Multidrug resistance protein-mediated transport of chlorambucil and melphalan conjugated to glutathione

The Nitrogen Mustards

The nitrogen mustards are powerful cytotoxic and lymphoablative agents and have been used for more than 60 years. They are employed in the treatment of cancers, sarcomas, and hematologic malignancies. Cyclophosphamide, the most versatile of the nitrogen mustards, also has a place in stem cell transplantation and the therapy of autoimmune diseases. Adverse effects caused by the nitrogen mustards on the central nervous system, kidney, heart, bladder, and gonads remain important issues. Advances in analytical techniques have facilitated the investigation of the pharmacokinetics of the nitrogen mustards, especially the oxazaphosphorines, which are prodrugs requiring metabolic activation. Enzymes involved in the metabolism of cyclophosphamide and ifosfamide are very polymorphic, but a greater understanding of the pharmacogenomic influences on their activity has not yet translated into a personalized medicine approach. In addition to damaging DNA, the nitrogen mustards can act through other mechanisms, such as antiangiogenesis and immunomodulation. The immunomodulatory properties of cyclophosphamide are an area of current exploration. In particular, cyclophosphamide decreases the number and activity of regulatory T cells, and the interaction between cyclophosphamide and the intestinal microbiome is now recognized as an important factor. New derivatives of the nitrogen mustards continue to be assessed. Oxazaphosphorine analogs have been synthesized in attempts to both improve efficacy and reduce toxicity, with varying degrees of success. Combinations of the nitrogen mustards with monoclonal antibodies and small-molecule targeted agents are being evaluated. SIGNIFICANCE STATEMENT: The nitrogen mustards are important, well-established therapeutic agents that are used to treat a variety of diseases. Their role is continuing to evolve.

Direct Detection of Glutathione Biosynthesis, Conjugation, Depletion and Recovery in Intact Hepatoma Cells

Nuclear magnetic resonance (NMR) spectroscopy was used to monitor glutathione metabolism in alginate-encapsulated JM-1 hepatoma cells perfused with growth media containing [3,3′-13C2]-cystine. After 20 h of perfusion with labeled medium, the 13C NMR spectrum is dominated by the signal from the 13C-labeled glutathione. Once 13C-labeled, the high intensity of the glutathione resonance allows the acquisition of subsequent spectra in 1.2 min intervals. At this temporal resolution, the detailed kinetics of glutathione metabolism can be monitored as the thiol alkylating agent monobromobimane (mBBr) is added to the perfusate. The addition of a bolus dose of mBBr results in rapid diminution of the resonance for 13C-labeled glutathione due to a loss of this metabolite through alkylation by mBBr. As the glutathione resonance decreases, a new resonance due to the production of intracellular glutathione-bimane conjugate is detectable. After clearance of the mBBr dose from the cells, intracellular glutathione repletion is then observed by a restoration of the 13C-glutathione signal along with wash-out of the conjugate. These data demonstrate that standard NMR techniques can directly monitor intracellular processes such as glutathione depletion with a time resolution of approximately < 2 min.

Targeting multidrug resistance-associated protein 1 (MRP1)-expressing cancers: Beyond pharmacological inhibition

Resistance to chemotherapy remains one of the most significant obstacles to successful cancer treatment. While inhibiting drug efflux mediated by ATP-binding cassette (ABC) transporters is a seemingly attractive and logical approach to combat multidrug resistance (MDR), small molecule inhibition of ABC transporters has so far failed to confer clinical benefit, despite considerable efforts by medicinal chemists, biologists, and clinicians. The long-sought treatment to eradicate cancers displaying ABC transporter overexpression may therefore lie within alternative targeting strategies. When aberrantly expressed, the ABC transporter multidrug resistance-associated protein 1 (MRP1, ABCC1) confers MDR, but can also shift cellular redox balance, leaving the cell vulnerable to select agents. Here, we explore the physiological roles of MRP1, the rational for targeting this transporter in cancer, the development of small molecule MRP1 inhibitors, and the most recent developments in alternative therapeutic approaches for targeting cancers with MRP1 overexpression. We discuss approaches that extend beyond simple MRP1 inhibition by exploiting the collateral sensitivity to glutathione depletion and ferroptosis, the rationale for targeting the shared transcriptional regulators of both MRP1 and glutathione biosynthesis, advances in gene silencing, and new molecules that modulate transporter activity to the detriment of the cancer cell. These strategies illustrate promising new approaches to address multidrug resistant disease that extend beyond the simple reversal of MDR and offer exciting routes for further research.

The role of photodynamic therapy on multidrug resistant breast cancer

Breast cancer heterogeneity allows cells with different phenotypes to co-exist, contributing to treatment failure and development of drug resistance. In addition, abnormal signal transduction and dysfunctional DNA repair genes are common features with breast cancer resistance. Chemo-resistance of breast cancer associated with multidrug resistance events utilizes ATP-binding cassette (ABC) efflux transporters to decrease drug intracellular concentration. Photodynamic therapy (PDT) is the treatment that involves a combination of a photosensitizer (PS), light and molecular oxygen to induce cell death. This treatment modality has been considered as a possible approach in combatting multidrug resistance phenomenon although its therapeutic potential towards chemo-resistance is still unclear. Attempts to minimize the impact of efflux transporters on drug resistance suggested concurrent use of chemotherapy agents, nanotechnology, endolysosomal release of drug by photochemical internalization and the use of structurally related compound inhibitors to block the transport function of the multidrug resistant transporters. In this review, we briefly summarize the role of membrane ABC efflux transporters in therapeutic outcomes and highlight research findings related to PDT and its applications on breast cancer with multidrug resistance phenotype. With the development of an ideal PS for photodynamic cancer treatment, it is possible that light activation may be used not only to sensitize the tumour but also to enable release of PS into the cytosol and as such bypass efflux membrane proteins and inhibit escape pathways that may lead to resistance.

Genetic disruption of multidrug resistance-associated protein 1 improves endothelial function and attenuates atherosclerosis in MRP1-/- LDLr-/- double knockout mice

INTRODUCTION

Multidrug resistance-associated protein 1 (MRP1) is an anion transporter which is implicated in the efflux of the intracellular antioxidant anion glutathione as well as leukotrienes. Pharmacological inhibition of MRP1 exhibits antioxidative and anti-atherosclerotic effects both in vitro and in vivo. However, pharmacological inhibitors of MRP1 lack selectivity, which prompted us to study the in vivo impact of a genetic disruption of MRP1 on endothelial dysfunction, reactive oxygen species formation and atherogenesis in an atherosclerotic mouse model.

MATERIAL AND METHODS

MRP1^-/- LDLr^-/- double knockout mice. were fed a high-fat and cholesterol-rich diet for 7 weeks. Thereafter, endothelial function was assessed in isolated aortic rings. Reactive oxygen species were quantified by L-012 chemiluminescence, and the atherosclerotic plaque burden was measured following oil red O staining.

RESULTS

Endothelium-dependent vasodilation of MRP1^-/- LDLr^-/- double knockout mice was significantly improved compared to MRP1-competent LDLr^-/- single knockout mice (0.56 ±0.06 vs. 0.78 ±0.08; n = 10; p = 0.048). This improvement was accompanied by a significant reduction in reactive oxygen species formation within the aortic tissue (102 ±27 RLU/s/mg vs. 315 ±78 RLU/s/mg, n = 9-11, p = 0.03). Moreover, the atherosclerotic plaque burden of MRP1^-/- LDLr^-/- double knockout mice was significantly reduced (0.06 ±0.01 vs. 0.12 ±0.02; n = 6; p = 0.047). Finally, arterial blood pressure was significantly reduced in MRP1^-/- LDLr^-/- double knockout mice (93 ±5 mm Hg vs. 128 ±4 mm Hg; n = 8-12; p < 0.001).

CONCLUSIONS

Genetic disruption of MRP1 appears to reduce blood pressure and vascular oxidative stress in vivo, which leads to improved endothelial function and a reduced plaque burden in atherosclerotic mice. Therefore, MRP1 might represent a promising therapeutic target to improve endothelial function in patients suffering from atherosclerosis.

