Point mutations in the mdr1 promoter of human osteosarcomas are associated with in vitro responsiveness to multidrug resistance relevant drugs

Roles of Hepatic Drug Transporters in Drug Disposition and Liver Toxicity

Hepatic drug transporters are mainly distributed in parenchymal liver cells (hepatocytes), contributing to drug's liver disposition and elimination. According to their functions, hepatic transporters can be roughly divided into influx and efflux transporters, translocating specific molecules from blood into hepatic cytosol and mediating the excretion of drugs and metabolites from hepatic cytosol to blood or bile, respectively. The function of hepatic transport systems can be affected by interspecies differences and inter-individual variability (polymorphism). In addition, some drugs and disease can redistribute transporters from the cell surface to the intracellular compartments, leading to the changes in the expression and function of transporters. Hepatic drug transporters have been associated with the hepatic toxicity of drugs. Gene polymorphism of transporters and altered transporter expressions and functions due to diseases are found to be susceptible factors for drug-induced liver injury (DILI). In this chapter, the localization of hepatic drug transporters, their regulatory factors, physiological roles, and their roles in drug's liver disposition and DILI are reviewed.

Mechanisms of methotrexate resistance in osteosarcoma cell lines and strategies for overcoming this resistance

The aim of the present study was to investigate the underlying mechanisms of methotrexate (MTX) resistance in the human osteosarcoma cell line, Saos-2/MTX4.4, and to evaluate various methods of overcoming the resistance to this chemotherapeutic agent. MMT assays were performed to determine the resistance of the primary (Saos-2) and resistant (Saos-2/MTX4.4) cell lines to MTX, cisplatin [cis-diamminedichloroplatinum II (DDP)], ifosfamide (IFO), Adriamycin (ADM), epirubicin (EPI) and theprubicin (THP). The Saos-2/MTX4.4 cells exhibited a low resistance to IFO, ADM, EPI and THP; however, no resistance to DDP was identified. Overall, the Saos-2/MTX4.4 cells exhibited a greater resistance to all the chemotherapeutic agents investigated compared with the Saos-2 cells. Rhodamine 123 (R123) fluorescence was measured in the Saos-2/MTX4.4 and Saos-2 cells 30 and 60 min after the addition of R123, and R123 plus verapamil (VER). VER administration increased the intracellular accumulation of R123. In addition, reverse transcription-quantitative polymerase chain reaction was performed to determine the mRNA expression levels of multidrug resistance gene 1 (MDR1) in the two cell lines. Although the Saos-2/MTX4.4 cells were more resistant to the chemotherapeutic agents than the Saos-2 cells, no significant difference was identified between the relative mRNA expression levels of MDR1 in the Saos-2/MTX4.4 and Saos-2 cells (0.4350±0.0354 vs. 0.3886±0.0456; P>0.05).

mRNA levels of related Abcb genes change opposite to each other upon histone deacetylase inhibition in drug-resistant rat hepatoma cells

The multidrug-resistant phenotype of tumor cells is acquired via an increased capability of drug efflux by ABC transporters and causes serious problems in cancer treatment. With the aim to uncover whether changes induced by epigenetic mechanisms in the expression level of drug transporter genes correlates with changes in the drug resistance phenotypes of resistant cells, we studied the expression of drug transporters in rat hepatoma cell lines. We found that of the three major rat ABC transporter genes Abcb1a, Abcb1b and Abcc1 the activity of only Abcb1b increased significantly in colchicine-selected, drug-resistant cells. Increased transporter expression in drug-resistant cells results primarily from transcriptional activation. A change in histone modification at the regulatory regions of the chromosomally adjacent Abcb1a and Abcb1b genes differentially affects the levels of corresponding mRNAs. Transcriptional up- and down-regulation accompany an increase in acetylation levels of histone H3 lysine 9 at the promoter regions of Abcb1b and Abcb1a, respectively. Drug efflux activity, however, does not follow tightly the transcriptional activity of drug transporter genes in hepatoma cells. Our results point out the need for careful analysis of cause-and-effect relationships between changes in histone modification, drug transporter expression and drug resistance phenotypes.

Cytokine-mediated reversal of multidrug resistance

The occurrence of the multidrug resistance phenotype still represents a limiting factor for successful cancer chemotherapy. Numerous efforts have been made to develop strategies for reversal and/or modulation of this major therapy obstacle through targeting at different levels of intervention. The phenomenon of MDR is often associated with overexpression of resistance-associated genes. Since the classical type of MDR in human cancers is mainly mediated by the P-glycoprotein encoded by the multidrug resistance gene 1, mdr1, the majority of reversal approaches target the expression and/or function of the mdr1 gene/P-glycoprotein. Due to the fact that the multidrug phenotype always represents the net effect of a panel of resistance-associated genes/gene products, other resistance genes, e.g. those encoding the multidrug resistance-associated protein MRP or the lung resistance protein LRP, were included in the studies. Cytokines such as tumor necrosis factor alpha and interleukin-2 have been shown to modulate the MDR phenotype in different experimental settings in vitro and in vivo. Several studies have been performed to evaluate their potential as chemosensitizers of tumor cells in the context of a combined application of MDR-associated anticancer drugs like doxorubicin and vincristine with cytokines. Moreover, the capability of cytokines to modulate the expression of MDR-associated genes was demonstrated, either by external addition or by transduction of the respective cytokine gene. Knowledge of the combination effects of cytokines and cytostatics and its link to their MDR-modulating capacity may contribute to a more efficient and to a more individualized immuno-chemotherapy of human malignancies.

