Multidrug resistance in the laboratory and clinic

Natural Products as Alternative Choices for P-Glycoprotein (P-gp) Inhibition

Multidrug resistance (MDR) is regarded as one of the bottlenecks of successful clinical treatment for numerous chemotherapeutic agents. Multiple key regulators are alleged to be responsible for MDR and making the treatment regimens ineffective. In this review, we discuss MDR in relation to P-glycoprotein (P-gp) and its down-regulation by natural bioactive molecules. P-gp, a unique ATP-dependent membrane transport protein, is one of those key regulators which are present in the lining of the colon, endothelial cells of the blood brain barrier (BBB), bile duct, adrenal gland, kidney tubules, small intestine, pancreatic ducts and in many other tissues like heart, lungs, spleen, skeletal muscles, etc. Due to its diverse tissue distribution, P-gp is a novel protective barrier to stop the intake of xenobiotics into the human body. Over-expression of P-gp leads to decreased intracellular accretion of many chemotherapeutic agents thus assisting in the development of MDR. Eventually, the effectiveness of these drugs is decreased. P-gp inhibitors act by altering intracellular ATP levels which are the source of energy and/or by affecting membrane contours to increase permeability. However, the use of synthetic inhibitors is known to cause serious toxicities. For this reason, the search for more potent and less toxic P-gp inhibitors of natural origin is underway. The present review aims to recapitulate the research findings on bioactive constituents of natural origin with P-gp inhibition characteristics. Natural bioactive constituents with P-gp modulating effects offer great potential for semi-synthetic modification to produce new scaffolds which could serve as valuable investigative tools to recognize the function of complex ABC transporters apart from evading the systemic toxicities shown by synthetic counterparts. Despite the many published scientific findings encompassing P-gp inhibitors, however, this article stand alones because it provides a vivid picture to the readers pertaining to Pgp inhibitors obtained from natural sources coupled with their mode of action and structures. It provides first-hand information to the scientists working in the field of drug discovery to further synthesise and discover new P-gp inhibitors with less toxicity and more efficacies.

Nitrobenzoxadiazole-based GSTP1-1 inhibitors containing the full peptidyl moiety of (pseudo)glutathione

CONTEXT

The inhibition of glutathione S-transferase P1-1 (GSTP1-1) is a sound strategy to overcome drug resistance in oncology practice.

OBJECTIVE

The nitrobenzoxadiazolyl (NBD) S-conjugate of glutathione and the corresponding γ-oxa-glutamyl isostere (compounds 1 and 5, respectively) have been disclosed as GST inhibitors. The rationale of their design is discussed in juxtaposition to non-peptide NBD thioethers.

MATERIALS AND METHODS

Synthesis of derivatives 1 and 5 and in vitro evaluation on human GSTP1-1 and M2-2 are reported.

RESULTS

Conjugates 1 and 5 were found to be low micromolar inhibitors of both isoforms. Furthermore, they display a threefold reduction in selectivity for GSTM2-2 over the P1-1 isozyme in comparison with the potent non-peptide inhibitor nitrobenzoxadiazolyl-thiohexanol (NBDHEX).

DISCUSSION AND CONCLUSIONS

Spectroscopic data are congruent with the formation of a stable sigma-complex between GSH and the inhibitors in the protein active site. Conjugate 5 is suitable for in vivo modulation of GST activity in cancer treatment.

Facile Synthesis of PEGylated PLGA Nanoparticles Encapsulating Doxorubicin and its In Vitro Evaluation as Potent Drug Delivery Vehicle

The advent of nanotechnology has bolstered a variety of nanoparticles based platforms for different biomedical applications. A better understanding for engineering novel nanoparticles for applications in cancer staging and therapy requires careful assessment of the nanoparticle's physico-chemical properties. Herein we report a facile synthesis method for PEGylated PLGA nanoparticles encapsulating anti-cancer drug doxorubicin for cancer imaging and therapy. The simple nanoprecipitation method reported here resulted in very robust PEGylated PLGA nanoparticles with close to 95% drug encapsulation efficiency. The nanoparticles showed a size of ~110 nm as characterized by TEM and DLS. The nanoparticles were further characterized by optical UV-Visible and fluorescence spectroscopy. The encapsulated doxorubicin showed a sustained release (>80%) from the nanoparticles matrix over a period of 8 days. The drug delivery efficiency of the nanoparticles was confirmed in vitro confocal imaging with PC3 and HeLa cell lines. In vitro quantitative estimation of drug accumulation in PC3 cell line showed a 22 times higher concentration of drug in case of nanoparticles based formulation in comparison to free drug and this was further reflected in the in vitro cytotoxicity assays. Overall the synthesis method reported here provides a simple and robust PLGA based platform for efficient drug delivery and imaging of cancer cells in vitro and in vivo.