Pyrrolopyrimidine derivatives and purine analogs as novel activators of Multidrug Resistance-associated Protein 1 (MRP1, ABCC1)

Multidrug resistance (MDR) is the main cause of diminished success in cancer chemotherapy. ABC transport proteins are considered to be one important factor of MDR. Besides P-glycoprotein (P-gp, ABCB1) and Breast Cancer Resistance Protein (BCRP, ABCG2), Multidrug Resistance-associated Protein 1 (MRP1, ABCC1) is associated with non-response to chemotherapy in different cancers. While considerable effort was spent in overcoming MDR during the last two decades, almost nothing is known with respect to activators of transport proteins. In this work we present certain pyrrolo[3,2-d]pyrimidine derivatives with variations at positions 4 and 5 and purine analogs with variations at position 6 as novel activators of MRP1-mediated transport of the MRP1 substrate calcein AM and the anticancer drug daunorubicin in low nanomolar concentration range. Two different MRP1 overexpressing cell lines were used, the doxorubicin-selected human lung cancer cell line H69 AR and the transfected Madin-Darby Canine Kidney cell line MDCK II MRP1. No effect was observed in the sensitive counterparts H69 and MDCK II wild type (wt). Derivatives with higher molecular weight possessed also inhibitory properties at low micromolar concentrations, although most compounds were rather poor MRP1 inhibitors. Purine analogs derived from potent MRP1 inhibitors of the pyrrolopyrimidine class showed equal activating, but no inhibiting effects at all. All tested compounds were non-toxic and had only minor impact on P-gp or BCRP, showing no inhibition or activation.

Comparison of the Lonidamine Potentiated Effect of Nitrogen Mustard Alkylating Agents on the Systemic Treatment of DB-1 Human Melanoma Xenografts in Mice

Previous NMR studies demonstrated that lonidamine (LND) selectively diminishes the intracellular pH (pHi) of DB-1 melanoma and mouse xenografts of a variety of other prevalent human cancers while decreasing their bioenergetic status (tumor βNTP/Pi ratio) and enhancing the activities of melphalan and doxorubicin in these cancer models. Since melphalan and doxorubicin are highly toxic agents, we have examined three other nitrogen (N)-mustards, chlorambucil, cyclophosphamide and bendamustine, to determine if they exhibit similar potentiation by LND. As single agents LND, melphalan and these N-mustards exhibited the following activities in DB-1 melanoma xenografts; LND: 100% tumor surviving fraction (SF); chlorambucil: 100% SF; cyclophosphamide: 100% SF; bendamustine: 79% SF; melphalan: 41% SF. When combined with LND administered 40 min prior to administration of the N-mustard (to maximize intracellular acidification) the following responses were obtained; chlorambucil: 62% SF; cyclophosphamide: 42% SF; bendamustine: 36% SF; melphalan: 10% SF. The effect of LND on the activities of these N-mustards is generally attributed to acid stabilization of the aziridinium active intermediate, acid inhibition of glutathione-S-transferase, which acts as a scavenger of aziridinium, and acid inhibition of DNA repair by O⁶-alkyltransferase. Depletion of ATP by LND may also decrease multidrug resistance and increase tumor response. At similar maximum tolerated doses, our data indicate that melphalan is the most effective N-mustard in combination with LND when treating DB-1 melanoma in mice, but the choice of N-mustard for coadministration with LND will also depend on the relative toxicities of these agents, and remains to be determined.

Pyrrolopyrimidine Derivatives as Novel Inhibitors of Multidrug Resistance-Associated Protein 1 (MRP1, ABCC1)

Five series of pyrrolo[3,2-d]pyrimidines were synthesized and evaluated with respect to potency and selectivity toward multidrug resistance-associated protein 1 (MRP1, ABCC1). This transport protein is a major target to overcome multidrug resistance in cancer patients. We investigated differently substituted pyrrolopyrimidines using the doxorubicin selected and MRP1 overexpressing small cell lung cancer cell line H69 AR in a calcein AM and daunorubicin cell accumulation assay. New compounds with high potency and selectivity were identified. Piperazine residues at position 4 bearing large phenylalkyl side chains proved to be beneficial for MRP1 inhibition. Its replacement by an amino group led to decreased activity. Aliphatic and aliphatic-aromatic variations at position 5 and 6 revealed compounds with IC50 values in high nanomolar range. All investigated compounds had low affinity toward P-glycoprotein (P-gp, ABCB1). Pyrrolopyrimidines with small substituents showed moderate inhibition against breast cancer resistance protein (BCRP, ABCG2).

Nrf2 Regulates the Sensitivity of Mouse Keratinocytes to Nitrogen Mustard via Multidrug Resistance-Associated Protein 1 (Mrp1)

Sulfur mustard and nitrogen mustard (mechlorethamine, HN2) are potent vesicants developed as chemical warfare agents. These electrophilic, bifunctional alkylating agents cause skin injury, including inflammation, edema, and blistering. HN2 covalently modifies macromolecules such as DNA, RNA, and proteins or is scavenged by glutathione, forming adducts that can contribute to toxicity. Multidrug resistance-associated protein 1 (Mrp1/MRP1) is a transmembrane ATPase known to efflux glutathione-conjugated electrophiles. In the present studies, we examined the effects of modulating Mrp1-mediated transport activity on the sensitivity of primary and PAM212 mouse keratinocytes to HN2. Primary keratinocytes, and to a lesser extent, PAM212 cells, express Mrp1 mRNA and protein and possess Mrp1 functional activity, as measured by calcein efflux. Sulforaphane, an activator of Nrf2, increased Mrp1 mRNA, protein, and functional activity in primary keratinocytes and PAM212 cells and decreased their sensitivity to HN2-induced growth inhibition (IC(50) = 1.4 and 4.8 µM in primary keratinocytes and 1 and 13 µM in PAM212 cells, in the absence and presence of sulforaphane, respectively). The Mrp1 inhibitor, MK-571, reversed the effects of sulforaphane on HN2-induced growth inhibition in both primary keratinocytes and PAM212 cells. In primary keratinocytes from Nrf2(-/-) mice, sulforaphane had no impact on Mrp1 expression or activity, or on sensitivity to HN2, demonstrating that its effects depend on Nrf2. These data suggest that Mrp1-mediated efflux is important in regulating HN2-induced keratinocyte growth inhibition. Enhancing HN2 efflux from keratinocytes may represent a novel strategy for mitigating vesicant-induced cytotoxicity.

The role of ABC transporters in ovarian cancer progression and chemoresistance

Over 80% of ovarian cancer patients develop chemoresistance which results in a lethal course of the disease. A well-established cause of chemoresistance involves the family of ATP-binding cassette transporters, or ABC transporters that transport a wide range of substrates including metabolic products, nutrients, lipids, and drugs across extra- and intra-cellular membranes. Expressions of various ABC transporters, shown to reduce the intracellular accumulation of chemotherapy drugs, are increased following chemotherapy and impact on ovarian cancer survival. Although clinical trials to date using ABC transporter inhibitors have been disappointing, ABC transporter inhibition remains an attractive potential adjuvant to chemotherapy. A greater understanding of their physiological functions and role in ovarian cancer chemoresistance will be important for the development of more effective targeted therapies. This article will review the role of the ABC transporter family in ovarian cancer progression and chemoresistance as well as the clinical attempts used to date to reverse chemoresistance.

Influence of multidrug resistance and drug transport proteins on chemotherapy drug metabolism

INTRODUCTION

Chemotherapy involving the use of anticancer drugs remains an important strategy in the overall management of patients with metastatic cancer. Acquisition of multidrug resistance remains a major impediment to successful chemotherapy. Drug transporters in cell membranes and intracellular drug metabolizing enzymes contribute to the resistance phenotype and determine the pharmacokinetics of anticancer drugs in the body.

AREAS COVERED

ATP-binding cassette (ABC) transporters mediate the transport of endogenous metabolites and xenobiotics including cytotoxic drugs out of cells. Solute carrier (SLC) transporters mediate the influx of cytotoxic drugs into cells. This review focuses on the substrate interaction of these transporters, on their biology and what role they play together with drug metabolizing enzymes in eliminating therapeutic drugs from cells.

