Ea, for the passage of the neutral form of these anthracyclines through the bilayers lie within 100 +/- 15 kJ x ml-1, except for pirarubicin and doxorubicin through anionic phospholipid-rich membranes (Ea = 57 kJ x mol-1) and cholesterol-rich membranes (Ea = 167 kJ x mol-1).

Human hepatoma cells rich in P-glycoprotein are sensitive to aclarubicin and resistant to three other anthracyclines

Drug resistance is a major obstacle to successful chemotherapy of primary liver cancer, which is associated with high expression of the multidrug resistance (MDR) gene product P-glycoprotein (Pgp), a multidrug efflux transporter. The most effective single agents in treatment of primary liver carcinoma belong to the anthracycline family, yet several anthracyclines are known to be substrates for Pgp. In the present study, we compared four anthracyclines with respect to cell growth inhibition, intracellular accumulation and cellular efflux using the HB8065/R human hepatoma cell line which is rich in Pgp, and the Pgp-poor parental line HB8065/S. The anthracyclines were also administered in conjunction with the Pgp-modifying agents verapamil and SDZ PSC 833 to assess modulation of resistance. The HB8065/R cells were sensitive to aclarubicin (ACL) and highly resistant to epirubicin (EPI), doxorubicin (DOX) and daunorubicin (DNR). SDZ PSC 833 enhanced accumulation, decreased efflux and increased cytotoxicity of EPI, DOX and DNR in the HB8065/R cells, but none of these effects was seen with ACL. In conclusion, ACL is apparently not transported by Pgp and retains its activity in a multidrug-resistant human hepatoma cell line; such properties can be exploited for clinical purposes.

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.

Kinetic transport analysis of daunorubicin by LoVo and LoVo/DX cells

We describe a fluorometric technique for the measurement of transport parameters of fluorescent drugs through cellular membranes. Unlike other procedures, this method gives an accurate measure of drug accumulated in the cells and measures the fraction of free and bound drug in the cell. The kinetic parameters of transport through cellular membranes are determined using a simple three-compartment model combined with fluorescence measurements performed on the extracellular medium and on Triton-permeabilized cells during daunorubicin incorporation. With this technique we found that LoVo cells have a greater daunorubicin uptake, a similar input rate constant and a lower output rate constant than the drug-resistant LoVo/DX cells.

Spectral shape modifications of anthracyclines bound to cell nuclei: a microspectrofluorometric study

Anthracyclines remain today the medications of choice against a wide spectrum of human cancers. Anthracyclines are fluorescent molecules and microfluorimetric methods are often used to determine their cellular distribution. The use of microspectrofluorometric techniques yields additional information because not only the fluorescence intensity but also the spectral modifications of the chromophore can be used to assess the intracellular drug concentration, its localisation and also eventually its metabolisation. It is well-documented that the shape of the fluorescence spectrum of anthracyclines changes markedly with the hydrophobicity of their environment. This change can be quantitatively measured by the ratio rho of the fluorescence emission intensities at 560 and 590 nm. We have observed that the shape of the fluorescent spectrum of adriamycin, daunorubicin and 4'-O-tetrahydropyranyladriamycin recorded from a small volume inside the cell nucleus was strongly dependent on the drug concentration and that the rho value decreases as the drug concentration increases. These data were compared with the rho variations when the drugs were either dissolved in different solvents or intercalated between the base pairs of DNA. We arrived at the conclusion that the shape variation of the drug spectra was not due to a change in their hydrophobicity environment but to an excitonic coupling of the electric dipolar transition moments of the pi --> pi* transition.

Co-operative, competitive and non-competitive interactions between modulators of P-glycoprotein

We measured the effects of individual modulators and of pairs of modulators of the multidrug resistance pump, P-glycoprotein, on the accumulation of labelled daunomycin into multidrug-resistant P388 leukemia cells at 37 degrees C and developed a kinetic analysis which enables such data to be modelled in terms of co-operative, competitive or non-competitive interaction between pairs of modulators. The modulators verapamil, cyclosporin and trifluoperazine interacted with P-glycoprotein as single molecules, while vinblastine, mefloquine, dipyridamole, tamoxifen and quinidine displayed Hill numbers close to 2, suggesting that pairs of modulator molecules need to act together in order to bring about effective reversal of P-glycoprotein. When the modulators were presented to P-glycoprotein in pairs, we found examples of both competitive and non-competitive behaviour. We interpret these results on a model in which two modulatory sites exit on the MDR pump. To one of these, mefloquine, vinblastine and tamoxifen bind preferentially; to the other, verapamil, dipyridamole, trifluoperazine and quinidine bind (but mefloquine and tamoxifen only weakly if at all). Cyclosporin A can interact with both sites.