Interactions between antidepressants and P-glycoprotein at the blood-brain barrier: clinical significance of in vitro and in vivo findings

The drug efflux pump P-glycoprotein (P-gp) plays an important role in the function of the blood-brain barrier by selectively extruding certain endogenous and exogenous molecules, thus limiting the ability of its substrates to reach the brain. Emerging evidence suggests that P-gp may restrict the uptake of several antidepressants into the brain, thus contributing to the poor success rate of current antidepressant therapies. Despite some inconsistency in the literature, clinical investigations of potential associations between functional single nucleotide polymorphisms in ABCB1, the gene which encodes P-gp, and antidepressant response have highlighted a potential link between P-gp function and treatment-resistant depression (TRD). Therefore, co-administration of P-gp inhibitors with antidepressants to patients who are refractory to antidepressant therapy may represent a novel therapeutic approach in the management of TRD. Furthermore, certain antidepressants inhibit P-gp in vitro, and it has been hypothesized that inhibition of P-gp by such antidepressant drugs may play a role in their therapeutic action. The present review summarizes the available in vitro, in vivo and clinical data pertaining to interactions between antidepressant drugs and P-gp, and discusses the potential relevance of these interactions in the treatment of depression.

P-glycoproteins and other multidrug resistance transporters in the pharmacology of anthelmintics: Prospects for reversing transport-dependent anthelmintic resistance

Parasitic helminths cause significant disease in animals and humans. In the absence of alternative treatments, anthelmintics remain the principal agents for their control. Resistance extends to the most important class of anthelmintics, the macrocyclic lactone endectocides (MLs), such as ivermectin, and presents serious problems for the livestock industries and threatens to severely limit current parasite control strategies in humans. Understanding drug resistance is important for optimizing and monitoring control, and reducing further selection for resistance. Multidrug resistance (MDR) ABC transporters have been implicated in ML resistance and contribute to resistance to a number of other anthelmintics. MDR transporters, such as P-glycoproteins, are essential for many cellular processes that require the transport of substrates across cell membranes. Being overexpressed in response to chemotherapy in tumour cells and to ML-based treatment in nematodes, they lead to therapy failure by decreasing drug concentration at the target. Several anthelmintics are inhibitors of these efflux pumps and appropriate combinations can result in higher treatment efficacy against parasites and reversal of resistance. However, this needs to be balanced against possible increased toxicity to the host, or the components of the combination selecting on the same genes involved in the resistance. Increased efficacy could result from modifying anthelmintic pharmacokinetics in the host or by blocking parasite transporters involved in resistance. Combination of anthelmintics can be beneficial for delaying selection for resistance. However, it should be based on knowledge of resistance mechanisms and not simply on mode of action classes, and is best started before resistance has been selected to any member of the combination. Increasing knowledge of the MDR transporters involved in anthelmintic resistance in helminths will play an important role in allowing for the identification of markers to monitor the spread of resistance and to evaluate new tools and management practices aimed at delaying its spread.

P-glycoprotein: tissue distribution, substrates, and functional consequences of genetic variations

P-glycoprotein (ABCB1, MDR1) belongs to the ABC transporter family transporting a wide range of drugs and xenobiotics from intra- to extracellular at many biological interfaces such as the intestine, liver, blood-brain barrier, and kidney. The ABCB1 gene is highly polymorphic. Starting with the observation of lower duodenal protein expression and elevated digoxin bioavailability in relation to the 3435C>T single nucleotide polymorphism, hundreds of pharmacokinetic and outcome studies have been performed, mostly genotyping 1236C>T, 2677G>T/A, and 3435C>T. Though some studies pointed out that intracellular concentrations of anticancer drugs, for example, within lymphocytes, might be affected by ABCB1 variants resulting in differential outcome, current knowledge of the functional significance genetic variants of ABC membrane transporters does not allow selection of a particular SNP to predict an individual's pharmacokinetics.