EXPERT OPINION

The majority of anticancer drugs are substrates for the ABC transporter and SLC transporter families. Together, these proteins have the ability to control the influx and the efflux of structurally unrelated chemotherapeutic drugs, thereby modulating the intracellular drug concentration. These interactions have important clinical implications for chemotherapy because ultimately they determine therapeutic efficacy, disease progression/relapse and the success or failure of patient treatment.

Effects of hyperglycemia on lonidamine-induced acidification and de-energization of human melanoma xenografts and sensitization to melphalan

We seek to exploit the natural tendency of melanomas and other tumors to convert glucose to lactate as a method for the selective intracellular acidification of cancer cells and for the potentiation of the activity of nitrogen-mustard antineoplastic agents. We performed this study to evaluate whether the induction of hyperglycemia (26 mM) could enhance the effects of lonidamine (LND, 100 mg/kg; intraperitoneally) on the induction of intracellular acidification, bioenergetic decline and potentiation of the activity of melphalan (LPAM) against DB-1 melanoma xenografts in mice. Intracellular pH (pHi ), extracellular pH (pHe ) and bioenergetics (β-nucleoside triphosphate to inorganic phosphate ratio, β-NTP/Pi) were reduced by 0.7 units (p < 0.001), 0.3 units (p > 0.05) and 51.4% (p < 0.05), respectively. The therapeutic response to LPAM (7.5 mg/kg; intravenously) + LND (100 mg/kg; intraperitoneally) was reduced by about a factor of three under hyperglycemic conditions relative to normoglycemia, producing a growth delay of 7.76 days (tumor doubling time, 5.31 days; cell kill, 64%) compared with LND alone of 1.70 days and LPAM alone of 0.29 days. Under normoglycemic conditions, LND plus LPAM produced a growth delay of 17.75 days, corresponding to a cell kill of 90% at the same dose for each of these agents. The decrease in tumor cell kill under hyperglycemic conditions correlates with an increase in tumor ATP levels resulting from increased glycolytic activity. However, hyperglycemia substantially increases lactic acid production in tumors by a factor of approximately six (p < 0.05), but hyperglycemia did not increase the effects of LND on acidification of the tumor, most probably because of the strong buffering action of carbon dioxide (the pKa of carbonic acid is 6.4). Therefore, this study demonstrates that the addition of glucose during treatment with LND diminishes the activity of this agent.

Protection of HepG2 cells against acrolein toxicity by 2-cyano-3,12-dioxooleana-1,9-dien-28-imidazolide via glutathione-mediated mechanism

Acrolein is an environmental toxicant, mainly found in smoke released from incomplete combustion of organic matter. Several studies showed that exposure to acrolein can lead to liver damage. The mechanisms involved in acrolein-induced hepatocellular toxicity, however, are not completely understood. This study examined the cytotoxic mechanisms of acrolein on HepG2 cells. Acrolein at pathophysiological concentrations was shown to cause apoptotic cell death and an increase in levels of protein carbonyl and thiobarbituric acid reactive acid substances. Acrolein also rapidly depleted intracellular glutathione (GSH), GSH-linked glutathione-S-transferases, and aldose reductase, three critical cellular defenses that detoxify reactive aldehydes. Results further showed that depletion of cellular GSH by acrolein preceded the loss of cell viability. To further determine the role of cellular GSH in acrolein-mediated cytotoxicity, buthionine sulfoximine (BSO) was used to inhibit cellular GSH biosynthesis. It was observed that depletion of cellular GSH by BSO led to a marked potentiation of acrolein-mediated cytotoxicity in HepG2 cells. To further assess the contribution of these events to acrolein-induced cytotoxicity, triterpenoid compound 2-cyano-3,12-dioxooleana-1,9-dien-28-imidazolide (CDDO-Im) was used for induction of GSH. Induction of GSH by CDDO-Im afforded cytoprotection against acrolein toxicity in HepG2 cells. Furthermore, BSO significantly inhibited CDDO-Im-mediated induction in cellular GSH levels and also reversed cytoprotective effects of CDDO-Im in HepG2 cells. These results suggest that GSH is a predominant mechanism underlying acrolein-induced cytotoxicity as well as CDDO-Im-mediated cytoprotection. This study may provide understanding on the molecular action of acrolein which may be important to develop novel strategies for the prevention of acrolein-mediated toxicity.

Renal human organic anion transporter 3 increases the susceptibility of lymphoma cells to bendamustine uptake

Chronic lymphatic leukemia (CLL) is often associated with nephritic syndrome. Effective treatment of CLL by chlorambucil and bendamustine leads to the restoration of renal function. In this contribution, we sought to elucidate the impact of organic anion transporters (OATs) on the uptake of bendamustine and chlorambucil as a probable reason for the superior efficacy of bendamustine over chlorambucil in the treatment of CLL. We examined the effects of structural analogs of p-aminohippurate (PAH), melphalan, chlorambucil, and bendamustine, on OAT1-mediated [(3)H]PAH uptake and OAT3- and OAT4-mediated [(3)H]estrone sulfate (ES) uptake in stably transfected human embryonic kidney-293 cells. Melphalan had no significant inhibitory effect on any OAT, whereas chlorambucil reduced OAT1-, OAT3-, and OAT4-mediated uptake of PAH or ES down to 14.6%, 16.3%, and 66.0% of control, respectively. Bendamustine inhibited only OAT3-mediated ES uptake, which was reduced down to 14.3% of control cells, suggesting that it interacts exclusively with OAT3. The IC50 value for OAT3 was calculated to be 0.8 μM. Real-time PCR experiments demonstrated a high expression of OAT3 in lymphoma cell lines as well as primary CLL cells. OAT3-mediated accumulation of bendamustine was associated with reduced cell proliferation and an increased rate of apoptosis. We conclude that the high efficacy of bendamustine in treating CLL might be partly contributed to the expression of OAT3 in lymphoma cells and the high affinity of bendamustine for this transporter.

Importance of ABCC1 for cancer therapy and prognosis

Multidrug resistance presents one of the most important causes of cancer treatment failure. Numerous in vitro and in vivo data have made it clear that multidrug resistance is frequently caused by enhanced expression of ATP-binding cassette (ABC) transporters. ABC transporters are membrane-bound proteins involved in cellular defense mechanisms, namely, in outward transport of xenobiotics and physiological substrates. Their function thus prevents toxicity as carcinogenesis on one hand but may contribute to the resistance of tumor cells to a number of drugs including chemotherapeutics on the other. Within 48 members of the human ABC superfamily there are several multidrug resistance-associated transporters. Due to the well documented susceptibility of numerous drugs to efflux via ABC transporters it is highly desirable to assess the status of ABC transporters for individualization of treatment by their substrates. The multidrug resistance associated protein 1 (MRP1) encoded by ABCC1 gene is one of the most studied ABC transporters. Despite the fact that its structure and functions have already been explored in detail, there are significant gaps in knowledge which preclude clinical applications. Tissue-specific patterns of expression and broad genetic variability make ABCC1/MRP1 an optimal candidate for use as a marker or member of multi-marker panel for prediction of chemotherapy resistance. The purpose of this review was to summarize investigations about associations of gene and protein expression and genetic variability with prognosis and therapy outcome of major cancers. Major advances in the knowledge have been identified and future research directions are highlighted.

