Purification and characterization of NF-R1 that regulates the expression of the human multidrug resistance (MDR1) gene

Resveratrol induces AMPK-dependent MDR1 inhibition in colorectal cancer HCT116/L-OHP cells by preventing activation of NF-κB signaling and suppressing cAMP-responsive element transcriptional activity

Resveratrol, a natural polyphenolic compound found in foods and beverages, has attracted increasing attention in recent years because of its potent chemopreventive and anti-tumor effects. In this study, the effects of resveratrol on the expression of P-glycoprotein/multi-drug resistance protein 1 (P-gp/MDR1), and the underlying molecular mechanisms, were investigated in oxaliplatin (L-OHP)-resistant colorectal cancer cells (HCT116/L-OHP). Resveratrol downregulated MDR1 protein and mRNA expression levels and reduced MDR1 promoter activity. It also enhanced the intracellular accumulation of rhodamine 123, suggesting that resveratrol can reverse multi-drug resistance by downregulating MDR1 expression and reducing drug efflux. Resveratrol treatment also reduced nuclear factor-κB (NF-κB) activity, reduced phosphorylation levels of IκBα, and reduced nuclear translocation of the NF-κB subunit p65. Moreover, downregulation of MDR1 expression and promoter activity was mediated by resveratrol-induced AMP-activated protein kinase (AMPK) phosphorylation. The inhibitory effects of resveratrol on MDR1 expression and cAMP-responsive element-binding protein (CREB) phosphorylation were reversed by AMPKα siRNA transfection. We found that the transcriptional activity of cAMP-responsive element (CRE) was inhibited by resveratrol. These results demonstrated that the inhibitory effects of resveratrol on MDR1 expression in HCT116/L-OHP cells were closely associated with the inhibition of NF-κB signaling and CREB activation in an AMPK-dependent manner.

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.

Haplotype-Oriented Genetic Analysis and Functional Assessment of Promoter Variants in theMDR1(ABCB1) Gene

Recently, a number of nucleotide variants have been described in the multidrug resistance 1 (MDR1/ABCB1) gene; however, most studies have focused on the coding region. In the present study, we identified promoter variants of the MDR1 gene and evaluated their phenotypic consequences using a reporter gene assay and the real-time polymerase chain reaction method. Ten allelic variants were detected in the promoter region (approximately 2 kilobases), seven of which were newly identified. Certain mutations occurred simultaneously, and a total of 10 haplotypes were observed. These promoter polymorphisms were found more frequently in Japanese than Caucasians. Some haplotypes were associated with changes in luciferase activity and placental and hepatic mRNA levels. We also determined DNA methylation status in the proximal promoter region of the MDR1 gene. The promoter region around potential binding sites for transcription factors was found to be hypomethylated and thus likely to be independent of the gene expression. Nucleotide and/or haplotype variants not only in the coding region but also in the promoter region of the MDR1 gene may be important for interindividual differences of P-glycoprotein expression.

CCAAT/Enhancer-Binding Protein β (Nuclear Factor for Interleukin 6) Transactivates the HumanMDR1 Gene by Interaction with an Inverted CCAAT Box in Human Cancer Cells

We investigated the mechanisms of MDR1 gene activation by CCAAT/enhancer binding protein beta (C/EBPbeta, or nuclear factor for interleukin 6) in human cancer cells. Transfection of the breast cancer cell line MCF-7 and its doxorubicin-selected variant MCF-7/ADR by either C/EBPbeta or C/EBPbeta-LIP (a dominant-negative form of C/EBPbeta) confirmed their roles in the activation or repression of the endogenous, chromosomally embedded MDR1 gene. Cotransfection experiments with promoter constructs revealed a C/EBPbeta interaction on the MDR1 promoter via the region within -128 to -75. Deletions within the putative AP-1 box (-123 to -111) increased MDR1 promoter activity when stimulated by C/EBPbeta, suggesting that the AP-1 site negatively regulates MDR1 activation by C/EBPbeta. Mutations within the inverted CCAAT box (Y box) (-82 to -73) abolished the C/EBPbeta-stimulated MDR1 promoter activity, indicating that the Y box is required for MDR1 activation by C/EBPbeta. Chromatin immunoprecipitation (ChIP) revealed that C/EBPbeta precipitates a transcription complex containing C/EBPbeta, the MDR1 promoter sequences (-250 to +54), and the hBrm protein. In conclusion, alteration of expression or function of C/EBPbeta plays an important role in MDR1 gene regulation. C/EBPbeta activates the endogenous MDR1 gene of MCF-7 cells, and this activation was associated with a novel C/EBPbeta interaction region within the proximal MDR1 promoter (-128 to -75). The mechanisms of MDR1 activation by C/EBPbeta include C/EBPbeta binding of the chromatin of the MDR1 gene and interactions of C/EBPbeta with the Y box and Y box-associated proteins.