Inhibition of mdr1 by G-quadruplex oligonucleotides and reversal of paclitaxel resistance in human ovarian cancer cells

The expression of ATP-dependent efflux pump P-glycoprotein (P-gp) in cancer cells generally results in multidrug resistance (MDR) to chemotherapeutic drugs, which is the main cause of chemotherapy failure in cancer treatment. The intracellular drug levels could be increased by some MDR reversal agents that inhibited the drug efflux activity of P-gp. The synthesized DNA nucleic acids of G-quadruplex represent a novel and unique class of anti-cancer agents. While there was no report on the roles of DNA G-quadruplex oligonucleotides (GQ-ODNs) in the MDR reversal, the present study was performed to investigate the ability of synthesized GQ-ODNs to reverse P-gp-mediated MDR and its mechanism in paclitaxel (PTX)-resistant SKOV3 (SKOV3/PTX) cells and their sensitive cell lines SKOV3. The ability of GQ-ODNs to reverse drug resistance was evaluated by MTS assay. The results showed that GQ-ODNs can reverse PTX resistance effectively. The potential of GQ-ODNs as reversal agents was evaluated with the nude mice tumor xenograft model and showed that the co-administration of the GQ-ODNs and PTX had better effects and was also more evident than treatment with only PTX. The P-gp expression was assessed by the Western blot; it showed that SKOV3/PTX cells showed highly expressed P-gp protein, while their sensitive cells scarcely showed P-gp. The presence of GQ-ODNs efficiently decreased the P-gp expression, showing that GQ-ODNs could reverse P-gp-mediated MDR by decreasing the expression of P-gp. This study indicated that GQ-ODNs could effectively reverse P-gp-mediated PTX resistance by inhibiting the expression of P-gp and by the co-administration of GQ-ODNs and PTX that could increase the apoptosis of SKOV3/PTX cells. Thus, the synthesized GQ-ODNs may be a potential inhibitor to overcome drug resistance.

Evolutionary triage governs fitness in driver and passenger mutations and suggests targeting never mutations

Genetic and epigenetic changes in cancer cells are typically divided into “drivers” and “passengers”. Drug development strategies target driver mutations, but inter- and intra-tumoral heterogeneity usually results in emergence of resistance. Here we model intratumoral evolution in the context of a fecundity/survivorship trade-off. Simulations demonstrate the fitness value, of any genetic change is not fixed but dependent on evolutionary triage governed by initial cell properties, current selection forces, and prior genotypic/phenotypic trajectories. We demonstrate spatial variations in molecular properties of tumor cells are the result of changes in environmental selection forces such as blood flow. Simulated therapies targeting fitness-increasing (driver) mutations usually decrease the tumor burden but almost inevitably fail due to population heterogeneity. An alternative strategy targets gene mutations that are never observed. Because up or down regulation of these genes unconditionally reduces cellular fitness, they are eliminated by evolutionary triage but can be exploited for targeted therapy.

Polysaccharide-based nucleic acid nanoformulations

Therapeutic application of nucleic acids requires their encapsulation in nanosized carriers that enable safe and efficient intracellular delivery. Before the desired site of action is reached, drug-loaded nanoparticles (nanomedicines) encounter numerous extra- and intracellular barriers. Judicious nanocarrier design is highly needed to stimulate nucleic acid delivery across these barriers and maximize the therapeutic benefit. Natural polysaccharides are widely used for biomedical and pharmaceutical applications due to their inherent biocompatibility. At present, there is a growing interest in applying these biopolymers for the development of nanomedicines. This review highlights various polysaccharides and their derivatives, currently employed in the design of nucleic acid nanocarriers. In particular, recent progress made in polysaccharide-assisted nucleic acid delivery is summarized and the specific benefits that polysaccharides might offer to improve the delivery process are critically discussed.

In vivo multiphoton imaging of mitochondrial structure and function during acute kidney injury

Mitochondrial dysfunction has been implicated in the pathogenesis of acute kidney injury due to ischemia and toxic drugs. Methods for imaging mitochondrial function in cells using confocal microscopy are well established; more recently, it was shown that these techniques can be utilized in ex vivo kidney tissue using multiphoton microscopy. We extended this approach in vivo and found that kidney mitochondrial structure and function can be imaged in anesthetized rodents using multiphoton excitation of endogenous and exogenous fluorophores. Mitochondrial nicotinamide adenine dinucleotide increased markedly in rat kidneys in response to ischemia. Following intravenous injection, the mitochondrial membrane potential-dependent dye TMRM was taken up by proximal tubules; in response to ischemia, the membrane potential dissipated rapidly and mitochondria became shortened and fragmented in proximal tubules. In contrast, the mitochondrial membrane potential and structure were better maintained in distal tubules. Changes in mitochondrial structure, nicotinamide adenine dinucleotide, and membrane potential were found in the proximal, but not distal, tubules after gentamicin exposure. These changes were sporadic, highly variable among animals, and were preceded by changes in non-mitochondrial structures. Thus, real-time changes in mitochondrial structure and function can be imaged in rodent kidneys in vivo using multiphoton excitation of endogenous and exogenous fluorophores in response to ischemia-reperfusion injury or drug toxicity.

Inhibition of ABCB1 (MDR1) expression by an siRNA nanoparticulate delivery system to overcome drug resistance in osteosarcoma

Background

The use of neo-adjuvant chemotherapy in treating osteosarcoma has improved patients' average 5 year survival rate from 20% to 70% in the past 30 years. However, for patients who progress after chemotherapy, its effectiveness diminishes due to the emergence of multi-drug resistance (MDR) after prolonged therapy.

Methodology/Principal Findings

In order to overcome both the dose-limiting side effects of conventional chemotherapeutic agents and the therapeutic failure resulting from MDR, we designed and evaluated a novel drug delivery system for MDR1 siRNA delivery. Novel biocompatible, lipid-modified dextran-based polymeric nanoparticles were used as the platform for MDR1 siRNA delivery; and the efficacy of combination therapy with this system was evaluated. In this study, multi-drug resistant osteosarcoma cell lines (KHOS_R2 and U-2OS_R2) were treated with the MDR1 siRNA nanocarriers and MDR1 protein (P-gp) expression, drug retention, and immunofluoresence were analyzed. Combination therapy of the MDR1 siRNA loaded nanocarriers with increasing concentrations of doxorubicin was also analyzed. We observed that MDR1 siRNA loaded dextran nanoparticles efficiently suppresses P-gp expression in the drug resistant osteosarcoma cell lines. The results also demonstrated that this approach may be capable of reversing drug resistance by increasing the amount of drug accumulation in MDR cell lines.