First evidence for toxic defense based on the multixenobiotic resistance (MXR) mechanism in Daphnia magna

The water flea Daphnia magna is widely used as test species in ecotoxicological bioassays. So far, there is no information available to which extent ATP binding cassette (ABC) transporter based multixenobiotic resistance (MXR) counteracts adverse chemical effects in this species. This, however, would be important for assessing to which extent the bio-active potential of a compound determined with this species depends on this cellular defense. We here present molecular, functional and toxicological studies that provide first evidence for ABC transporter-based MXR in D. magna. We cloned putatively MXR-related partial abcb1, abcc1/3, abcc4 and abcc5 coding sequences; respective transcripts were constitutively expressed in different D. magna life stages. MXR associated efflux activity was monitored in D. magna using the fluorescent substrate dyes rhodamine 123, rhodamine B and calcein-AM combined with inhibitors of human ABCB1 and/or ABCC transporter activities reversin 205, MK571 and cyclosporin A. With inhibitors present, efflux of dye substrates was reduced in D. magna in a concentration-dependent mode, as indicated by elevated accumulation of the dyes in D. magna tissues. In animals pre-exposed to mercury, pentachlorophenol or dacthal applied as inducers of ABC transporter expression, levels of some ABC transporter transcripts were increased in some cases showing that these genes can be chemically induced. Likewise, pre-exposure of animals to these chemicals decreased dye accumulation in tissue, indicating enhanced MXR transporter activity, likely associated with higher transporter protein levels. Toxicity assays with toxic transporter substrates mitoxantrone and chlorambucil that were applied singly and in combination with inhibitors were performed to study the tolerance role of Abcb1 and Abcc efflux transporters in D. magna. Joint toxicities of about half of the binary combinations of test compounds applied (substrate/inhibitor, substrate/substrate, inhibitor/inhibitor) were greater than joint effects predicted with mixture toxicity models, which can be explained by chemosensitization through MXR efflux transporter interference. Our data indicate the presence of an MXR efflux system in D. magna. It needs to be considered when assessing the bioactive potential of test compounds with this species. Further, chemosensitization may explain joint toxicities of compound mixtures to D. magna that are higher than expected.

Multidrug resistance: Physiological principles and nanomedical solutions

Multidrug resistance (MDR) is a pathophysiological phenomenon employed by cancer cells which limits the prolonged and effective use of chemotherapeutic agents. MDR is primarily based on the over-expression of drug efflux pumps in the cellular membrane. Prominent examples of such efflux pumps, which belong to the ATP-binding cassette (ABC) superfamily of proteins, are Pgp (P-glycoprotein) and MRP (multidrug resistance-associated protein), nowadays officially known as ABCB1 and ABCC1. Over the years, several strategies have been evaluated to overcome MDR, based not only on the use of low-molecular-weight MDR modulators, but also on the implementation of 1-100(0) nm-sized drug delivery systems. In the present manuscript, after introducing the most important physiological principles of MDR, we summarize prototypic nanomedical strategies to overcome multidrug resistance, including the use of carrier materials with intrinsic anti-MDR properties, the use of nanomedicines to modify the mode of cellular uptake, and the co-formulation of chemotherapeutic drugs together with low- and high-molecular-weight MDR inhibitors within a single drug delivery system. While certain challenges still need to be overcome before such constructs and concepts can be widely applied in the clinic, the insights obtained and the progress made strongly suggest that nanomedicine formulations hold significant potential for improving the treatment of multidrug-resistant malignancies.

Transporters and drug-drug interactions: important determinants of drug disposition and effects

Uptake and efflux transporters determine plasma and tissue concentrations of a broad variety of drugs. They are localized in organs such as small intestine, liver, and kidney, which are critical for drug absorption and elimination. Moreover, they can be found in important blood-tissue barriers such as the blood-brain barrier. Inhibition or induction of drug transporters by coadministered drugs can alter pharmacokinetics and pharmacodynamics of the victim drugs. This review will summarize in particular clinically observed drug-drug interactions attributable to inhibition or induction of intestinal export transporters [P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)], to inhibition of hepatic uptake transporters [organic anion transporting polypeptides (OATPs)], or to inhibition of transporter-mediated [organic anion transporters (OATs), organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATEs), P-gp] renal secretion of xenobiotics. Available data on the impact of nutrition on transport processes as well as genotype-dependent, transporter-mediated drug-drug interactions will be discussed. We will also present and discuss data on the variable extent to which information on the impact of transporters on drug disposition is included in summaries of product characteristics of selected countries (SPCs). Further work is required regarding a better understanding of the role of the drug metabolism-drug transport interplay for drug-drug interactions and on the extrapolation of in vitro findings to the in vivo (human) situation.

Inhibition and induction of glutathione S-transferases by flavonoids: possible pharmacological and toxicological consequences

Many studies reviewed herein demonstrated the potency of some flavonoids to modulate the activity and/or expression of glutathione S-transferases (GSTs). Because GSTs play a crucial role in the detoxification of xenobiotics, their inhibition or induction may significantly affect metabolism and biological effects of many drugs, industrials, and environmental contaminants. The effect of flavonoids on GSTs strongly depends on flavonoid structure, concentration, period of administration, as well as on GST isoform and origin. Moreover, the results obtained in vitro are often contrary to the vivo results. Based on these facts, the revelation of important flavonoid-drug or flavonoid-pollutant interaction has been complicated. However, it should be borne in mind that ingestion of certain flavonoids in combination with drugs or pollutants (e.g., acetaminophen, simvastatin, cyclophosphamide, cisplatine, polycyclic aromatic hydrocarbons, chlorpyrifos, acrylamide, and isocyanates), which are GST substrates, could have significant pharmacological and toxicological consequences. Although reasonable consumptions of a flavonoids-rich diet (that may lead to GST induction) are mostly beneficial, the uncontrolled intake of high concentrations of certain flavonoids (e.g., quercetin and catechins) in dietary supplements (that may cause GST inhibition) may threaten human health.

Multidrug resistance proteins (MRPs, ABCCs): importance for pathophysiology and drug therapy

The nine multidrug resistance proteins (MRPs) represent the major part of the 12 members of the MRP/CFTR subfamily belonging to the 48 human ATP-binding cassette (ABC) transporters. Cloning, functional characterization, and cellular localization of most MRP subfamily members have identified them as ATP-dependent efflux pumps with a broad substrate specificity for the transport of endogenous and xenobiotic anionic substances localized in cellular plasma membranes. Prototypic substrates include glutathione conjugates such as leukotriene C(4) for MRP1, MRP2, and MRP4, bilirubin glucuronosides for MRP2 and MRP3, and cyclic AMP and cyclic GMP for MRP4, MRP5, and MRP8. Reduced glutathione (GSH), present in living cells at millimolar concentrations, modifies the substrate specificities of several MRPs, as exemplified by the cotransport of vincristine with GSH by MRP1, or by the cotransport of GSH with bile acids or of GSH with leukotriene B(4) by MRP4.The role of MRP subfamily members in pathophysiology may be illustrated by the MRP-mediated release of proinflammatory and immunomodulatory mediators such as leukotrienes and prostanoids. Pathophysiological consequences of many genetic variants leading to a lack of functional MRP protein in the plasma membrane are observed in the hereditary MRP2 deficiency associated with conjugated hyperbilirubinemia in Dubin-Johnson syndrome, in pseudoxanthoma elasticum due to mutations in the MRP6 (ABCC6) gene, or in the type of human earwax and osmidrosis determined by single nucleotide polymorphisms in the MRP8 (ABCC8) gene. The hepatobiliary and renal elimination of many drugs and their metabolites is mediated by MRP2 in the hepatocyte canalicular membrane and by MRP4 as well as MRP2 in the luminal membrane of kidney proximal tubules. Therefore, inhibition of these efflux pumps affects pharmacokinetics, unless compensated by other ATP-dependent efflux pumps with overlapping substrate specificities.

Evaluation of cervical lymph nodes with CT perfusion in patients with hypopharyngeal and laryngeal squamous cell cancer

BACKGROUND

In patients with head and neck squamous cell cancer, metastases in cervical lymph nodes still remain the single most important negative predicting factor. Their presence reduces overall 5-year survival by 50%. The aim of the study was to evaluate the role of computed tomography perfusion (CTP) for differentiation between metastatic and non-metastatic cervical lymph nodes in patients with squamous cell cancer of hypopharynx and larynx.

MATERIAL/METHODS

This was a prospective single center study of 18 consecutive patients. Eleven patients with squamous cell cancer of the hypopharynx and seven patients with laryngeal cancer underwent CT examination of the neck followed by CTP. Group II, III, and V of lymph nodes were evaluated. Perfusion maps of basic parameters (blood flow [BF], blood volume [BV], mean transit time [MTT] and permeability surface [PS]) were reconstructed for all patients. In all patients resection of primary tumour along with neck dissection was performed. Lymph nodes underwent histopathological examinations for presence of metastases. CTP parameters were related with histological analysis of resected nodes.