Heat-responsive gene expression for gene therapy

Therapy-inducible vectors are useful for conditional expression of therapeutic genes in gene therapy, which is based on the control of gene expression by conventional treatment modalities. By this approach, combination of chemotherapy, radiation or hyperthermia with gene therapy can result in considerable, additive or synergistic improvement of therapeutic efficacy. This concept has been successfully tested in particular for gene therapy of cancer. The identification of efficient heat-responsive gene promoters provided the rationale for heat-regulated gene therapy. The objective of this review is to provide insights into the cellular mechanisms of heat-shock response, as prerequisite for therapeutic actions of hyperthermia and into the field of heat-responsive gene therapy. Furthermore, the major strategies of heat-responsive gene therapy systems in particular for cancer treatment are summarized. The developments for heat-responsive vector systems for in vitro and in vivo approaches are discussed. This review will provide an overview for this gene therapy strategy and its potential for multimodal therapeutic concepts in the clinic.

Pharmacogenetics/genomics of membrane transporters in cancer chemotherapy

Inter-individual variability in drug response and the emergence of adverse drug reactions are main causes of treatment failure in cancer therapy. Recently, membrane transporters have been recognized as an important determinant of drug disposition, thereby affecting chemosensitivity and -resistance. Genetic factors contribute to inter-individual variability in drug transport and targeting. Therefore, pharmacogenetic studies of membrane transporters can lead to new approaches for optimizing cancer therapy. This review discusses genetic variations in efflux transporters of the ATP-binding cassette (ABC) family such as ABCB1 (MDR1, P-glycoprotein), ABCC1 (MRP1), ABCC2 (MRP2) and ABCG2 (BCRP), and uptake transporters of the solute carrier (SLC) family such as SLC19A1 (RFC1) and SLCO1B1 (SLC21A6), and their relevance to cancer chemotherapy. Furthermore, a pharmacogenomic approach is outlined, which using correlations between the growth inhibitory potency of anticancer drugs and transporter gene expression in multiple human cancer cell lines, has shown promise for determining the relevant transporters for any given drugs and predicting anticancer drug response.

Heat-inducible in vivo gene therapy of colon carcinoma by human mdr1 promoter-regulated tumor necrosis factor-alpha expression

The promoter of the human multidrug resistance gene (mdr1) harbors defined heat-responsive elements, which could be exploited for construction of heat-inducible expression vectors. To analyze the hyperthermia inducibility of the mdr1 promoter in vitro and in vivo, we used the pcDNA3-mdrp-hTNF vector construct for heat-induced tumor necrosis factor alpha (TNF-alpha) expression in transfected HCT116 human colon carcinoma cells at mRNA level by quantitative real-time reverse transcription-PCR and at protein level by TNF-alpha ELISA. For the in vitro studies, the pcDNA3-mdrp-hTNF-transfected tumor cells were treated with hyperthermia at 43 degrees C for 2 h. In the animal studies, stably transfected or in vivo jet-injected tumor-bearing Ncr:nu/nu mice were treated for 60 min at 42 degrees C to induce TNF-alpha expression. Both the in vitro and in vivo experiments show that hyperthermia activates the mdr1 promoter in a temperature- and time-dependent manner, leading to an up to 4-fold increase in mdr1 promoter-driven TNF-alpha expression at mRNA and an up to 3-fold increase at protein level. The in vivo heat-induced TNF-alpha expression combined with Adriamycin (8 mg/kg) treatment leads to the inhibition of tumor growth in the animals. These experiments support the idea that heat-induced mdr1 promoter-driven expression of therapeutic genes is efficient and feasible for combined cancer gene therapy approaches.

Role of pharmacogenetics of ATP-binding cassette transporters in the pharmacokinetics of drugs

Interindividual differences of drug response are an important cause of treatment failures and adverse drug reactions. The identification of polymorphisms explaining distinct phenotypes of drug metabolizing enzymes contributed in part to the understanding of individual variations of drug plasma levels. However, bioavailability also depends on a major extent from the expression and activity of drug transport across biomembranes. In particular efflux transporters of the ATP-binding cassette (ABC) family such as ABCB1 (P-glycoprotein, P-gp), the ABCC (multidrug resistance-related protein, MRP) family and ABCG2 (breast cancer resistance protein, BCRP) have been identified as major determinants of chemoresistance in tumor cells. They are expressed in the apical membranes of many barrier tissue such as the intestine, liver, blood-brain barrier, kidney, placenta, testis and in lymphocytes, thus contributing to plasma, liquor, but also intracellular drug disposition. Since expression and function exhibit a broad variability, it was hypothesized that hereditary variances in the genes of membrane transporters could explain at least in part interindividual differences of pharmacokinetics and clinical outcome of a variety of drugs. This review focuses on the functional significance of single nucleotide polymorphisms (SNP) of ABCB1, ABCC1, ABCC2, and ABCG2 in in vitro systems, in vivo tissues and drug disposition, as well as on the clinical outcome of major indications.