The rise of DNA methylation and the importance of chromatin on multidrug resistance in cancer

In recent years, the different classes of drugs and regimens used clinically have provided an improvement in tumour management. However, treatment is often palliative for the majority of cancer patients. Transformed cells respond poorly to chemotherapy mainly due to the development of the multidrug resistance (MDR) phenotype. Response to treatment does not generally result in complete remission and disease cure is uncommon for patients presenting with advanced stage cancer. Successful treatment of cancer requires a clearer understanding of chemotherapeutic resistance. Here, we examine what is known of one of the most extensively studied mechanisms of cellular drug resistance. The human multidrug resistance gene 1 (MDR1) is associated with expression of p-glycoprotein (Pgp). A transmembrane protein, Pgp acts as an efflux pump and reduces intracellular drug levels and thus its effectiveness as an antitumor agent. The precise mechanism of transcriptional regulation has been unclear due to the complex regulatory nature of the gene. It has become increasingly apparent that trans-activation or genetic amplification is by no means the only mechanism of activation. Consequently, alternative pathways have received more attention in the area of epigenetics to help explain transcriptional competence at a higher level of organization. The goal of this article is to highlight important findings in the field of methylation and explain how they impinge on MDR1 gene regulation. In this review, we cover the current information and postulate that epigenetic modification of MDR1 chromatin influences gene transcription in leukaemia. Finally, we explore transcriptional regulation and highlight recent progress with engineered ZFP's (zinc finger proteins).

NF-kappaB transcription factor induces drug resistance through MDR1 expression in cancer cells

The ubiquitous NF-kappaB transcription factor has been reported to inhibit apoptosis and to induce drug resistance in cancer cells. Drug resistance is the major reason for cancer therapy failure and neoplastic cells often develop multiple mechanisms of drug resistance during tumor progression. We observed that NF-kappaB or P-glycoprotein inhibition in the HCT15 colon cancer cells led to increased apoptotic cell death in response to daunomycin treatment. Interestingly, NF-kappaB inhibition through transfection of a plasmid coding for a mutated IkappaB-alpha inhibitor increased daunomycin cell uptake. Indeed, the inhibition of NF-kappaB reduced mdr1 mRNA and P-glycoprotein expression in HCT15 cells. We identified a consensus NF-kappaB binding site in the first intron of the human mdr1 gene and demonstrated that NF-kappaB complexes could bind with this intronic site. Moreover, NF-kappaB transactivates an mdr1 promoter luciferase construct. Our data thus demonstrate a role for NF-kappaB in the regulation of the mdr1 gene expression in cancer cells and in drug resistance.

Transcriptional regulators of the human multidrug resistance 1 gene: recent views

The multidrug resistance (MDR) phenotype is the major cause of failure of cancer chemotherapy. This phenotype is mainly due to the overexpression of the human MDR1 (hMDR1) gene. Several studies have shown that transcriptional regulation of this gene is unexpectedly complex and is far from being completely understood. Current work is aimed mainly at defining unclear and new control regions in the hMDR1 gene promoter as well as clarifying corresponding signaling pathways. Such studies provide new insights into the mechanisms by which xenobiotic molecules might modify the physiological hMDR1 expression as well as the possible role of oncogenes in the pathological dysregulation of the gene. Here we report recent findings on the regulation of hMDR1 which may help define specific targets aimed at modulating its transcription.