Conclusions/Significance

Lipid-modified dextran-based polymeric nanoparticles are a promising platform for siRNA delivery. Nanocarriers loaded with MDR1 siRNA are a potential treatment strategy for reversing MDR in osteosarcoma.

Cyclohexyl-linked tricyclic isoxazoles are potent and selective modulators of the multidrug resistance protein (MRP1)

Structure-activity relationship (SAR) studies on the tricyclic isoxazole series of MRP1 modulators have resulted in the identification of potent and selective inhibitors containing cyclohexyl-based linkers. These studies ultimately identified compound 21b, which reverses drug resistance to MRP1 substrates, such as doxorubicin, in HeLa-T5 cells (EC(50)=0.093microM), while showing no inherent cytotoxicity. Additionally, 21b inhibits ATP-dependent, MRP1-mediated LTC(4) uptake into membrane vesicles prepared from the MRP1-overexpressing HeLa-T5 cells (EC(50)=0.064microM) and shows selectivity (1115-fold) against the related transporter, P-glycoprotein, in HL60/Adr and HL60/Vinc cells. Finally, when dosed in combination with the oncolytic MRP1 substrate vincristine, 21b showed tumor regression and growth delay in MRP1-overexpressing tumors in vivo.

Inhibition of ABCB1 (MDR1) and ABCB4 (MDR3) expression by small interfering RNA and reversal of paclitaxel resistance in human ovarian cancer cells

Ovarian cancer is currently the most lethal gynecologic malignancy in developed countries, and paclitaxel is a cornerstone in the treatment of this malignancy. Unfortunately, the efficacy of paclitaxel is limited by the development of drug resistance. Clinical paclitaxel resistance is often associated with ABCB1 (MDR1) overexpression, and in vitro paclitaxel resistance typically demonstrates overexpression of the ABCB1 gene. In this study, we demonstrate that paclitaxel-resistant cell lines overexpress both ABCB1 and ABCB4 (MDR3). To evaluate the role of these transporters in paclitaxel-resistant ovarian cancer cells, small interference RNAs (siRNAs) were used to target ABCB1 and ABCB4 RNA in the paclitaxel-resistant SKOV-3TR and OVCAR8TR ovarian cancer cell lines. Treatment of these lines with either chemically synthesized siRNAs or transfection with specific vectors that express targeted siRNAs demonstrated decreased mRNA and protein levels of ABCB1 or ABCB4. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays of siRNA-treated cells demonstrated 7- to 12.4-fold reduction of paclitaxel resistance in the lines treated with the synthesized siRNA of ABCB1 and 4.7- to 7.3-fold reduction of paclitaxel resistance in the cell lines transfected with siRNA of ABCB1 expressing vectors. ABCB4 siRNA-treated cell lines showed minor reduction in paclitaxel resistance. These results indicate that siRNA targeted to ABCB1 can sensitize paclitaxel-resistant ovarian cancer cells in vitro and suggest that siRNA treatment may represent a new approach for the treatment of ABCB1-mediated drug resistance.

Mechanism of cytosine arabinoside-mediated apoptosis: role of Rel A (p65) dephosphorylation

Nuclear transcription factor kappa B (NF-kappaB) has been shown both to block apoptosis and to promote cell proliferation, and hence has been considered an important target for anticancer drug development. The pyrimidine analogue cytosine arabinoside (araC) is among the most effective agents used in the treatment of acute leukemia, and we demonstrate in this study that its chemotherapeutic activity may be mediated by its inhibition of NF-kappaB. We found that in Jurkat cells, although tumor necrosis factor (TNF), araC, or ceramide induced NF-kappaB, the induction was only transient in the case of araC. In both HuT-78 and serum-activated LPS-stimulated Jurkat (SA-LPS/Jkt) cells that expressed NF-kappaB, TNF or ceramide treatments did not affect the NF-kappaB expression whereas araC downregulated it. AraC, but not TNF or ceramide was able to induce apoptosis in these cells as detected by assays for lipid peroxidation, reactive oxygen intermediates generation, caspase activation, cytotoxicity, Bcl-2 degradation, and DNA fragmentation. AraC also potentiated apoptosis mediated by cis-platin, etoposide, or taxol in these cells. AraC was able to induce protein phosphatases (PP) 2A and 2B-A, and phosphorylation of p65 subunit of NF-kappaB in the HuT-78 and SA-LPS/Jkt cells was downregulated by araC treatment. Furthermore, calyculin A, a specific phospho-serine/phospho-threonine phosphatase inhibitor, protected HuT-78 and SA-LPS/Jkt cells from araC-mediated NF-kappaB downregulation and apoptosis. These observations collectively suggest that araC induces apoptosis in NF-kappaB-expressing cells by dephosphorylating the p65 subunit of NF-kappaB.