MDR-1, but not MDR-3 gene expression, is associated with unmutated IgVH genes and poor prognosis chromosomal aberrations in chronic lymphocytic leukemia

Two P-glycoprotein (P-gp) genes, MDR-1 (ABCB1) and MDR-3 (ABCB4), have been identified in humans. This study was designed to investigate whether associations exist between expression of MDR-1 and MDR-3 P-gp and other markers of poor prognosis and/or prior exposure to therapeutic agents in chronic lymphocytic leukemia (CLL). IgVH mutational status, gene usage, CD38 positivity, FISH analysis and clinical information were available on all patients. Twenty-one of 101 patients tested showed MDR-3 P-gp positivity. Associations with markers of poor prognosis or prior chemotherapy did not reach statistical significance, but MDR-3 P-gp positive patients had significantly shorter survivals than MDR-3 P-gp negative patients. MDR-1 P-gp expression (18/25) showed a strong association with unmutated IgVH genes and adverse prognosis cytogenetics (p = 0.015, p = 0.014, respectively), but was independent of prior exposure to chemotherapeutic agents. These results suggest a role for MDR-1 and MDR-3 in chemoresistant disease. This study highlights the value of determining MDR phenotype in CLL patients prior to treatment, to allow the design of novel drug regimens containing agents that reverse MDR function.

Significant transcriptional down-regulation of the human MDR1 gene by β-naphthoflavone: A proposed hypothesis linking potent CYP gene induction to MDR1 inhibition

Previous work has established the existence of a co-ordinate response in induction between Phase I xenobiotic metabolism, cytochrome P450 (CYP) and the multidrug resistance (MDR1) genes in hepatocytes and some tumor cells. Further correlation was obtained between development of multidrug resistance in cancer cells and a concomitant decrease in inducibility of CYP1A and CYP3A drug metabolizing genes. In the present study, a human MDR1 promoter reporter gene construct was designed to investigate the reverse effect in which selected activators of the major CYP (1-3) genes were tested for potential inhibition of transcriptional activity of the MDR1 gene. beta-naphthoflavone (BNF), a potent CYP1A1 inducer, significantly (P<0.05) down-regulated MDR1 transcriptional activity at 10 microM concentration, causing a 33-fold decrease relative to vector control values. Chemotherapeutic relevance of BNF's transcriptional down-regulation of MDR1 promoter activity was further demonstrated by its restoring 45.86%, and 79.34% drug sensitivity to the resistant MCF-7/Adr cells at 10- and 20 microM concentrations, respectively (P<0.05). A functional linkage between potent induction of the major CYP (1-3) genes and transcriptional down-regulation of MDR1 gene in drug-resistant tumor cells is hereby hypothesized. Steroid and xenobiotic nuclear receptor (SXR) is proposed to mediate the cross-talk between the two genes and to recruit potent CYP gene inducers as co-repressor ligands in effecting its transcriptional down-regulation of MDR1 gene. Implications for the multidrug resistance phenomenon are discussed.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

MDR1 genotype-related pharmacokinetics: fact or fiction?

Multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. A number of various types of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics. The first investigation of the effects of MDR1 genotypes on pharmacotherapy was reported in 2000; a silent single nucleotide polymorphism (SNP), C3435T in exon 26, was found to be associated with the duodenal expression of MDR1, and thereby the plasma concentration of digoxin after oral administration. In the last 5 years, clinical studies have been conducted around the world on the association of MDR1 genotype with MDR1 expression and function in tissues, and with the pharmacokinetics and pharmacodynamics of drugs; however, there are still discrepancies in the results on C3435T. In 1995, a novel concept to predict in vivo oral pharmacokinetic performance from data on in vivo permeability and in vitro solubility has been proposed, and this Biopharmaceutical Classification System strongly suggested that the effects of intestinal MDR1 on the intestinal absorption of substrates is minimal in the case of commercially available oral drugs, and therefore MDR1 genotypes are little associated with the pharmacokinetics after oral administration. This review summarizes the latest reports for the future individualization of pharmacotherapy based on MDR1 genotyping, and attempts to explain discrepancies.

Polymorphism in the P-glycoprotein drug transporter MDR1 gene in colon cancer patients

OBJECTIVE

The P-glycoprotein, a product of MDR1 (multiple drug resistance 1) gene, is a membrane efflux pump localized in epithelial cells in the small and large intestine, a part of the gastrointestinal barrier that protects cells against xenobiotics from our diet, bacterial toxins, drugs and other biologically active compounds, possibly carcinogens. In the present study, an association of MDR1 gene polymorphism and the occurrence of colon cancer were evaluated.

METHODS

The study population consisted of 184 unrelated sporadic colon cancer patients and 188 healthy unrelated controls. Colon cancer patients were also subdivided into two subgroups, i.e., diagnosed before and after 50 years of age, and compared with age-stratified controls. The C3435T MDR1 gene polymorphism was identified using the polymerase chain reaction-restriction fragment length polymorphism method.