Cellular uptake, cytotoxicity, and transport kinetics of anthracyclines in human sensitive and multidrug-resistant K562 cells

Multidrug resistance in tumor cells is often associated with the presence of an approximately 170 kDa plasma membrane glycoprotein (Pgp) that acts as a drug-efflux pump and decreases intracellular antitumor drug concentration. We measured the uptake of seven anthracyclines (daunorubicin, doxorubicin, 4'-epi-doxorubicin, 4'-deoxy-doxorubicin, iododoxorubicin, 3'-(3-methoxymorpholino)-doxorubicin (FCE23762) and 4-demethoxy-daunorubicin) into K562 cells sensitive and resistant (K562/DNR) to daunorubicin. The K562/DNR subline expresses Pgp at the membrane surface, whereas its sensitive counterpart does not. Laser flow cytometry was used to quantitate intracellular anthracycline content. Uptake of daunorubicin, doxorubicin, 4'-epi-doxorubicin, and 4'-deoxy-doxorubicin was minimal in the K562/DNR subline as compared to their uptake in sensitive cells. On the contrary, iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin accumulate to nearly the same extent into sensitive and resistant K562 cells. Growth inhibition data indicated that the resistance factor for iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin is markedly decreased as compared to the other drugs. Fluorescence measurements were carried out to determine the kinetic parameters associated with the influx and efflux of the drugs into and out of K562 cells. Kinetic data indicated that iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin are not actively rejected from resistant cells, suggesting that they are poor substrates for Pgp-mediated transport. This observation is related to their ability to overcome the multidrug-resistant phenotype of K562/DNR cells in vitro.

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).

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

The basic distinguishing feature of all cells expressing functional P-glycoprotein-multidrug resistance (P-gp-MDR) is a decrease in steady-state accumulation drug levels as compared to drug-sensitive controls. In an attempt to identify mechanism(s) by which MDR can be circumvented, we examined the cellular accumulation, in resistant cells, of 4'-O-tetrahydropyranyl-doxorubicin (pirarubicin) alone and in conjunction with various molecules belonging to three different classes: the crown ethers, the tetraalkylammonium salts, and the polyoxethylene amphiphiles. The present study was performed using a spectrofluorometric method which enabled us to follow the uptake and release of fluorescent molecules by living cells while the cells were being incubated with the drug. Erythroleukemia K562 cell lines were used. Our data show that the compounds of these three completely different classes were able to increase the incorporation of pirarubicin provided they had a minimum degree of lipophilicity. Study of the growth inhibitory activity of these compounds revealed that cross-resistance to the tetraalkyl ammonium salt increased with the lipophilicity and was equal to 58 for tetraoctylammonium salt, the most lipophilic compound of this series. This demonstrates that neither the presence of a positive charge nor an aromatic moiety is required for MDR recognition.

Reversal of multidrug resistance by verapamil analogues

The basic distinguishing feature of multidrug resistant (MDR) cells is a decrease in steady-state drug levels as compared to drug-sensitive controls. It is well-known that verapamil increases the sensitivity of MDR cells to drugs, thus reverting drug resistance. A limiting factor for its clinical use is the pronounced cardiovascular effects of the calcium channel antagonist which occur at the high plasma concentrations required to block P-glycoprotein transport efficiently. From a clinical point of view, it is important to find verapamil derivatives with low calcium channel blocking activity and high reverting activity. This was the aim of the present study. In this context we have investigated the ability of 20 verapamil analogues with restricted molecular flexibility to increase cellular accumulation of anticancer drugs and overcome resistance, and their inotropic, chronotropic, and slow calcium channel antagonistic activity. In this study an anthracycline derivative 4'-O-tetrahydropyranyl adriamycin, and an erythroleukaemia K562 cell line were used. Three of the 20 derivatives checked were completely devoid of calcium channel blocking activity while exhibiting MDR reverting ability comparable to that of verapamil. These derivatives could be useful for the treatment of MDR in cancer patients and for the design and development of other verapamil derivatives.