Monitoring daunorubicin-induced alterations in protein expression in pancreas carcinoma cells by two-dimensional gel electrophoresis

Tumors of the pancreas are characterized by a high intrinsic potency to develop chemoresistance towards cytotoxic drugs, which is the main cause of ineffective treatment. The phenomenon of multidrug resistance is known to be a multifactorial event in which several mechanisms act simultaneously. We investigated the response of pancreas tumor cells after exposure to the anthracycline daunorubicin (DRC), a well-known antitumor agent in chemotherapy, by two-dimensional gel electrophoresis (2-DE). DRC is known to cause DNA damage and to affect tumor cell growth. Importantly, we aimed at investigating alterations in the protein expression pattern after first contact of the tumor cells with DRC, thus simulating a situation close to clinical chemotherapy and elucidating cell survival strategies following initial drug exposure. A concentration dependent up-regulation of a variety of proteins was observed, indicating that cell response to DRC involves multiple signaling events. Since the p53 tumor suppressor is essentially involved in the regulation of cell growth and controlled cell death (apoptosis) after cellular stress (like DNA damage), we investigated the role of p53 in DRC-resistant and -sensitive pancreas carcinoma cells by measuring p53 transcriptional transactivation activities. No differences in p53 activities were observed in response to DRC treatment in both pancreas cell lines, whereas mamma carcinoma cells (MCF-7), possessing wild-type p53, demonstrated the expected increase in p53 transcriptional transactivation activity. Hence, the tested pancreas carcinoma cells harbor a mutant, nonfunctional p53. We additionally analyzed the steady state protein levels of the cyclin dependent kinase inhibitor p21(CIP1), which is known to be involved in cell cycle control. Interestingly, p21(CIP1 )was induced by DRC in sensitive cells in a concentration dependent manner and was highest in resistant cells. In conclusion, our results suggest that the induction of proteins by DRC in pancreas carcinoma cells, as observed by 2-DE, occurs independently from p53 signaling events, but is probably associated with increased levels of p21(CIP1).

Tricyclic isoxazoles are novel inhibitors of the multidrug resistance protein (MRP1)

Tricyclic isoxazoles were identified from a screen as a novel class of selective multidrug resistance protein (MRP1) inhibitors. From a screen lead, SAR efforts resulted in the preparation of LY 402913 (9h), which inhibits MRP1 and reverses drug resistance to MRP1 substrates, such as doxorubicin, in HeLa-T5 cells (EC(50)=0.90 microM), while showing no inherent cytotoxicity. Additionally, LY 402913 inhibits ATP-dependent, MRP1-mediated LTC(4) uptake into membrane vesicles prepared from the MRP1-overexpressing HeLa-T5 cells (EC(50)=1.8 microM). LY 402913 also shows selectivity ( approximately 22-fold) against the related transporter, P-glycoprotein, in HL60/Adr and HL60/Vinc cells. Finally, when dosed in combination with the oncolytic MRP1 substrate vincristine, LY 402913 delays the growth of MRP1-overexpressing tumors in vivo.

Phase I and pharmacokinetic study of the novel MDR1 and MRP1 inhibitor biricodar administered alone and in combination with doxorubicin

PURPOSE

To evaluate the safety, tolerability, and pharmacokinetics of biricodar (VX-710), an inhibitor of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP1), alone and with doxorubicin in patients with advanced malignancies. The effect of VX-710 on the tissue distribution of (99m)Tc-sestamibi, a P-gp and MRP1 substrate, was also evaluated.

PATIENTS AND METHODS

Patients with solid malignancies refractory to standard therapy first received a 96-hour infusion of VX-710 alone at 20 to 160 mg/m(2)/h. After a 3-day washout, a second infusion of VX-710 was begun, on the second day of which doxorubicin 45 mg/m(2) was administered. Cycles were repeated every 21 to 28 days. (99m)Tc-sestamibi scans were performed before and during administration of VX-710 alone.

RESULTS

Of the 28 patients who enrolled, 25 patients were eligible for analysis. No dose-limiting toxicity (DLT) was observed in the nine assessable patients who received 120 mg/m(2)/h or less. Among seven patients receiving VX-710 160 mg/m(2)/h, two DLTs were seen: reversible CNS toxicity and febrile neutropenia. All other adverse events were mild to moderate and reversible. Plasma concentrations of VX-710 in patients who received at 120 and 160 mg/m(2)/h were two- to fourfold higher than concentrations required to fully reverse drug resistance in vitro. VX-710 exhibited linear pharmacokinetics with a harmonic mean half-life of 1.1 hours. VX-710 enhanced hepatic uptake and retention of (99m)Tc-sestamibi in all patients.

CONCLUSION

A 96-hour infusion of VX-710 at 120 mg/m(2)/h plus doxorubicin 45 mg/m(2) has acceptable toxicity in patients with refractory malignancies. The safety and pharmacokinetics of VX-710 plus doxorubicin warrant efficacy trials in malignancies expressing P-gp and/or MRP1.