RESULTS

The distribution of wild-type and mutated genotypes was similar in the colon cancer patients and in the healthy controls. However, when patients diagnosed before 50 years of age were compared with the healthy population, carriers of MDR1 3435TT genotype or 3435T allele were at 2.7-fold (P<0.05) and 1.7-fold (P<0.05) higher risk of the disease development, respectively.

CONCLUSIONS

Genetic testing for C3435T MDR1 gene polymorphism may be a suitable test to evaluate the risk for colon cancer in patients under 50 years of age.

The MDR1 (ABCB1) gene polymorphism and its clinical implications

There has been an increasing appreciation of the role of drug transporters in the pharmacokinetic and pharmacodynamic profiles of certain drugs. Among various drug transporters, P-glycoprotein, the MDR1 gene product, is one of the best studied and characterised. P-glycoprotein is expressed in normal human tissues such as liver, kidney, intestine and the endothelial cells of the blood-brain barrier. Apical (or luminal) expression of P-glycoprotein in these tissues results in reduced drug absorption from the gastrointestinal tract, enhanced drug elimination into bile and urine, and impeded entry of certain drugs into the central nervous system. The clinical relevance of P-glycoprotein depends on the localisation in human tissues (i.e. vectorial or directional movement), the therapeutic index of the substrate drug and the inherent inter- and intra-individual variability. With regard to the variability, polymorphisms of the MDR1 gene have recently been reported to be associated with alterations in disposition kinetics and interaction profiles of clinically useful drugs, including digoxin, fexofenadine, ciclosporin and talinolol. In addition, polymorphism may play a role in patients who do not respond to drug treatment. Moreover, P-glycoprotein is an important prognostic factor in malignant diseases, such as tumours of the gastrointestinal tract.A growing number of preclinical and clinical studies have demonstrated that polymorphism of the MDR1 gene may be a factor in the overall outcome of pharmacotherapy for numerous diseases. We believe that further understanding the physiology and biochemistry of P-glycoprotein with respect to its genetic variations will be important to establish individualised pharmacotherapy with various clinically used drugs.

[MDR1 genotypes related to pharmacokinetics and MDR1 expression]

The multidrug-resistant transporter encoded by the MDR1 gene belongs to the ATP-binding cassette superfamily of membrane transporters. It is involved not only in the acquisition of multidrug-resistance phenotypes in cancer cells but also in normal tissues such as the brain, kidneys, liver, and intestines. This transporter has the potential to export unnecessary or toxic exogenous substances or metabolites, and in the intestine it is thought to play a role in limiting the oral absorption of a number of structurally unrelated drugs. In 2000, Hoffmeyer et al. performed a systemic screening for MDR1 polymorphisms and suggested that a single-nucleotide polymorphism (SNP) in exon 26 of the MDR1 gene (C3435T) was associated with a lower level of intestinal MDR1 expression, and thereby with lower plasma concentrations of digoxin after oral administration. At present, over 20 SNPs have been found in the MDR1 gene. Clinical studies on the effects of C3435T on MDR1 expression and function in the tissues, and consequently on the pharmacokinetics, have been performed worldwide. In this review, the latest reports concerning the relationship of MDR1 genotypes with pharmacokinetics and MDR1 expression are summarized. Our experimental results demonstrate the importance of genetic polymorphisms at positions 3435 and 2677 in the MDR1 gene on pharmacokinetics and intestinal MDR1 expression. In the future, haplotype analysis of the MDR1 gene and subsequent classification of subjects are needed for individualized pharmacotherapy based on MDR1 genotyping.

Transcriptional regulation of ABC drug transporters

P-glycoprotein, the founding member of the ATP-binding cassette (ABC) family of drug transporters, was first identified almost three decades ago and shown to confer resistance to multiple chemotherapeutic agents when overexpressed in human tumors. Subsequent years have witnessed a tremendous effort to characterize the function and regulation of P-glycoprotein, initially spurred by the hope that its inhibition was the key to overcoming clinical resistance to multiple anticancer agents. However, the identification of MRP1, another member of the ABC drug transporter family, led to the realization that the multidrug resistance (MDR) phenotype is considerably more complex than initially believed. Indeed, at the present time at least 10 members of the ABC transporter family have been implicated in an MDR phenotype, and it is likely that more will be added to this list as studies progress. With this complexity comes the imperative to improve our understanding of the function of individual transporters, as well as to delineate the mechanisms underlying their expression in normal and tumor cells, particularly those that may be amenable to therapeutic intervention. Several articles within this volume address the structure and function of drug transporters. This review will focus on our current understanding of the regulation of ABC drug transporters at the level of transcription.

The association of P-glycoprotein with response to chemotherapy and clinical outcome in patients with osteosarcoma. A meta-analysis

BACKGROUND

There is controversy regarding whether P-glycoprotein (Pgp) may be a prognostic factor for the response to chemotherapy and clinical disease progression in patients with osteosarcoma.