Transcriptional regulation of MDR genes

The emergence of resistance in a tumor population is most often associated with a disregulation of gene expression, usually at the level of transcription. A major goal in the field of cancer chemotherapy is to define the mechanisms underlying transcriptional regulation of drug resistance genes in an effort to identify targets for therapeutic intervention. Recently, considerable progress has been made in identifying the molecular mechanisms involved in the transcriptional regulation of the P-glycoprotein (Pgp) gene. When overexpressed in tumor cells, Pgp confers resistance to a variety of chemotherapeutic agents; this resistance has been termed MDR (multidrug resistance). Moreover, Pgp is a normal component of a variety of highly differentiated cell types and, as such, is regulated by both internal and external environmental stimuli. In this review, we will discuss the current knowledge regarding the DNA elements and protein factors involved in both constitutive and inducible regulation of Pgp transcription in normal and tumor cells.

Demethylation of the human MDR1 5' region accompanies activation of P-glycoprotein expression in a HL60 multidrug resistant subline

Chemotherapy is frequently limited by the development of multidrug resistance, a major cause of which is activation of the P-glycoprotein-encoding MDR1 gene. We have previously developed a P-glycoprotein-expressing multidrug resistant subline (HL60/E8) from the non-P-glycoprotein-expressing human HL60 promyelocytic leukemia cell line. A possible cause of MDR1 silencing in HL60 cells is methylation of the promoter proximal region, thus demethylation occurring as a result of drug treatment may be responsible for MDR1 activation in the multidrug resistant subline. Using the bisulphite genomic sequencing technique we demonstrated that HL60 DNA is methylated at multiple sites within two distinct areas, one upstream and one downstream of the transcription start point. Only a single site in each area was methylated in all strands examined, with the remaining adjacent sites showing partial methylation. In contrast, DNA from the multidrug resistant HL60/E8 subline was unmethylated at essentially all sites in both areas. Thus the development of the P-glycoprotein-expressing multidrug resistant subline was associated with demethylation of the MDR1 5' region.

Biochemical, genetic, and metabolic adaptations of tumor cells that express the typical multidrug-resistance phenotype. Reversion by new therapies

Among the genetic and metabolic alterations that cancer cells undergo, several allow their survival under extreme environmental conditions. The resulting aberrant metabolism is compatible with tumor progression at the expenses of high energy needs, especially for maintaining high division rate. When treated with chemotherapeutic drugs many cancer cells take advantage of their ability to develop a resistance phenotype, as part of an adaptative mechanism. Two main actors of this multidrug phenotype (MDR) are represented by the P-glycoprotein and by the more recently discovered multidrug-resistance associated protein (MRP), two membrane proteins of the ABC superfamily of transporters that can extrude chemotherapeutic drugs under an ATP-dependent mechanism. We will briefly review the major metabolic aberrations that several cancers develop, followed by the molecular, genetic, structural, and functional aspects related mainly to P-glycoprotein, with a concern for the regulation of mdr gene expression. We will point out the role that membrane cholesterol may play in the MDR phenotype, relate this phenotype to bioenergetic considerations, and review the ways of modulating it by the use of new therapeutic approaches.

Tumor necrosis factor-alpha and expression of the multidrug resistance-associated genes LRP and MRP

BACKGROUND AND PURPOSE

Cancer cells that express P-glycoprotein, multidrug resistance-associated protein (MRP), or lung resistance protein (LRP) have demonstrated resistance to a wide variety of chemotherapeutic drugs. Recently, we reported that human colon carcinoma cells that express all three proteins exhibit reduced P-glycoprotein gene expression and a loss of multidrug resistance after exposure to tumor necrosis factor-alpha, a hormone-like protein produced by cells of the immune system. In this study, we examined the effects of tumor necrosis factor-alpha on MRP and LRP gene expression in the same colon carcinoma cells.