Altered DNA-cleavage activity of topoisomerase II from WEHI-3B leukemia cells with specific resistance to ciprofloxacin

In order to investigate the mechanisms of drug resistance arising in tumor cells, we investigated the capacity of fluoroquinolones to inhibit the in vitro growth of WEHI-3B monomyelocytic leukemia cells and then we established a variant of this line (currently maintained in the absence of drug). The line, named WEHI-3B/CPX, expresses a specific resistance to ciprofloxacin (CPX; resistance index=17.3+/-2.2), and does not show cross-resistance with other fluoroquinolones, camptothecin and topoisomerase II inhibitors such as doxorubicin, etoposide and teniposide. Although a little decrease in intracellular accumulation of CPX is observed in WEHI-3B/CPX cells, these cells do not express MDR or LRP markers, and the resistance is not circumvented by verapamil. Purified nuclear extracts from WEHI-3B and WEHI-3B/CPX cells were tested for topoisomerase I catalytic activity and checking in vitro topoisomerase I sensitivity to CPX and camptothecin inhibition, but no difference was observed. As the treatment with CPX showed that the resistant cell line suffers a significantly lower number of breaks in the DNA molecule we also addressed our investigations to the topoisomerase II-dependent DNA cleavage that, in the resistant clone, was found dramatically less susceptible to be enhanced by CPX both in pre-strand and post-strand DNA passage conditions. WEHI-3B/CPX cells do not express any character of multidrug resistance and represent a rare case of specific drug resistance to CPX. The specific resistance to CPX observed in these cells is related to a functional decrease of topoisomerase II cleavage activity. It could be consequent to a decreased binding affinity of CPX for the topoisomerase II--DNA complex or to a decreased affinity or specificity of topoisomerase II for its DNA cleavage sites.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

Reversal of MRP-mediated multidrug resistance in human lung cancer cells by the antiprogestatin drug RU486

Multidrug resistance-associated protein (MRP) and P-glycoprotein (P-gp) are drug efflux pumps conferring multidrug resistance to tumor cells. RU486, an antiprogestatin drug known to inhibit P-gp function, was examined for its effect on MRP activity in MRP-overexpressing lung tumor GLC4/Sb30 cells. In such cells, the antihormone compound was found to increase intracellular accumulation of calcein, a fluorescent compound transported by MRP, in a dose-dependent manner, through inhibition of cellular export of the dye; in contrast, it did not alter calcein levels in parental GLC4 cells. RU486, when used at 10 microM, a concentration close to plasma concentrations achievable in humans, strongly enhanced the sensitivity of GLC4/Sb30 cells towards two known cytotoxic substrates of MRP, the anticancer drug vincristine and the heavy metal salt potassium antimonyl tartrate. Vincristine accumulation levels were moreover up-regulated in RU486-treated GLC4/Sb30 cells. In addition, such cells were demonstrated to display reduced cellular levels of glutathione which is required for MRP-mediated transport of some anticancer drugs. These findings therefore demonstrate that RU486 can down-modulate MRP-mediated drug resistance, in addition to that linked to P-gp, through inhibition of MRP function.

Inhibition of multidrug resistance-associated protein (MRP) activity by rifampicin in human multidrug-resistant lung tumor cells

The multidrug resistance-associated protein (MRP) is a drug efflux membrane pump conferring multidrug resistance on tumor cells. In order to look for compounds that can lead to reversal of such a resistance, the antituberculosis compound rifampicin, belonging to the chemical class of rifamycins, was examined for its effect on MRP activity in human multidrug resistant lung cancer GLC4/ADR cells. Rifampicin was shown to increase accumulation of the MRP substrate calcein in GLC4/ADR cells in a dose-dependent manner by inhibiting its MRP-mediated efflux from the cells; it also enhanced intracellular retention of another substrate of MRP such as the anticancer drug vincristine in the resistant cells. By contrast, the antituberculosis drug did not alter cellular levels of accumulation of either calcein or vincristine in parental drug-sensitive GLC4 cells. Other rifamycins such as rifamycin B and rifamycin SV were also demonstrated to increase intracellular accumulation of calcein in GLC4/ADR cells. These results therefore indicate that rifamycins, including rifampicin, probably constitute a new chemical class of modulators down-regulating MRP-mediated drug transport.

2-Deoxy-D-glucose preferentially kills multidrug-resistant human KB carcinoma cell lines by apoptosis

The aim of this study was to determine the mechanism of cell death associated with the preferential killing of multidrug-resistant (MDR) cells by the glycolytic inhibitor 2-deoxy-D-glucose (2DG) in a range of MDR human KB carcinoma cell lines selected in different drugs. The D10 values for KB-V1, KB-C1 and KB-A1 (selected in vinblastine, colchicine and doxorubicin respectively) were 1.74, 1.04 and 0.31 mM, respectively, compared with 4.60 mM for the parental cell line (KB-3-1). The mechanism of cell death was identified as apoptosis, based on nuclear morphology, annexin V binding and poly(ADP-ribose) polymerase (PARP) cleavage. 2DG induced apoptosis in the three MDR cell lines in a dose- and time-dependent manner and did not induce necrosis. PARP cleavage was detected in KB-C1 cells within 2 h of exposure to 50 mM 2DG and slightly later in KB-A1 and KB-V1 cells. The relative levels of 2DG sensitivity did not correlate with the levels of multidrug resistance or with the reduced levels of the glucose transporter GLUT-1 in these cells. We speculate that a 2DG-stimulated apoptotic pathway in MDR KB cells differs from that in normal KB cells.