Direct evaluation of intracellular accumulation of free and polymer-bound anthracyclines

Nanoparticulate carriers of anthracyclines are being developed with the aim of improving the pharmacokinetic or pharmacodynamic behavior of these drugs. To understand how the drug reaches its nuclear targets, we have developed two methods that allow the quantification of the interaction between an anthracycline and cellular DNA: (1) by direct evaluation of the quenching of anthracycline fluorescence due to the intercalation of the drug into DNA and (2) by the measurement of Hoechst 33258 fluorescence associated with its displacement from DNA-binding sites for which it competes with the anthracycline. We show that the intracellular accumulation and DNA binding of doxorubicin encapsulated in polyisohexylcyanoacrylate nanospheres (dox-NS) and of daunorubicin bound to polyglutamic acid are reduced by 30%-40% in comparison with those obtained for free doxorubicin (dox) and daunorubicin, respectively. The results obtained with dox or NS-dox are not modified by prior incubation with either of these compounds. The two methods yielded similar results, and we conclude that either technique is applicable to the evaluation of the interaction of carrier-bound anthracyclines with cellular DNA.

P-glycoprotein-mediated efflux of hydroxyrubicin, a neutral anthracycline derivative, in resistant K562 cells

Hydroxyrubin (OH-Dox), a neutral doxorubicin derivative that is only slightly cross-resistant to doxorubicin (Dox), can be actively pumped out of resistant K562 cells by P-glycoprotein (P-gp). This efflux is saturable and can be inhibited by verapamil. The Michaelis constant is equal to 2 +/- 0.5 microM. However, the efficiency of P-gp in pumping out the drugs is 2.5 times less for OH-Dox than for Dox. This shows that in order to be pumped out by P-gp a molecule does not necessarily have to have a basic center. The mean influx coefficient for the drug is 5 times higher for OH-Dox than for Dox. In conclusion, the degree of resistance of analogs is related not only to their ability to be recognized and transported by P-gp but also, and probably essentially, to their kinetics of uptake. Both parameters have to be taken into account in the rational design of new compounds capable of overcoming multidrug resistance.

Mobile ionophores are a novel class of P-glycoprotein inhibitors. The effects of ionophores on 4'-O-tetrahydropyranyl-adriamycin incorporation in K562 drug-resistant cells

The decrease of the intracellular concentration of drug in resistant cells compared to sensitive cells is, in most cases, correlated with the presence, in the membrane of resistant cells, of a 170-kDa P-glycoprotein responsible for an active efflux of the drug. In an attempt to identify mechanism(s) by which multidrug resistance can be circumvented, we have examined the cellular accumulation of 4'-O-tetrahydropyranyl-adriamycin, alone and in conjunction with various ionophores on the one hand and with cyclosporin A on the other hand. The present study was performed using a spectrofluorometric method with which it is possible to follow continuously the uptake and release of fluorescent molecules by living cells, as the incubation of the cells with the drug proceeds. Erythroleukemia K562 cell lines were used. Using experimental conditions in which these ionophores were unable to modify either the intracellular pH, or the transmembrane potential, or to induce an intracellular ATP depletion, we have shown that mobile ionophores as well as cyclosporin inhibit the P-glycoprotein-mediated efflux of 4'-O-tetrahydropyranyl-adriamycin in K562 resistant cells, whereas gramicidin, a channel-forming ionophore, does not. The concentration that must be used to inhibit 50% of the efflux was 0.7 microM for valinomycin, 0.4 microM for nonactin, 0.2 microM for nigericin, 1.1 microM for monensin, 0.4 microM for lasalocid, 1.2 microM for calcimycin and 0.4 microM for cyclosporin. Due to the high toxicity of the ionophores, the observation that they increased 4'-O-tetrahydropyranyl-adriamycin accumulation in the multidrug-resistant cells is not correlated with an effect of these compounds on drug resistance. However, the correlation exists in the case of cyclosporin. From our data showing that lipophilic neutral complexes, formed between carboxylic ionophores and metal ions, are both able to inhibit the P-glycoprotein-mediated efflux of anthracycline we can infer that the lipophilicity but not the cationic charge is an important physical property.