METHODS

HCT15 and HCT116 colon carcinoma cells were incubated with tumor necrosis factor-alpha at 100 U/mL for 2, 12, 24, 48, or 72 hours; alternatively, cells transfected with an expression vector containing a human tumor necrosis factor-alpha complementary DNA were studied. The effects of tumor necrosis factor-alpha on MRP and LRP messenger RNA expression were evaluated by means of reverse transcription and the polymerase chain reaction; effects on MRP and LRP protein expression were examined by use of specific monoclonal antibodies and flow cytometry. The flow cytometry data were analyzed by use of the two-sided, nonparametric Mann-Whitney rank sum test.

RESULTS

Treatment with exogenous tumor necrosis factor-alpha reduced the level of LRP messenger RNA in both cell types in an apparently time-dependent fashion; in HCT15 cells, almost no LRP messenger RNA was detected after 48 hours of treatment. In contrast, the level of MRP messenger RNA was increased in HCT116 cells by such treatment, but the level in HCT15 cells was unchanged. Treatment with exogenous tumor necrosis factor-alpha induced changes in LRP and MRP protein expression in the two cell types that paralleled the changes found for messenger RNA. In transfected cells, the endogenous production of tumor necrosis factor-alpha reduced LRP gene expression (both messenger RNA and protein) and increased MRP gene expression (both messenger RNA and protein), regardless of cell type.

CONCLUSION

In human colon carcinoma cells, tumor necrosis factor-alpha influences MRP and LRP gene expression in opposite ways. The findings for LRP gene expression parallel our earlier findings for P-glycoprotein expression in these cells.

IMPLICATION

In developing strategies for overcoming multidrug resistance in tumor cells, the possibility that an agent can suppress one or more mechanisms of drug resistance and enhance others should be considered.

DNA elements recognizing NF-Y and Sp1 regulate the human multidrug-resistance gene promoter

Regulation of the human multidrug resistance gene (hMDR1) was studied by mapping DNA elements in the proximal promoter necessary for efficient transcription. Transient transfection analysis in tumor cell lines (HCT116, HepG2, and Saos2) of promoter deletions identified several regulatory domains. These cell lines expressed hMDR1 mRNA. Removal of an element between +25 and +158 reduced promoter activity by 2-3-fold, whereas deletion of sequences from approximately -5000 to -138 base pairs gave a approximately 2-fold increase. The activity of the hMDR1 promoter (-137 to +25) was comparable in activity to the SV40 early promoter and enhancer combination. Deletion of the hMDR1 promoter between -86 and -44 reduced activity by 5-10-fold, identifying an important regulatory region. This minimal region (-88 to -37) activated transcription when inserted upstream of a synthetic promoter, suggesting that it acts independently of other regulatory sequences. Two DNA elements within 85 base pairs of the transcriptional start site were required to confer efficient gene expression. A double-point mutation in the Y box (inverted CCAAT box) between -70 and -80 reduced activity of the promoter by 5-10-fold, and a single-point mutation at -52 within a GC-rich element reduced activity by 3-fold. Thus, both the Y-box and GC elements must each remain intact for optimal promoter activity. DNA-binding analyses suggest that the transcription factor NF-Y, but not YB-1 or c/EBP, is most likely responsible for controlling the activity of the Y-box element in these tumor cell lines. DNA-binding analyses also suggest that Sp1, alone or in combination with other nuclear factors, likely controls the activity of the GC element.

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.

Regulation of multidrug resistance through the cAMP and EGF signalling pathways

The development of cross-resistance to many natural product anticancer drugs, termed multidrug resistance (MDR), is a serious limitation to cancer chemotherapy. MDR is often associated with overexpression of the MDR1 gene product, P-glycoprotein, a multifunctional drug transporter. Understanding the mechanisms that regulate the transcriptional activation of MDR1 may afford a means of reducing or eliminating MDR. We have found that MDR1 expression can be modulated by type I cAMP-dependent protein kinase (PKA). This suggests that MDR may be modulated by selectively downregulating PKA activity to effect inhibition of PKA-dependent trans-activating factors which may be involved in MDR1 transcription. High levels of type I PKA occur in primary breast carcinomas and patients exhibiting this phenotype show decreased survival. The selective type I PKA inhibitors, 8-Cl-cAMP and Rp8-Cl-cAMP[S], may be particularly useful for downregulating PKA, and inhibit transient expression of a reporter gene under the control of MDR1 promoter elements. Thus, investigations of the signalling pathways involved in transcriptional regulation of MDR1 may lead to a greater understanding of the mechanisms governing the expression of MDR and provide a focus for pharmacological intervention.