Constitutive activation of NF-kappaB causes resistance to apoptosis in human cutaneous T cell lymphoma HuT-78 cells. Autocrine role of tumor necrosis factor and reactive oxygen intermediates

How tumor cells develop resistance to apoptosis induced by cytokines and chemotherapeutic agents is incompletely understood. In the present report, we investigated apoptosis induction by tumor necrosis factor (TNF) in two human T cell lines, Jurkat and HuT-78. While TNF inhibited the growth of Jurkat cells and activated caspase-3, it had no effect on HuT-78 cells. It was further found that HuT-78 cells constitutively expressed the nuclear transcription factor NF-kappaB. TNF activated NF-kappaB in Jurkat cells but not in HuT-78 cells. HuT-78 cells were also resistant to NF-kappaB activation induced by phorbol ester, H2O2, ceramide, endotoxin, and interleukin-1. Despite the presence of preactivated NF-kappaB, HuT-78 cells also expressed high levels of IkappaB-alpha, the inhibitory subunit of NF-kappaB and, unlike Jurkat cells, were resistant to TNF-induced degradation of IkappaB-alpha. Its half-life in HuT-78 cells was 12 h as opposed to 45 min in Jurkat cells. Antibodies against TNF blocked the constitutive activation of NF-kappaB and proliferation of HuT-78 cells but had no significant effect on Jurkat cells, suggesting an autocrine role for TNF. The antioxidant pyrrolidine dithiocarbamate also suppressed constitutive NF-kappaB activation and it reversed the cell's sensitivity to TNF-induced cytotoxicity and activation of caspase-3. Overall, these results suggest that constitutive activation of NF-kappaB, TNF, and prooxidant pathway in certain T cell lymphomas causes resistance to apoptosis, and this can be reversed by antioxidants.

Induction of multidrug resistance gene expression in rat liver cells in response to acute treatment by the DNA-damaging agent methyl methanesulfonate

Expression of multidrug resistance (mdr) genes encoding the P-glycoprotein (P-gp) drug efflux pump was analysed in cultured rat liver epithelial cells acutely treated by the DNA-damaging agent methyl methanesulfonate (MMS). Exposure to this alkylating agent used at 30 microg/ml for 12 or 24 h was shown to enhance mdr mRNA levels in rat liver cells without alteration of cell viability. Induction of mdr transcripts occurred through increased expression of the mdr1b gene as indicated by reverse transcriptase-polymerase chain reaction analysis using rat mdr gene-specific primers and was not associated with up-regulation of cytochrome P-450 1A1, thereby suggesting that this detoxifying enzyme and P-gp were not coordinately regulated by MMS. In addition, the DNA-damaging agent was found to enhance in a dose-dependent manner cellular efflux of the P-gp substrate rhodamine 123, which was inhibited by the P-gp inhibitor verapamil, thus providing evidence that exposure to MMS led to increased P-gp-related drug transport in rat liver cells. The up-regulation of functional P-gp expression occurring in MMS-treated liver cells may be interpreted as a part of the cellular response to DNA damage.

Results of a phase III, double-blind, placebo-controlled trial of megestrol acetate modulation of P-glycoprotein-mediated drug resistance in the first-line management of small-cell lung carcinoma

The objective of this study was to determine if the addition of megestrol acetate (MA), a modulator of P-glycoprotein-mediated drug resistance, to first-line cytotoxic therapy in patients with limited and advanced stage small-cell lung cancer (SCLC) would improve median time to disease progression and median overall survival. Secondary outcomes evaluated were response rates and patient symptom profile. Between 1992 and 1995, 130 eligible patients were randomized in a double-blind fashion to receive standard first-line therapy consisting of alternating courses of cyclophosphamide/doxorubicin/vincristine and etoposide/cisplatin (and thoracic radiotherapy for limited stage patients), along with either placebo or MA 160 mg t.i.d. for 8 days commencing 3 days before initiation of each cycle of chemotherapy. Treatment was continued for a maximum of six cycles. A total of 130 eligible patients were randomized, 65 to each arm. Fifty-two per cent of patients had limited disease and 48% had advanced disease. The median time to disease progression in limited stage disease was 46 weeks in the placebo arm and 43 weeks in the MA arm (P = 0.71) and in advanced stage disease was 28 weeks in the placebo arm and 27 weeks in the MA arm (P = 0.92). The median overall survival in limited stage disease was 75 weeks in the placebo arm and 75 weeks in the MA arm (P = 0.56) and in advanced stage disease was 41 weeks in the placebo arm and 39 weeks in the MA arm (P = 0.96). There was no consistent statistical difference in response rates or patient symptom profiles between the two treatment arms. The addition of MA, in the dose and schedule used, to standard first-line cytotoxic therapy in SCLC did not result in a significant improvement in response rates, symptom profile, median time to disease progression or overall survival.

In vivo model systems in P-glycoprotein-mediated multidrug resistance

In this article we review the in vivo model systems that have been developed for studying P-glycoprotein-mediated multidrug resistance (MDR) in the preclinical setting. Rodents have two mdr genes, both of which confer the MDR phenotype: mdr 1a and mdr 1b. At gene level they show strong homology to the human MDR1 gene and the tissue distribution of their gene product is very similar to P-glycoprotein expression in humans. In vivo studies have shown the physiological roles of P-glycoprotein, including protection of the organism from damage by xenobiotics. Tumors with intrinsic P-glycoprotein expression, induced MDR or transfected with an mdr gene, can be used as syngeneic or xenogenic tumor models. Ascites, leukemia, and solid MDR tumor models have been developed. Molecular engineering has resulted in transgenic mice that express the human MDR1 gene in their bone marrow and in knockout mice missing a murine mdr gene. The data on pharmacokinetics, efficacy, and toxicity of chemosensitizers of P-glycoprotein in vivo are described. Results from studies using monoclonal antibodies directed against P-glycoprotein and other miscellaneous approaches for modulation of MDR are mentioned. The importance of in vivo studies prior to clinical trials is being stressed and potential pitfalls due to differences between species are discussed.