RESULTS

CTP and histological findings of 65 nodes were correlated. 24 of them were metastatic and 41 were non-metastatic. Metastatic nodes showed significant hyperperfusion, comparing to non-metastatic ones. An average value of BF in metastatic nodes was 136.4 ml/100 g/min, BV was 7.7 ml/100 g, MTT was 4.4 s and PS was 19.4 ml/100 g/min. The average values for non-metastatic nodes were: BF was 80.7 ml/100 g/min, BV was 4.7 ml/100 g, MTT was 5.6 s and PS was 12.8 ml/100 g/min. The differences were significantly higher for BF, BV and PS values (p<0.05).

CONCLUSIONS

CTP may be useful in differentiation between metastatic and non-metastatic lymph nodes, based on evaluation of the value of BF, BV and PS.

Influx and efflux transport as determinants of melphalan cytotoxicity: Resistance to melphalan in MDR1 overexpressing tumor cell lines

There is a considerable variation in efficacy of melphalan therapy in multiple myeloma (MM) and other hematopoietic tumors. We hypothesized that this may be due to variations in the expression of influx and efflux transporters of melphalan. We measured the expression of the influx transporters LAT1, LAT2, and TAT1 and the efflux transporters MDR1, MRP1 and BCRP by quantitative RT-PCR and related their expression to the intracellular accumulation and cytotoxicity of melphalan in 7 MM and 21 non-MM hematopoietic tumor cell lines. Variation in the intracellular accumulation accounted for nearly half of the variation in the cytotoxicity of melphalan in MM cell lines (r(2)=0.47, P=0.04). High expression of the efflux transporter MDR1 was associated with low intracellular accumulation and low cytotoxicity of melphalan (r(2)=0.56, P=0.03 and r(2)=0.62, P=0.02, respectively). The effect was reversed by the MDR1 inhibitor cyclosporine. In addition, the MDR1 overexpressing HL-60 cell line showed 10-fold higher resistance to melphalan than the non-MDR1 expressing one. Again, the resistance was reversed by cyclosporine and by MDR1-specific shRNA. LAT1 was the major influx transporter in tumor cell lines with 4000-fold higher expression than LAT2. Down-regulation of LAT1 by siRNA reduced the melphalan uptake by 58% and toxicity by 3.5-fold, but natural variation in expression between the tumor cell lines was not associated with accumulation or cytotoxicity of melphalan. In conclusion, tumor-specific variations in the expression of the efflux transporter MDR1, but not of the influx transporter LAT1, affect the intracellular accumulation of melphalan and thus determine its cytotoxicity.

Structure and function of the MRP2 (ABCC2) protein and its role in drug disposition

The multi-drug resistance protein 2 (MRP2; ABCC2) is an ATP-binding cassette transporter playing an important role in detoxification and chemoprotection by transporting a wide range of compounds, especially conjugates of lipophilic substances with glutathione, glucuronate and sulfate, which are collectively known as phase II products of biotransformation. In addition, MRP2 can also transport uncharged compounds in cotransport with glutathione, and thus can modulate the pharmacokinetics of many drugs. The other way around, its expression and activity are also altered by certain drugs and disease states. Unlike other members of the MRP/ABCC family, MRP2 is specifically expressed on the apical membrane domain of polarised cells as hepatocytes, renal proximal tubular cells, enterocytes and syncytiotrophoblasts of the placenta. Several naturally occurring mutations leading to the absence of functional MRP2 protein from the apical membrane have been described causing the human Dubin-Johnson syndrome associated with conjugated hyperbilirubinaemia. Experimental mutation studies have revealed critical amino acids for substrate binding in the MRP2 molecule. This review is, therefore, focused on the structure and function of MRP2, the substrates transported and the clinical relevance of MRP2.

Curcumin as chemosensitizer

This overview presents curcumin as a significant chemosensitizer in cancer chemotherapy. Although the review focuses on curcumin and its analogues on multidrug resistance (MDR) reversal, the relevance of curcumin as a nuclear factor (NF)-KB blocker and sensitizer of many chemoresistant cancer cell lines to chemotherapeutic agents will also be discussed. One of the major mechanisms of MDR is the enhanced ability of tumor cells to actively efflux drugs, leading to a decrease in cellular drug accumulation below toxic levels. Active drug efflux is mediated by several members of the ATP-binding cassette (ABC) superfamily of membrane transporters, which have now been subdivided into seven families designated A through G. Among these ABC families, the classical MDR is attributed to the elevated expression of ABCB1 (Pgp), ABCC1 (MRP1), and ABCG2 (MXR). The clinical importance of Pgp, MRP1, and MXR for MDR and cancer treatment has led to the investigation of the inhibiting properties of several compounds on these transporters. At present, due in part to the disappointing results associated with the many side effects of synthetic modulators that have been used in clinical trials, current research efforts are directed toward the identification of novel compounds, with attention to dietary natural products. The advantage is that they exhibit little or virtually no side effects and do not further increase the patient's medication burden.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

Enhanced glutathione S-transferase expression in 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine-resistant IEC-18 cells

In the present study we show that repeated exposure of the rat intestinal epithelial cell line IEC-18 to 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP), from a toxicological point of view the most relevant phase-1 metabolite of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP, the main heterocyclic aromatic amine present in processed meat), led to the selection of N-OH-PhIP-resistant IEC-18 cells. This phenomenon was accompanied by a fivefold increase in total glutathione S-transferase (GST) activity, measured with the broad-spectrum substrate 1-chloro-2,4-dinitrobenzene, in the N-OH-PhIP-resistant IEC-18 cells. Furthermore, a Western blotting analysis revealed that the expression of GST subunits A1, A3, A4, M1 and P1 was enhanced in the N-OH-PhIP-resistant IEC-18 cells.

Mechanism of selectivity of an angiogenesis inhibitor from screening a genome-wide set of Saccharomyces cerevisiae deletion strains

BACKGROUND

The synthetic tripeptide arsenical 4-(N-(S-glutathionylacetyl)amino) phenylarsenoxide (GSAO) is an angiogenesis inhibitor that targets the mitochondria of actively dividing but not quiescent endothelial cells, arresting their proliferation and causing apoptosis. Normal endothelial cells are much more sensitive to GSAO than tumor cells. To elucidate the mechanism of tumor cell resistance, we identified yeast genes that are necessary for resistance to GSAO.

METHODS

We screened a genome-wide set of 4546 Saccharomyces cerevisiae deletion strains to identify GSAO-sensitive strains. We then examined GSAO accumulation in and proliferation activity of endothelial cells (BAECs) and tumor cells treated with GSAO and modulators of pathways and proteins identified in the yeast screen. We also examined GSAO effects on proliferation of mammalian cells transfected with transporter protein constructs.

RESULTS

Eighty-eight deletion strains were sensitive to GSAO. The most sensitive strains had deletions of genes whose products are involved in vacuolar function (corresponding to drug transport in mammalian cells) and glutathione synthesis. BAECs were more sensitive to GSAO than tumor cells, and cell sensitivity to GSAO was approximately proportional to cellular glutathione levels. Treatment of BAECs and tumor cells with MK-571, an inhibitor of multidrug resistance-associated protein (MRP), or with buthionine sulfoximine, an inhibitor of glutathione synthesis, increased their sensitivity to GSAO. Mammalian cells transfected with MRP1 or MRP2 were resistant to GSAO, whereas cells transfected with MRP3, MRP4, MRP5, P-glypoprotein, or breast cancer resistance protein were not.

CONCLUSIONS

Differences in MRP activity and cellular glutathione levels contribute to the selectivity of GSAO for endothelial versus tumor cells. MRP1 and/or MRP2 may transport GSAO from resistant cells, with glutathione acting as a cotransporter. Genetic screening in yeast is a powerful tool for understanding drug action in mammalian cells.

A Fluorometric Screening Assay for Drug Efflux Transporter Activity in the Blood-Brain Barrier

PURPOSE

To examine the capability of a fluorometric assay to identify and characterize the drug efflux interactions of a broad spectrum of drug agents in an in vitro model of the blood-brain barrier (BBB).