Characterization of the basal and carcinogen regulatory elements of the rat mdr1b promoter

In this report we characterized the transcriptional regulation of the rat mdr1b gene by xenobiotics. The expression of this gene was increased in primary rat hepatocytes and in the H4-II-E hepatoma cell line by exposure to carcinogens such as aflatoxin B1, N-acetoxy-2-acetylaminofluorene, and methyl methanesulfonate. Nuclear run-on experiments indicated that the higher steady-state levels of mdr1b mRNA were due to an increase in transcription. The 5'-flanking region of the mdr1b gene was isolated, sequenced, and functionally characterized in transient and stable transfection assays. A single transcription start site was identified for this gene; no alternate start sites were used after induction with aflatoxin B1. Deletion analysis of this promoter demonstrated that the sequence between nt -214 and -178 was critical for basal promoter activity. This region did not contain any consensus-binding sites for previously identified transcription factors. A negative regulatory region was also identified between nt -940 and -250. No specific carcinogen-responsive element was identified; the xenobiotic response required a large part of the promoter. These data suggest that the carcinogen induction of mdr1b expression is mediated through sequences that overlap or that are identical to the basal promoter element.

Drug-induced changes in the expression of MDR-associated genes: investigations on cultured cell lines and chemotherapeutically treated leukemias

The induced expression of multiple drug resistance (MDR)-associated genes as a direct response of tumor cells to antineoplastic drugs could be an important factor influencing the success of cancer chemotherapy. We investigated the effects of such compounds on mdr1/P-glycoprotein (P-gp) gene expression and drug sensitivities in the T-lymphoblastoid human cell line CCRF-CEM and MDR sublines. Thereby, we observed that actinomycin D or adriamycin administered at sublethal concentrations induced increases of mdr1 mRNA levels and resistance within 72 h. Furthermore, on leukemia cell samples collected before and after chemotherapy we checked by a complementary DNA polymerase chain reaction (cDNA-PCR) approach for similar alterations in the relative expression levels of the MDR-associated genes (a) mdr1/P-gp (b) mrp (MDR related protein), and (c) the topoisomerase II isoforms alpha and beta. We found a concomitant increase in mdr1 and mrp gene expression combined with a decreased expression of topoisomerase II alpha in the course of the second relapse of an acute lymphoblastic leukemia (ALL). This points to the emergence of at least three different MDR mechanisms in this type of leukemia unresponsive to chemotherapy. A chronic myeloid leukemia (CML) in blast crisis, however, showed combined increases in mdr1 (about 20-fold) and mrp (about four fold) gene expression after intense but unsuccessful chemotherapy over a 6-month period. Our results indicate the occurrence of induced resistance in vitro and in vivo and suggest a contribution of the newly identified ATP-binding cassette (ABC) transporter MRP in MDR.