METHODS

The authors conducted a meta-analysis of 14 studies (n = 631 patients) that evaluated the correlation between Pgp and histologic response to chemotherapy and clinical disease progression (death, metastasis, or recurrence). Data were synthesized in receiver operating characteristic curves and with fixed-effects and random-effects likelihood ratios and risk ratios.

RESULTS

Pgp had no discriminating ability for identifying poor responders versus good responders to chemotherapy: The positive likelihood ratio was 1.15 (95% confidence interval [95% CI], 0.93-1.43), and the negative likelihood ratio was 0.88 (95% CI, 0.65-1.18; random-effects calculations). There was some between-study heterogeneity, but no study showed strong discriminating ability. Conversely, Pgp positivity increased the risk of disease progression 1.92-fold (95% CI, 1.18-3.13; random-effects calculations) with some between-study heterogeneity that disappeared when only studies that employed immunohistochemistry were considered (risk ratio, 2.23; 95% CI, 1.37-3.64). The results were robust in various sensitivity analyses, although smaller studies tended to show stronger associations with the risk of disease progression compared with larger studies (P = 0.03).

CONCLUSIONS

The available evidence showed conclusively that Pgp was not associated with the histologic response of patients with osteosarcoma to combination chemotherapy regimens. Conversely, Pgp positivity, as determined by immunohistochemistry, was a strong correlate of more rapid disease progression, although there was heterogeneity across the performed studies that, to some extent, may have reflected bias, differential measurements of Pgp, or confounding with other risk factors.

Pharmacogenetics of MDR1 and its impact on the pharmacokinetics and pharmacodynamics of drugs

The multi-drug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette (ABC) superfamily of membrane transporters. MDR1 was originally isolated from resistant tumor cells as part of the mechanism of multi-drug resistance, but over the last decade, it has been elucidated that human MDR1 is also expressed throughout the body to confer intrinsic resistance to the tissues by exporting unnecessary or toxic exogeneous substances or metabolites. A number of various types of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics and pharmacodynamics. In 2000, Hoffmeyer et al. performed a systemic screening for MDR1 polymorphisms and indicated that a single nucleotide polymorphism (SNP), C3435T in exon 26, which caused no amino acid change, was associated with the duodenal expression of MDR1 and thereby the plasma concentrations of digoxin after oral administration. Interethnic differences in genotype frequencies of C3435T have been clarified, and, at present, a total of 28 SNPs have been found at 27 positions on the MDR1 gene. Clinical studies on the effects of C3435T on MDR1 expression and function in the tissues, and also on the pharmacokinetics and pharmacodynamics have been performed around the world; however, there are still discrepancies in the results, suggesting that the haplotype analysis of the gene should be included instead of SNP detection, and the design of clinical trials must be carefully planned to avoid misinterpretations. A polymorphism of C3435T is also reported to be a risk factor for a certain class of diseases such as the inflammatory bowel diseases, Parkinson's disease and renal epithelial tumor, and this might also be explained by the effects on MDR1 expression and function. In this review, the latest reports are summarized for the future individualization of pharmacotherapy based on MDR1 genotyping.

Basal expression of the multidrug resistance gene 1 (MDR-1) is associated with the TT genotype at the polymorphic site C3435T in mammary and ovarian carcinoma cell lines

Resistance to established drugs for cancer therapy is in many cases associated with overexpression of the multidrug resistance gene 1 (MDR-1). Regulation of basal expression of MDR-1 and mechanisms of induction as a result of exposure to cytotoxic substances are still not completely understood. Recent reports have suggested an association of the C3435T polymorphism in exon 26 of the MDR-1 gene with MDR-1 expression in duodenal mucosa cells of healthy individuals. We analyzed the C3435T and G2677T genotypes of 38 mammary and ovarian carcinoma cell lines and measured basal MDR-1 expression by real-time reverse transcriptase-polymerase chain reaction. Cell lines were classified as non-expressing or showing weak basal expression that was found to be significantly associated (six/seven versus 13/31 expressing cell lines; P=0.0448, Fisher's exact test) with the TT genotype at position 3435 of the MDR-1 gene.

Cancer chemoresistance: the relationship between p53 and multidrug transporters

Extensive studies indicate that both p53 and multidrug transporters play important roles in chemoresistance. Since the initial reports a decade ago demonstrating a transcriptional dependence of the ABCB1 gene (MDR) promoter by p53, much data have been accumulated. However, despite being the subject of intense study, this p53-MDR relationship remains unclear in human cancers. The data are confounded by variable and contrasting results when considering the in vitro regulation and attempting to draw parallels in tissue specimens. The original model suggested that wild-type p53 downregulates the ABCB1 promoter, whereas mutant p53 increases expression of ABCB1. This review summarizes the data for and against this hypothesis. What emerges from these studies is a complex picture, where data have been obtained in support of this hypothesis, but there are also many circumstances where it is not supported. Taken together, these data suggest that the relationship between p53 and multidrug transporters is conditional. It is dependent on cellular environment, the drug used, and the nature of the p53 mutation.