METHODS

Various concentrations of drug agent (1, 10, and 100 microM) were evaluated for their effect on the cellular accumulation of the P-glycoprotein (P-gp) probe R123 (3.2 microM), and the mixed P-gp and multidrug resistance-associated protein (MRP) probe, BCECF (1 microM), in bovine brain microvessel endothelial cell (BBMEC) monolayers. Drugs demonstrating a significant effect were further quantitated using an expanded concentration range and a nonlinear regression curve fit to determine the potency (IC50) and efficacy (Imax) of the drug for P-gp and/or MRP.

RESULTS

Several of the 36 therapeutic agents examined showed drug efflux transporter interactions in BBMEC monlayers. Melphalan and risperidone significantly enhanced the accumulation of R123 over control (1.47- and 1.82-fold, respectively) with resulting IC50s of 1.4 and 14.6 microM, respectively. Chlorambucil and valproic acid significantly enhanced the accumulation of BCECF compared to control monolayers (2.02- and 4.01-fold, respectively) with resulting IC50s of 146.1 and 768.5 microM, respectively.

CONCLUSIONS

The current study demonstrates the feasibility of a fluorometric assay consisting of R123 and BCECF in assessing the drug efflux interactions of a variety of drugs in the BBB.

Pharmacogenomics of MDR and MRP subfamilies

Drug-metabolizing enzymes, drug transporters and drug targets play significant roles as determinants of drug efficacy and toxicity. Their genetic polymorphisms often affect the expression and function of their products and are expected to become surrogate markers to predict the response to drugs in individual patients. With the sequencing of the human genome, it has been estimated that approximately 500–1200 genes code for drug transporters and, recently, there have been significant and rapid advances in the research on the relationships between genetic polymorphisms of drug transporters and interindividual variation of drug disposition. At present, the clinical studies of multi-drug resistance protein 1 (MDR1, P-glycoprotein, ABCB1), which belongs to the ATP-binding cassette (ABC) superfamily, are the most comprehensive among the ABC transporters, but clinical investigations on other drug transporters are currently being performed around the world. MDR1 can be said to be the most important drug transporter, since clinical reports have suggested that it regulates the disposition of various types of clinically important drugs, but in vitro investigations or animal experiments have strongly suggested that the members of the multi-drug resistance-associated protein (MRP) subfamily can also become key molecules for pharmacotherapy. In addition to those, breast cancer resistance protein (BCRP, ABCG2), another ABC transporter, is well known as a key molecule of multi-drug resistance to several anticancer agents. However, this review focuses on the latest information on the pharmacogenetics of the MDR and MRP subfamilies, and its impact on pharmacotherapy is discussed.

The beta-D-glucoside and sodium-dependent glucose transporter 1 (SGLT1)-inhibitor phloridzin is transported by both SGLT1 and multidrug resistance-associated proteins 1/2

Phloridzin, a glucoside of the flavonoid-like polyphenol phloretin, has long been known to be a specific nontransportable inhibitor of the sodium-dependent glucose transporter SGLT1. The objective of this study was to determine whether efflux by multidrug resistance-associated protein (MRP) transporters might have masked the absorption by SGLT1 in previous studies. Various cells used as transport models were incubated with phloridzin (50 microM) in the absence and presence of 50 microM 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK-571), a highly selective MRP1/MRP2 inhibitor, and the cellular uptake of phloridzin was measured by high performance liquid chromatography. The uptake of phloridzin by SGLT1-transfected Chinese hamster ovary (CHO) (G6D3) cells was 1.7-fold higher than that by parent CHO cells (p < 0.01). In the presence of MK-571, the uptake of phloridzin by CHO cells increased 3.7-fold (p < 0.001). MK-571 caused an 8.0-fold increase in the uptake of phloridzin by G6D3 cells (p < 0.0001). Thus, in the absence of MRP1 efflux, transport of phloridzin by SGLT1 was clearly demonstrated. Similar results were obtained for the glycosides of the flavonoids quercetin, genistein, and diosmetin. A significantly lower accumulation of phloridzin in MRP2-transfected Madin-Darby canine kidney (MDCK) cells compared with parent MDCK cells demonstrated that phloridzin was a substrate also for MRP2 (p < 0.05). This conclusion was further strengthened when MK-571 increased the uptake by MRP2-MDCK cells as much as 3.6-fold (p < 0.01). These results demonstrate that phloridzin, in contrast to previous notions, is transported by SGLT1. In addition, they demonstrate that this and other flavonoid glycosides unexpectedly are efficiently effluxed by both MRP1 and MRP2.

Cellular uptake and efflux of the tea flavonoid (-)epicatechin-3-gallate in the human intestinal cell line Caco-2

(-)-Epicatechin gallate (ECG) is one of the flavonoids in green tea, which has been demonstrated to have cancer-preventive properties in many model systems. However, the extent and mechanisms of accumulation of these flavonoids in cells is unknown. The objectives of this study were to determine the accumulation of ECG by the intestinal epithelial cell Caco-2 and to characterize the transport mechanism involved. The cells were exposed to ECG +/- various transport inhibitors and incubated at 37 degrees C. Absorbed flavonoids were extracted and quantified by high-performance liquid chromatography. The uptake of ECG included a nonsaturable initial rapid process as well as a much slower saturable process. The saturable ECG uptake by the Caco-2 cells was sodium-independent but clearly dependent on a pH gradient. Phloretin and benzoic acid, inhibitors of the monocarboxylate transporter (MCT), significantly reduced ECG uptake. The uptake of ECG in the Caco-2 cells increased 2-fold in the presence of 50 microM 3-[(3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl)-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK-571), suggesting the involvement of multidrug-associated protein (MRP)2 in efflux of ECG. This was confirmed using Madin-Darby canine kidney cells transfected with MRP2. Also P-glycoprotein was responsible for some ECG efflux. MK-571 also caused a dramatic increase in ECG accumulation in Chinese hamster ovary cells, suggesting that ECG was also a substrate for MRP1. Together, these observations demonstrate important roles of membrane transporters, i.e., MCT, MRP2, P-glycoprotein, and MRP1, in the cellular accumulation and potential effects of ECG.

Multidrug resistance protein-6 (MRP6) in human dermal fibroblasts. Comparison between cells from normal subjects and from Pseudoxanthoma elasticum patients

Multidrug resistance protein-6 (MRP6) is a membrane transporter whose deficiency leads to the connective tissue disorder Pseudoxanthoma elasticum (PXE). In vitro dermal fibroblasts from normal and PXE subjects, homozygous for the R1141X mutation, were compared for their ability to accumulate and to release fluorescent calcein, in the absence and in the presence of inhibitors and competitors of the MDR-multidrug resistance protein (MRP) systems, such as 3-(3-(2-(7-choro-2 quinolinyl) ethenyl)phenyl ((3-dimethyl amino-3-oxo-propyl)thio) methyl) propanoic acid (MK571), verapamil (VPL), vinblastine (VBL), chlorambucil (CHB), benzbromarone (BNZ) and indomethacin (IDM). In the absence of chemicals, calcein accumulation was significantly higher and the release significantly slower in PXE cells compared to controls. VBL and CHB reduced calcein release in both cell strains, without affecting the differences between PXE and control fibroblasts. VPL, BNZ and IDM consistently delayed calcein release from both control and PXE cells; moreover, they abolished the differences between normal and MRP6-deficient fibroblasts observed in the absence of chemicals. These findings suggest that VPL, BNZ and IDM interfere with MRP6-dependent calcein extrusion in in vitro human normal fibroblasts. Interestingly, MK571 almost completely abolished calcein release from PXE cells, whereas it induced a strong but less complete inhibition in control fibroblasts, suggesting that MRP6 is not inhibited by MK571. Data show that MRP6 is active in human fibroblasts, and that its sensitivity to inhibitors and competitors of MDR-MRPs' membrane transporters is different from that of other translocators, namely, MRP1. It could be suggested that MRP1 and MRP6 transport different physiological substances and that MRP6 deficiency cannot be overcome by other membrane transporters, at least in fibroblasts. These data further support the hypothesis that MRP6 deficiency may be relevant for fibroblast metabolism and responsible for the metabolic alterations of these cells at the basis of connective tissue clinical manifestations of PXE.