Transcriptional regulation of multidrug resistance in breast cancer

The development of cross-resistance to many natural product anticancer drugs, termed multidrug resistance (MDR), is one of the major reasons why cancer chemotherapy ultimately fails. This type of MDR is often associated with over-expression of the MDR1 gene product, P-glycoprotein (Pgp), a multifunctional drug transporter. The expression of MDR in breast tumors is related to their origination from a tissue that constitutively expresses Pgp as well as to the development of resistance during successive courses of chemotherapy. Therefore, understanding the mechanisms that regulate the transcriptional activation of MDR1 may afford a means of reducing or eliminating MDR. We have found that MDR1 expression can be modulated by type I cAMP-dependent protein kinase (PKA), opening up the possibility of modulating MDR by selectively down-regulating the activity of PKA-dependent transcription factors which upregulate MDR1 expression. High levels of type I PKA occurs in primary breast carcinomas and patients exhibiting this phenotype show decreased survival. The selective type I cAMP-dependent protein kinase (PKA) inhibitors, 8-Cl-cAMP and Rp8-Cl-cAMP[S] may be particularly useful for downregulating PKA-dependent MDR-associated transcription factors, and we have found these compounds to downregulate transient expression of a reporter gene under the control of several MDR1 promoter elements. Thus, investigations of this nature should not only lead to a greater understanding of the mechanisms governing the expression of MDR, but also provide a focus for pharmacologic intervention by a new class of inhibitors.

Multidrug resistance during cancer chemotherapy--biotechnological solutions to a clinical problem

Tumour cells can be resistant to a variety of chemotherapeutic drugs of different structure (multidrug resistance) by expressing a transmembrane pump (P-glycoprotein) on their cell surface. This situation can lead to a failure of cancer chemotherapy as the P-glycoprotein acts by actively pumping the drugs out of cells, thus lowering the intracellular concentration of the drug and, hence, its cytotoxic effectiveness. This review summarizes present and proposed approaches to preventing or circumventing the action of this drug-transporting protein.

The multidrug resistance P-glycoprotein

P-glycoprotein plays an important role in the resistance of cancers to chemotherapy. Thus, an understanding of the mechanism by which it functions, and its 'normal' physiological role, is of clinical relevance as well as intrinsic interest. Considerable progress towards this goal has been made in the last year or so. In addition, the finding that P-glycoprotein is associated with both a channel and a transporter activity has, potentially, far-reaching implications for an understanding of the nature of channels and transporters.

Purification and characterization of NF-R2 that regulates the expression of the human multidrug resistance (MDR1) gene

NF-R2 is a DNA-binding protein that interacts with the MDR1 gene proximal promoter sequence. We previously reported that NF-R2 binds within the promoter's -126 and -102 regions, which contain the ATTCAGTCA motif. In the present study, we have purified NF-R2 from the nuclear extract of K562/ADM cells, a multidrug-resistant cell line derived from human myelogenous leukemia K562 cells, using sequential chromatography on Sephacryl S-300, DEAE-Sepharose, heparin-Sepharose and a DNA affinity column consisting of a repetitive synthetic ATTCAGTCA motif coupled to Sepharose. NF-R2 runs as a single protein of 75 kDa on SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis). CAT (chloramphenicol acetyltransferase) expression assay and gel mobility shift competition assay with mutated promoters revealed that the ATTCAGTCA motif is a positive regulatory element of MDR1 gene and that the motif is important for NF-R2 binding. These results suggest that NF-R2 may be involved in the positive regulation of the MDR1 gene transcription.

Pharmacologic circumvention of multidrug resistance

The ability of malignant cells to develop resistance to chemotherapeutic drugs is a major obstacle to the successful treatment of clinical tumors. The phenomenon multidrug resistance (MDR) in cancer cells results in cross-resistance to a broad range of structurally diverse antineoplastic agents, due to outward efflux of cytotoxic substrates by the mdr1 gene product, P-glycoprotein (P-gp). Numerous pharmacologic agents have been identified which inhibit the efflux pump and modulate MDR. The biochemical, cellular and clinical pharmacology of agents used to circumvent MDR is analyzed in terms of their mechanism of action and potential clinical utility. MDR antagonists, termed chemosensitizers, may be grouped into several classes, and include calcium channel blockers, calmodulin antagonists, anthracycline and Vinca alkaloid analogs, cyclosporines, dipyridamole, and other hydrophobic, cationic compounds. Structural features important for chemosensitizer activity have been identified, and a model for the interaction of these drugs with P-gp is proposed. Other possible cellular targets for the reversal of MDR are also discussed, such as protein kinase C. Strategies for the clinical modulation of MDR and trials combining chemosensitizers with chemotherapeutic drugs in humans are reviewed. Several novel approaches for the modulation of MDR are examined.