Effects of multidrug resistance-related ATP-binding-cassette transporter proteins on the cytoskeletal activity of cytochalasins

Cytochalasins are microfilament-active mould metabolites, widely utilized to study the involvement of the actin cytoskeleton in cellular processes as well as in genotoxicity and cell kinetic research. In this study we have investigated whether multidrug-resistance phenotypes, caused by overexpression of the ATP-binding-cassette transporter proteins P-glycoprotein (P-gp) or multidrug-resistance-associated protein (MRP), influence the microfilament-depolymerizing effect of cytochalasins. Using four well-characterized multidrug-resistance cell models, we have shown that both the microfilament-disrupting (phalloidine staining) and the cytotoxic (MTT-assay) activity of cytochalasins are reduced in parallel with increased P-gp expression and restorable by P-gp-modulating agents. This also applied to the cytochalasin D-mediated induction of polykaryons (microscopic evaluation) which arise as a consequence of impaired cytokinesis but unaffected karyokinesis. The reduced cellular activity of cytochalasins in P-gp-positive cell lines was correlated with decreased intracellular accumulation ([3H]cytochalasin B accumulation) which was also restorable by P-gp modulators. Moreover, the dose-dependent inhibition of P-gp photoaffinity labeling ([3H]-azidopine) suggested cytochalasins as P-gp-binding agents. In contrast, MRP overexpression had no effect on either cytochalasin microfilament activity or cytotoxicity. In conclusion, data indicate that the microfilament-destructive effects of cytochalasins are impaired due to a reduction of the intracellular cytochalasin accumulation by P-gp but not by MRP. Results are discussed with regard to P-gp as a resistance factor when cytochalasins are utilized to study microfilament dynamics, cell cycle kinetics or chromosomal damage. Moreover, the polykaryon-inducing activity of cytochalasin D is suggested as a specific indicator for a P-gp-mediated multidrug-resistance phenotype and the reversing potency of chemosensitizers.

Expression of the multidrug resistance-associated protein (MRP) and chemoresistance of human non-small-cell lung cancer cells

Human non-small-cell lung cancer (NSCLC) is considered to be a chemotherapy-refractory malignancy. The underlying mechanisms remain rather obscure. The multidrug resistance-associated protein (MRP), mediating a multidrug resistance (MDR) phenotype, has been reported to be overexpressed in several drug-selected lung cancer cell lines. A few previous studies have described intrinsic MRP expression in both NSCLC and normal lung tissues. However, the drug-transporting activity as well as the correlation with chemoresistance is unclear. Using 15 unselected cell lines, we show that MRP (mRNA and protein as detected by reverse transcriptase polymerase chain reaction and immunoblot) is frequently expressed intrinsically, with markedly varying intensity, in NSCLC. Two cell lines expressed high MRP levels, one comparable to the drug-selected controls (GLC4/ADR, HL-60/AR) without, however, amplification of the MRP gene (Southern hybridization). Using 3H-daunomycin (3H-DM) and calcein as MRP substrates and probenecid (PRO), genistein (GEN), benzbromarone (BB), N-ethylmaleimide (NEM) and verapamil (VP) as MRP modulators, drug accumulation studies revealed a transporting activity of MRP that correlated significantly with the gene expression data. Moreover, a significant correlation between MRP expression and chemoresistance against daunomycin (DM), doxorubicin (DOX), etoposide (VP-16) and vinblastine (VBL), but not cisplatin (CDDP) and bleomycin (Bleo) (MTT-based survival assay), was detected. Correlations mainly rested on the pronounced chemoresistance of 2 highly MRP-expressing cell lines and did not reach significance when these cell lines were excluded.

Up-regulation of P-glycoprotein expression in rat liver cells by acute doxorubicin treatment

Expression of P-glycoprotein, a plasma-membrane glycoprotein involved in multidrug resistance and encoded by mdr genes, was investigated in cultured rat liver cells acutely exposed to doxorubicin. This anticancer drug was shown to increase mdr mRNA levels in a dose-dependent manner in both rat liver epithelial (RLE) cells and primary rat hepatocytes. This induction of mdr transcripts was detected as early as a 4-h exposure to doxorubicin used at 0.5 microg/ml. It occurred through increased expression of the mdr1 gene as assessed by northern blot analysis using rat mdr-gene-specific probes. In addition, RLE cells exposed to doxorubicin displayed an overexpression of a 140-kDa P-glycoprotein as demonstrated by western blotting. Moreover, doxorubicin-treated RLE cells displayed enhanced cellular efflux of the P-glycoprotein substrate rhodamine 123 that was inhibited by the P-glycoprotein blocker verapamil, thus providing evidence that doxorubicin-induced P-glycoprotein was functional in liver cells. Doxorubicin-mediated mdr mRNA induction was found to be fully inhibited by actinomycin D, thus indicating its dependence on RNA synthesis; it was demonstrated to be not associated with alteration of protein synthesis, suggesting it differed from the known mdr mRNA overexpression occurring in response to cycloheximide. In contrast to P-glycoprotein, other liver detoxification pathways such as cytochromes P-450 1A were not induced by doxorubicin treatment. These data indicate that doxorubicin can act as a potent acute inducer of functional P-glycoprotein in rat liver cells and therefore may modulate both chemosensitivity of hepatic cells and P-glycoprotein-mediated biliary secretion of xenobiotics.