Use of the human MDR1 promoter for heat-inducible expression of therapeutic genes

The promoter of the human multidrug resistance gene (mdr1) harbors stress-responsive elements, which can be induced e.g., by heat or cytostatic drugs. In previous studies the drug-responsiveness of the mdr1 promoter was successfully used for the drug-inducible expression of the human TNF-alpha gene in vitro and in vivo. Beside the drug-responsive elements of the mdr1 promoter, heat-shock responsive elements have also been identified, which could be exploited for construction of heat-inducible expression vectors. To analyze the hyperthermia-inducibility of the mdr1 promoter we used the pmdr-p-CAT and pM3mdr-p-hTNF vector constructs. Both constructs carry the mdr1 promoter fragment spanning from -207 to +153 to drive expression of the CAT-reporter or TNF-alpha gene. We tested the heat-induced CAT-reporter and TNF-alpha expression in vitro in transduced HCT15 and HCT116 human colon carcinoma cells. For the studies the transduced tumor cells were treated with hyperthermia at 41.5 degrees C or 43 degrees C for 2 hr to induce CAT or TNF-alpha expression. Cells and supernatants were harvested before hyperthermia and at certain time points (0-120 hr) after heat shock. The heat-induced CAT-reporter expression or TNF-alpha secretion was determined by specific ELISA. The experiments indicate that hyperthermia activates the mdr1 promoter in a temperature and time dependent manner. This induction leads to an 2- to 4-fold increase in CAT-reporter or 2- to 7-fold increase in TNF alpha expression in the tumor cell lines. These experiments reveal that the mdr1 promoter driven expression of therapeutic genes can be employed for combined cancer gene therapy approaches.

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.

Regulation of MDR1 gene expression: emerging concepts

Drug resistance genes, such as MDR1, involved in drug efflux, and their regulation have been the subject of intense research efforts in the past 10 years. Many factors and cellular signalling pathways play a role in the regulation of MDR1 gene expression. Commonly used chemotherapeutic agents activate in vitro and in vivo general stress response pathways, potential targets of which include MDR1 and other drug resistance genes. The contribution of these agents to the emergence of drug-resistant tumour cells is of concern. Recent evidence points to a role for the epigenetic regulation of MDR1 gene expression. The identification of key components in the DNA methylation/chromatin system of gene regulation may in time lead to more informed and targeted approaches to treating drug-resistant tumours. Copyright 2000 Harcourt Publishers Ltd.

Strength and specificity of different gene promoters in oral cancer cells

Gene therapy of oral cancer will require expression of genes by promoters that are both powerful and relatively tumor specific. We compared the level of expression of a reporter gene from promoters of human cytomegalovirus (CMV), SV40 virus, mouse mammary tumor virus (MMTV), human papillomaviruses (HPV) types 16 and 18, and the human multi-drug-resistance gene (mdr1), in several lines of oral cancer cells. In the oral cancer cell line 686LN the rank order of expression levels was: CMV > SV40 > HPV > mdr1 > MMTV. Unlike in previous reports the mdr1 promoter was no more active in two cancer cell lines with mutations in the p53 gene than in two other lines with wild-type p53, and its expression level could not be increased by either doxorubicin or taxol. On the other hand, expression from the MMTV promoter was increased over 10-fold by the presence of 1 microM dexamethasone. Thus, by an appropriate choice of promoter and inducer a wide variety of expression levels, over a 3-log range, could be attained in 686LN cells. The oral cancer-specificity of each promoter was judged by comparing expression in the neuroblastoma line IMR32. The most specific promoters were those from papillomaviruses, which were up to 45 times more active in the oral cancer cells, and the least specific was the CMV promoter. In order to find if an HPV-derived promoter was sufficient to produce expression of a suicide phenotype the 686 promoter was cloned adjacent to the thymidine kinase gene of herpes simplex and the construct was expressed from an adenovirus vector. The vector reduced the growth of 686LN cells over a 5-day period by up to 32% when optimal concentrations of virus and ganciclovir were used. These data will be valuable in the design of new constructs for gene therapy of oral cancer.