Increased resistance to nitrogen mustards and antifolates following in vitro selection of murine fibroblasts and primary hematopoietic cells transduced with a bicistronic retroviral vector expressing the rat glutathione S-transferase A3 and a mutant dihydrofolate reductase

We have constructed a retroviral bicistronic vector, MFG/GID, that transduces the expression of both the A3 isoform of the rat glutathione S-transferase (GST A3), and the tyr-22 variant of the human dihydrofolate reductase (DHFR(L22Y)). Transduction of murine 3T3 fibroblasts with this vector increased their in vitro resistance to chlorambucil (1.8-fold) and trimetrexate (TMTX) (748-fold). TMTX selection of a mixed population of 20% GID-transduced NIH 3T3 cells and 80% control cells resulted in a marked increase in the GST peroxidase activity associated with the GST A3 isoform (17.7-fold). MFG/GID-transduced primary clonogenic murine hematopoietic progenitor cells were likewise more resistant to TMTX and chlorambucil than control beta-gal-transduced cells. Selecting GID-transduced hematopoietic cells with a combination of TMTX and a nucleoside transport inhibitor resulted in a marked increase in resistance upon re-exposure to TMTX (99% survival). Similarly, GID-transduced hematopoietic cells selected with TMTX were more resistant to chlorambucil, with 40% survival at a drug concentration that killed practically all control cells. These results suggest that antifolate-mediated selection of MFG/GID-transduced hematopoietic cells could be used as a mean to enrich the population of transduced cells prior to or following transplantation, thus potentially conferring in vivo chemoprotection to nitrogen mustards and antifolates.

Effect of flavonoids on MRP1-mediated transport in Panc-1 cells

The purpose of this study was to identify the effects of dietary flavonoids, which are present in fruits, vegetables, and plant-derived beverages, on the transport of daunomycin (DNM) and vinblastine (VBL) in Panc-1 cells. Panc-1 is a human pancreatic adenocarcinoma cell line, which expresses Multidrug Resistance-Associated Protein1 (MRP1). The 2-h accumulation of (3)H-DNM and (3)H-VBL was determined in the presence and absence of 22 flavonoids. Biochanin-A, genistein, quercetin, chalcone, silymarin, phloretin, morin, and kaempferol, at 100 microM concentrations, all significantly increased the accumulation of both DNM and VBL in Panc-1 cells, with morin increasing DNM and VBL accumulation by 546 +/- 50% (mean +/- SE, n = 9) and 553 +/- 37% (n = 9), respectively. Fisetin treatment significantly decreased the accumulation of both DNM and VBL. Concentration-dependent studies demonstrated significant effects on VBL accumulation at 50 microM, but not at 10 microM concentrations, except for chalcone that was effective at a 10 microM concentration. Following a 24-h incubation, there were no changes in MRP1 membrane expression or glutathione-S-transferase activity in cells. Cellular glutathione (GSH) concentrations were significantly decreased following a 2-h incubation with biochanin A, chalcone, genistein, phloretin, quercetin, and silymarin, and following a 24-h incubation with biochanin A, chalcone, genistein, and phloretin. These results therefore indicate that the flavonoids morin, chalcone, silymarin, phloretin, genistein, quercetin, biochanin A, and kaempferol can inhibit MRP1-mediated drug transport, effects that may involve binding interactions with MRP1, as well as modulation of GSH concentrations.

Synthesis and in vitro evaluation of quaternary ammonium derivatives of chlorambucil and melphalan, anticancer drugs designed for the chemotherapy of chondrosarcoma

To enhance affinity for malignant cartilaginous tumors (chondrosarcomas), quaternary ammonium (QA) conjugates of chlorambucil and melphalan were prepared by linking the QA moiety to nitrogen mustards via an amide bond. They exhibited closely similar and sometimes more favorable values than their parent compounds. In the cell lines tested, the two QA conjugates displayed appreciable cytotoxicity, the QA conjugate of chlorambucil even showing an enhanced efficiency against chondrosarcoma compared with chlorambucil.

Cellular glutathione content, in vitro chemoresponse, and the effect of BSO modulation in samples derived from patients with advanced ovarian cancer

OBJECTIVES

The objective was to assess the relationship between glutathione content and drug sensitivity with glutathione modulation in ovarian cancer in a pilot study using 31 samples of freshly obtained ovarian tumor material from 26 patients with advanced disease.

METHODS

Processed tumor samples were screened to determine the glutathione content using an enzyme recycling assay modified for use in a 96-well plate format. Chemosensitivity testing (MTT assay) was used to assess sensitivity to cisplatin and doxorubicin and modulation using buthionine sulfoximine. Multidrug-resistance-associated protein MRP1 (putative drug-glutathione conjugate transporter) expression was also assessed.

RESULTS

There was a significant increase in the tumor cell GSH levels in samples from patients who had received previous chemotherapy (9) versus those from chemotherapy-naïve patients (20), P = 0.005. In vitro chemosensitivity testing with doxorubicin and cisplatin (using LC(50) values, i.e., drug dose causing 50% reduction in cell survival relative to untreated control) failed to show a relationship with glutathione levels. Coincubation of cisplatin and doxorubicin with buthionine sulfoximine resulted in a significant increase in sensitivity to both of these drugs overall (cisplatin, P = 0.05; doxorubicin, P = 0.025), with 20 samples showing sensitization to a drug to which they were previously resistant. MRP1 expression failed to show a correlation with drug sensitivity and glutathione levels.

CONCLUSIONS

Our study supports the use of glutathione modulation using agents such as buthionine sulfoximine in patients with heavily pretreated, drug-resistant ovarian cancer.

Transport of leukotriene C4 and structurally related conjugates

Transport proteins control the release of the endogenous glutathione conjugate leukotriene C4 (LTC4) from leukotriene-synthesizing cells as well as its hepatobiliary and renal elimination. The photolabile conjugated triene structure of LTC4 has enabled direct photoaffinity labeling of the multidrug resistance protein 1 (MRP1, symbol ABC C1) in membranes from mastocytoma cells, leading to the identification of the function of this protein as an ATP-dependent export pump for LTC4 and structurally related conjugates. MRP1 is assigned to the C branch of the superfamily of ATP-binding cassette (ABC) transporters and was originally identified by virtue of its association with drug resistance in tumor cells. Besides LTC4, which is a high-affinity substrate, a variety of conjugates of hydrophobic endogenous or xenobiotic substances with glutathione, glucuronate, or sulfate are transported by MRP1. In addition, hydrophobic compounds may undergo cotransport with glutathione. Effective inhibitors of MRP1-mediated transport include structural analogs of LTC4 and of other cysteinyl leukotrienes. The ATP-dependent transport system which transports cysteinyl leukotrienes across the hepatocyte canalicular membrane into bile was cloned and characterized as the second isoform or paralog of the mammalian MRP family, MRP2 (ABC C2). MRP2 is localized to the apical membrane of polarized cells. The overall substrate specificities of MRP1 and MRP2 are similar, despite an amino acid identity of only 48%. The transport proteins mediating the uptake of LTC4 into hepatocytes across the basolateral membrane are members of the organic anion transporter (OATP) branch of the solute carrier (SLC) superfamily and are thus distinct from the ATP-dependent export pumps of the MRP family.