Possible role of the multidrug resistance-associated protein (MRP) in chemoresistance of human melanoma cells

Human malignant melanoma is characterised by unresponsiveness to conventional chemotherapy. Melanoma-derived cell lines are often markedly chemoresistant, suggesting that cellular mechanisms mediate the multidrug resistance (MDR) phenotype. The multidrug resistance-associated protein (MRP) is a drug transporter protein associated with resistance to a broad spectrum of lipophilic drugs. To investigate whether MRP is involved in intrinsic drug resistance of human melanoma, we analysed expression and functional activity of MRP as well as its impact on chemoresistance in 40 melanoma cell lines (35 established by us from primary and metastatic lesions and 5 obtained from international sources), as well as in one dysplastic naevus-derived cell line and in normal melanocytes. By reverse transcriptase-polymerase chain reaction various levels of MRP mRNA were detected in all melanoma cell lines, and by immunoblot the corresponding protein in a high percentage of them. Functional activity of MRP was assayed by analysing cellular accumulation of 3H-daunomycin (3H-DM) and calcein in response to MRP-modulators by beta-spectrometric and fluorescence-activated cell sorter analysis, respectively. Probenecid (PRO), N-ethylmaleimide (NEM) and benzbromarone (BB) moderately (< or = 1.43-fold) but significantly enhanced intracellular accumulation of MRP substrate probes corresponding to MRP expression. Moreover, the sensitivity of melanoma cell lines to daunomycin (DM) and doxorubicin (DOX), but not to vinblastine (VBL), etoposide (VP-16) and cisplatin (CDDP), analysed by an MTT-based survival assay, were inversely correlated with MRP-gene expression. Our results imply that MRP may be a component of the intrinsic chemoresistance phenotype characteristic of human malignant melanoma.

The P-glycoprotein multidrug transporter

1. P-glycoprotein (P-gp) is a transmembrane protein involved in ATP-dependent efflux of various structurally unrelated anticancer drugs. Its overexpression in cancer cells decreases intracellular drug concentrations and, thus, confers a multidrug resistance phenotype. 2. P-gp is encoded by MDR genes, which constitute a small gene family comprising two genes in humans and three genes in rodents. Only the MDR1 gene in humans and mdr1 and mdr3 genes in rodents have been demonstrated to be involved in drug resistance. 3. P-gp encoded by the human MDR1 gene is a phosphorylated and glycosylated protein 1289 amino acids long, and consists of 2 halves that share a high degree of similarity. 4. A wide variety of cancers have been shown to express P-gp, including solid tumors and hematological malignancies. This P-gp positivity can be evidenced at the time of diagnosis prior to chemotherapy or at relapse after treatment, and has been correlated with treatment failure and poor prognosis in several types of cancer. In addition, P-gp is also expressed by some normal tissues, such as liver and kidney. 5. P-gp expression is regulated by various factors, including xenobiotics and hormones. 6. P-gp-mediated multidrug resistance can be reversed by various unrelated compounds called chemosensitizers or reversing agents. These drugs act through inhibition of P-gp function and have entered clinical trials.

Comparison of drug accumulation in P-glycoprotein-expressing and MRP-expressing human leukaemia cells

P-glycoprotein- and multidrug resistance-associated protein (MRP)-mediated multidrug resistance is associated with decreased drug accumulation. The P-glycoprotein-expressing CCRF-CEM/VLB100 subline and the MRP-expressing CCRF-CEM/E1000 subline are both 50-fold resistant to daunorubicin. However, accumulation of daunorubicin and rhodamine 123 was > 85% reduced in the P-glycoprotein-expressing subline compared to 40-50% in the MRP-expressing subline. Further, the CCRF-CEM/E1000 cells were 30-fold resistant to idarubicin, without reduced accumulation. Verapamil and SDZ PSC 833 restored daunorubicin and rhodamine 123 accumulation, while buthionine sulphoximine affected only the CCRF-CEM/ E1000 subline. We conclude that the verapamil associated change in rhodamine 123 accumulation provides a sensitive functional assay for both P-glycoprotein- and MRP-mediated MDR.

Modulation of multidrug resistance with dexniguldipine hydrochloride (B8509-035) in the CC531 rat colon carcinoma model

The chemosensitizing potency of dexniguldipine hydrochloride (B8509-035) on epidoxorubicin was assessed in a multidrug-resistant (MDR) tumour model, the intrinsic MDR rat colon carcinoma CC531. In vitro in the sulphorhodamine B cell-viability assay the cytotoxicity of epidoxorubicin was increased approximately 15-fold by co-incubation with 50 ng/ml dexniguldipine. In vivo concentrations of dexniguldipine 5 h after a single oral dose of 30 mg/kg were 72 (+/- 19 SD) ng/ml in plasma and 925 (+/- 495 SD) ng/g in tumour tissue. Levels of the metabolite of dexniguldipine, M-1, which has the same chemosensitizing potential, were 26 (+/- 6 SD) ng/ml and 289 (+/- 127 SD) ng/g respectively. The efficacy of treatment with 6 mg/kg epidoxorubicin applied intravenously combined with 30 mg kg-1 day-1 dexniguldipine administered orally for 3 days prior to epidoxorubicin injection was evaluated on tumours grown under the renal capsule. Dexniguldipine alone did not show antitumour effects in vivo. Dexniguldipine modestly, but consistently, potentiated the tumour-growth-inhibiting effect of epidoxorubicin, reaching statistical significance in two out of four experiments. In conclusion, these experiments show that dexniguldipine has potency as an MDR reverter in vitro and in vivo in this solid MDR tumour model.