A mutation in the promoter of the multidrug resistance gene (MDR1) in human hematological malignancies may contribute to the pathogenesis of resistant disease

The overexpression of the multidrug resistance gene MDR1 has been found to be associated with therapy-resistance in hematological malignancies. Yet the cellular mechanisms underlying this increased expression are completely unknown. Point mutations in the MDR1 promoter have been found in osteogenic sarcoma (Stein et al., Eur J of Cancer, 30A: 1541-1545, 1994). We therefore analyzed DNA from hematological malignancies for MDR1 promoter point mutations. Two pairs of overlapping PCR primers were designed which did not amplify the MDR3 gene. Amplified DNA was screened using single strand conformation polymorphism (SSCP). 139 patients and 93 normal controls were studied. Fifteen patients (11%) were found to have abnormal bands on the SSCP analysis. Of these, 9 had acute myeloid leukemia (AML), 4 chronic lymphocytic leukemia (CLL), 1 acute lymphocytic leukemia (ALL), and 1 nonHodgkin's lymphoma (NHL). Sequence analysis revealed that all patients were heterozygous for a point mutation in the promoter (T-C transition at +8). Four normals (4%) were found to be heterozygous for the mutation. Confirmation of the mutation was performed by oligonucleotide probe hybridization. All but two of the AML patients have died due to chemoresistant disease (one is lost to followup). Of the CLL patients, one is alive with progressive disease, and the others have died. Further studies will assess the effect of this mutation on MDR1 gene transcription.

Overexpression of lung-resistance protein and increased P-glycoprotein function in acute myeloid leukaemia cells predict a poor response to chemotherapy and reduced patient survival

We investigated the role of the drug resistance-related proteins LRP, MRP and Pgp and the apoptotic suppressor, bcl-2, in relation to other clinical characteristics, with respect to response and survival in 91 patients with newly diagnosed AML, treated with standard chemotherapy. Multivariate analysis showed that poor response to chemotherapy was associated with increasing age (P=0.0004), LRP expression (P=0.0001) and Pgp function (P=0.015). The significant predictors of both leukaemia-free survival (LFS) and overall survival (OS) were LRP (LFS, P=0.01; OS, P=0.0001), Pgp function (LFS, P=0.0001; OS, P=0.0003) and cytogenetic abnormalities (LFS, P=0.0001; OS. P=0.0005). Patients with the lowest expression of LRP and Pgp function and favourable karyotype (group I) had an LFS of 30.2 months compared to 8 5 months in the group with the highest expression of LRP and Pgp and poor prognosis karyotype (group III, P=0.002). OS decreased from 75.4 months in group I to 7.9 months in group III patients (P <0.0001). Neither MRP nor bcl-2 were significantly associated with chemotherapy response and survival. Correlations were found between increasing expression of LRP and older age (P=0.05) and an unfavourable karyotype (P=0.005), but these variables were independent of each other in analysis of treatment response and patient survival. Our findings suggest that both LRP and Pgp are clinically relevant drug-resistance proteins and it may be necessary to modulate both LRP and Pgp functions in order to reverse the multidrug resistance phenotype in AML.

Targeted vectors for gene therapy of cancer and retroviral infections

Gene therapy has developed to a technology which rapidly moved from the laboratory bench to the bedside in the clinic. This implies safe, efficient and targeted gene transfer systems for suitable application to the patient. Beside the development of such gene transfer vectors of viral or nonviral origin, improvement of cell type specific and inducible gene expression is pivotal for successful gene therapy leading to targeted gene action. Numerous gene therapy approaches for treatment of cancer and retroviral infections utilize cell type specific and/or regulatable promoter and enhancer sequences for the selective expression of therapeutic genes in the desired cell populations and tissues. In this article the recent developments and the potential of expression targeting are reviewed for gene therapy approaches of cancer and retroviral infections.

Vincristine induction of mutant and wild-type human multidrug-resistance promoters is cell-type-specific and dose-dependent

To investigate multidrug-resistance gene (MDR1) promoter efficacy and drug inducibility in cells with different multidrug-resistance phenotypes, multidrug-resistant HCT15 and drug-sensitive KM12 human colon carcinoma cell lines were transfected with constructs incorporating the chloramphenicol acetyltransferase (CAT) reporter gene, driven by wild-type and point-mutated MDR1 promoter regions. The basal CAT expression level in HCT15 cells was markedly elevated compared to KM12 cells. CAT induction by vincristine was dose-dependent over a broad concentration range (40-500 ng/ml) in both lines. The induction levels were related to the cells' MDR phenotype, with the multidrug-resistant HCT15 cells showing the greater effect. In both cell types, basal and drug-induced CAT expression were significantly enhanced by the point-mutated promoter regions. The findings support the possible exploitation of the MDR1 promoter for construction of drug-inducible and MDR-cell-targeted expression vectors for use in gene therapy.

MDR1 gene expression: evaluation of its use as a molecular marker for prognosis and chemotherapy of bone and soft tissue sarcomas

Successful chemotherapeutic treatment of malignant tumours is often limited by the intrinsic or acquired multidrug resistance (MDR). The classical MDR phenotype is characterised by reduced drug accumulation within the cell, caused by overexpression of the MDR1 gene encoded P-glycoprotein. Some reports have been published evaluating MDR1 expression as a molecular marker for response to chemotherapy in human bone and soft tissue sarcomas. In this review, an attempt is made to summarise the accuracy of the measurement of MDR1 expression for use in prognosis, as well as in decisions on chemotherapeutic treatment of sarcomas. In addition, general problems for the performance of such studies is discussed.