Induction of rat organic anion transporting polypeptide 2 by pregnenolone-16alpha-carbonitrile is via interaction with pregnane X receptor

The rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) is a liver transporter that mediates the uptake of a variety of structurally diverse compounds, and has a high affinity for cardiac glycosides. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN), a ligand for the rodent pregnane X receptor (PXR), significantly enhances the rat oatp2 gene expression. To understand the molecular mechanism of oatp2 induction by PCN, rat oatp2 gene was cloned. The rat oatp2 gene consists of 16 exons; alternative splicing of the second noncoding exon gives rise to the two published rat oatp2 cDNAs. Approximately 8700 base pairs (bp) of the 5'-flanking region of the rat oatp2 gene were linked to the luciferase reporter gene and used in transient transfection assays in H4IIE cells. Treatment of PCN induced the expression of the reporter gene in a dose-dependent manner. Four potential PXR response elements (PXREs) were identified in the 5'-flanking region of the rat oatp2 gene. One element (DR3-1) is located approximately -5000 bp with three more (DR3-2, DR3-3, and DR3-4) clustered at about -8000 bp. Results from electrophoretic mobility shift assays showed that the PXR-retinoid X receptor alpha heterodimer binds to the DR3-2 with the highest affinity, to the DR3-4 and DR3-1 with a lower affinity, and weakly or not at all to the DR3-3. Furthermore, a series of partial deletions of the 5'-flanking region illustrated that both the proximal and distal clusters of PXREs are required for maximal induction of rat oatp2 by PCN. In conclusion, these data elucidate the molecular mechanism by which PCN treatment induces rat oatp2 gene expression. In addition, this study identifies rat oatp2 as a direct PXR-targeted gene and further supports the hypothesis that activation of PXR affects a network of genes that is involved in either metabolism or transport of drugs, steroids, and bile acids.

Role of multidrug resistance protein 1 (MRP1) and glutathione S-transferase A1-1 in alkylating agent resistance. Kinetics of glutathione conjugate formation and efflux govern differential cellular sensitivity to chlorambucil versus melphalan toxicity

We investigated the role of phase II (conjugation) and phase III (efflux) detoxification of the anticancer drugs melphalan (MLP) and chlorambucil (CHB). Although both drugs are substrates of Alpha-class glutathione S-transferases (GST) and the monoglutathionyl conjugates formed in these enzymatic reactions are transported by MRP1, we found that GSTA1-1 and MRP1 acted in synergy to confer resistance to CHB but not to MLP (Morrow, C. S., Smitherman, P. K., Diah, S. K., Schneider, E., and Townsend, A. J. (1998) J. Biol. Chem. 273, 20114-20120). To explain this selectivity of MRP1/GST-mediated resistance, we report results of side-by-side experiments comparing the kinetics of MLP- versus CHB-glutathione conjugate: formation, product inhibition of GSTA1-1 catalysis, and transport by MRP1. The monoglutathionyl conjugate of CHB, CHB-SG, is a very strong competitive inhibitor of GSTA1-1 (K(i) 0.14 microM) that is >30-fold more potent than that of the corresponding conjugate of MLP, MLP-SG (K(i) 4.7 microM). The efficiency of GSTA1-1-mediated monoglutathionyl conjugate formation is more than 4-fold higher for CHB than MLP. Lastly, both CHB-SG and MLP-SG are efficiently transported by MRP1 with similar V(max) although the K(m) for CHB-SG (0.37 microm) is significantly lower than for MLP-SG (1.1 microM). These results indicate that MRP1 is required for GSTA1-1-mediated resistance to CHB in order to relieve potent product inhibition of the enzyme by intracellular CHB-SG formed. The kinetic properties of MRP1 are well suited to eliminate CHB-SG at pharmacologically relevant concentrations. For MLP detoxification, where product inhibition of GSTA1-1 is less important, GSTA1-1 does not confer resistance because of the relatively poorer catalytic efficiency of MLP-SG formation. Similar analyses can be useful for predicting the pharmacological and toxicological consequences of MRP and GST expression on cellular sensitivity to various other electrophilic xenobiotics.

The drug resistance proteins, multidrug resistance-associated protein and P-glycoprotein, do not confer resistance to Fas-induced cell death

BACKGROUND

Multidrug resistance (MDR) is mediated by the drug resistance proteins, the multidrug resistance-associated protein (MRP) and P-glycoprotein, both of which confer resistance by the active efflux of chemotherapeutic drugs from the cell. Reduced Fas (CD95/APO-1) expression and resistance to Fas-mediated apoptosis have also been correlated with P-glycoprotein-mediated MDR.

METHODS

We investigated cell surface Fas expression (using anti-Fas monoclonal antibody DX2.1) in a series of MRP-expressing drug-resistant leukemia sublines, and P-glycoprotein-expressing leukemia sublines, and their susceptibility to apoptosis induced by anti-Fas treatment (CH-11 monoclonal antibody). Caspase-3 activation was detected by Western blot and apoptosis was determined by flow cytometry with 7-aminoactinomycin D (7-AAD) staining of cells.

RESULTS

Fas expression was not reduced in either the MRP- or P-glycoprotein-expressing drug-resistant cell lines, although expression was reduced by 15% in one low-level drug-resistant subline. Expression of MRP or P-glycoprotein did not confer resistance to caspase-3 activation or to anti-Fas-induced cell death.

CONCLUSIONS

MDR mediated by the drug transport proteins MRP and P-glycoprotein does not correlate with resistance to Fas-mediated cell death or resistance to caspase-3 activation.

ABC drug transporters: hereditary polymorphisms and pharmacological impact in MDR1, MRP1 and MRP2

Transport by ATP-dependent efflux pumps, such as P-glycoprotein (PGP) and multi-drug resistance related proteins (MRPs), influences bioavailability and disposition of drugs. These efflux pumps serve as defence mechanisms and determine bioavailability and CNS concentrations of many drugs. However, despite the fact that substantial data have been accumulated on the structure, function and pharmacological role of ABC transporters and even though modification of PGP function is an important mechanism of drug interactions and adverse effects in humans, there is a striking lack of data on variability of the underlying genes. This review focuses on the human drug transporter proteins PGP (MDR1) and the multi-drug resistance proteins MRP1 and MRP2. An overview is provided of pharmacologically relevant genetic, structural and functional data as well as on hereditary polymorphisms, their phenotypical consequences and pharmacological implications.

The (patho)physiological functions of the MRP family

The identification of certain members of the large superfamily of ATP binding cassette transport proteins such as MDR1 -P-glycoprotein and the multidrug resistance protein MRP1 as ATP-dependent drug efflux pumps has been a major contribution in our understanding of the multidrug resistance phenotype of cancer cells. Importantly, both transport proteins that exhibit only low structural homology have a very different substrate specificity but confer resistance to a similar spectrum of natural product chemotherapeutic drugs. In contrast to the drug transporter MDR1, MRP1 mainly transports anionic Phase II-conjugates. In addition MRP1-mediated drug resistance is highly dependent on high intracellular glutathione levels which may be linked to the apparent physiological involvement of MRP1 in glutathione-related cellular processes. This review summarizes the current knowledge about functional aspects of MRP1 and its five recently cloned homologues MRP2-MRP6 and discusses their substrate specificities and cellular localization with emphasis on drug resistance. Copyright 2000 Harcourt Publishers Ltd.

Development of multidrug-resistance convertors: sense or nonsense?

This review describes the clinical relevance of the two drug transporters P-glycoprotein (Pgp) and multidrug resistance-associated protein (MRP) and the in vitro phenomenon which is referred to as multidrug resistance (MDR). The attempts to try to block these resistance mechanisms are summarized with specific attention for the intentionally designed "second generation" MDR-convertors. Potential explanations of the limited clinical success rate are given and recommendations for the design of future studies provided.

The effect of glutathione on the ATPase activity of MRP1 in its natural membranes

The transport mechanism by which the multidrug resistance protein 1 (MRP1) effluxes cytotoxic agents out of cells is still not completely understood. However, the cellular antioxidant glutathione (GSH) has been shown to have an important role in MRP1-mediated drug transport. In this study we show that GSH stimulates the ATPase activity of MRP1 in a natural plasma membrane environment. This stimulation was dose-dependent up to 5 mM. The MRP1 substrates vincristine and daunorubicin do not induce MRP1 ATPase activity. In addition, the effect of GSH on the MRP1 ATPase activity is not increased by daunorubicin or by vincristine. In contrast, a GSH conjugate of daunorubicin (WP811) does induce the ATPase activity of MRP1. In the presence of GSH the effect of WP811 was not significantly increased. Finally, (iso)flavonoid-induced MRP1 ATPase activity is not synergistically increased by the presence of GSH. In conclusion, we show that GSH has no apparent influence on the ATPase reaction induced by several MRP1 substrates and/or modulators. The subclasses of molecules had different effects on the MRP1 ATPase activity, which supports the existence of different drug binding sites.