What does Multidrug Resistance (MDR) Expression Mean in the Clinic?

Potential of Natural Products in the Treatment of Glioma: Focus on Molecular Mechanisms

Despite the waning of traditional treatments for glioma due to possible long-term issues, the healing possibilities of substances derived from nature have been reignited in the scientific community. These natural substances, commonly found in fruits and vegetables, are considered potential alternatives to pharmaceuticals, as they have been shown in prior research to impact pathways surrounding cancer progression, metastases, invasion, and resistance. This review will explore the supposed molecular mechanisms of different natural components, such as berberine, curcumin, coffee, resveratrol, epigallocatechin-3-gallate, quercetin, tanshinone, silymarin, coumarin, and lycopene, concerning glioma treatment. While the benefits of a balanced diet containing these compounds are widely recognized, there is considerable scope for investigating the efficacy of these natural products in treating glioma.

Circulating biosignatures in multiple myeloma and their role in multidrug resistance

A major obstacle to chemotherapeutic success in cancer treatment is the development of drug resistance. This occurs when a tumour fails to reduce in size after treatment or when there is clinical relapse after an initial positive response to treatment. A unique and serious type of resistance is multidrug resistance (MDR). MDR causes the simultaneous cross resistance to unrelated drugs used in chemotherapy. MDR can be acquired through genetic alterations following drug exposure, or as discovered by us, through alternative pathways mediated by the transfer of functional MDR proteins and nucleic acids by extracellular vesicles (M Bebawy V Combes E Lee R Jaiswal J Gong A Bonhoure GE Grau, 23 9 1643 1649, 2009).Multiple myeloma is an incurable cancer of bone marrow plasma cells. Treatment involves high dose combination chemotherapy and patient response is unpredictable and variable due to the presence of multisite clonal tumour infiltrates. This clonal heterogeneity can contribute to the development of MDR. There is currently no approved clinical test for the minimally invasive testing of MDR in myeloma.Extracellular vesicles comprise a group of heterogeneous cell-derived membranous structures which include; exosomes, microparticles (microvesicles), migrasomes and apoptotic bodies. Extracellular vesicles serve an important role in cellular communication through the intercellular transfer of cellular protein, nucleic acid and lipid cargo. Of these, microparticles (MPs) originate from the cell plasma membrane and vary in size from 0.1-1um. We have previously shown that MPs confer MDR through the transfer of resistance proteins and nucleic acids. A test for the early detection of MDR would benefit clinical decision making, improve survival and support rational drug use. This review focuses on microparticles as novel clinical biomarkers for the detection of MDR in Myeloma and discusses their role in the therapeutic management of the disease.

Multifunctional Molecular Beacon Micelles for Intracellular mRNA Imaging and Synergistic Therapy in Multidrug-Resistant Cancer Cells

Multidrug resistance (MDR) resulting from overexpression of P-glycoprotein (Pgp) transporters increases the drug efflux and thereby limits the chemotherapeutic efficacy. It is desirable to administer both an MDR1 gene silencer and a chemotherapeutic agent in a sequential way to generate a synergistic therapeutic effect in multidrug-resistant cancer cells. Herein, we rationally designed an anti-MDR1 molecular beacon (MB)-based micelle (a-MBM) nanosystem, which is composed of a diacyllipid core densely packed with an MB corona. One of Pgp-transportable agents, doxorubicin (DOX), was encapsulated in the hydrophobic core of the micelle and in the stem sequence of MB. The a-MBM-DOX nanosystem showed an efficient self-delivery, enhanced enzymatic stability, excellent target selectivity, and high drug-loading capacity. With its relatively high enzymatic stability, a-MBM-DOX initially facilitated intracellular MDR1 mRNA imaging to distinguish multidrug-resistant and non-multidrug-resistant cells and subsequently downregulated the MDR1 gene expression owing to an antisense effect. After that, the MB corona was degraded, destroying the micellar nanostructure and releasing DOX, which resulted in a high accumulation of DOX in OVCAR8/ADR cells and a high chemotherapeutic efficacy owing to successful restoration of drug sensitivity. This micelle approach has the potential for both visualizing MDR1 mRNA and overcoming MDR in a sequential and synergistic way.

Nanomedicine-based combination anticancer therapy between nucleic acids and small-molecular drugs

Anticancer therapy has always been a vital challenge for the development of nanomedicine. Repeated single therapeutic agent may lead to undesirable and severe side effects, unbearable toxicity and multidrug resistance due to complex nature of tumor. Nanomedicine-based combination anticancer therapy can synergistically improve antitumor outcomes through multiple-target therapy, decreasing the dose of each therapeutic agent and reducing side effects. There are versatile combinational anticancer strategies such as chemotherapeutic combination, nucleic acid-based co-delivery, intrinsic sensitive and extrinsic stimulus combinational patterns. Based on these combination strategies, various nanocarriers and drug delivery systems were engineered to carry out the efficient co-delivery of combined therapeutic agents for combination anticancer therapy. This review focused on illustrating nanomedicine-based combination anticancer therapy between nucleic acids and small-molecular drugs for synergistically improving anticancer efficacy.

Turning the gun on cancer: Utilizing lysosomal P-glycoprotein as a new strategy to overcome multi-drug resistance

Oxidative stress plays a role in the development of drug resistance in cancer cells. Cancer cells must constantly and rapidly adapt to changes in the tumor microenvironment, due to alterations in the availability of nutrients, such as glucose, oxygen and key transition metals (e.g., iron and copper). This nutrient flux is typically a consequence of rapid growth, poor vascularization and necrosis. It has been demonstrated that stress factors, such as hypoxia and glucose deprivation up-regulate master transcription factors, namely hypoxia inducible factor-1α (HIF-1α), which transcriptionally regulate the multi-drug resistance (MDR), transmembrane drug efflux transporter, P-glycoprotein (Pgp). Interestingly, in addition to the established role of plasma membrane Pgp in MDR, a new paradigm of intracellular resistance has emerged that is premised on the ability of lysosomal Pgp to transport cytotoxic agents into this organelle. This mechanism is enabled by the topological inversion of Pgp via endocytosis resulting in the transporter actively pumping agents into the lysosome. In this way, classical Pgp substrates, such as doxorubicin (DOX), can be actively transported into this organelle. Within the lysosome, DOX becomes protonated upon acidification of the lysosomal lumen, causing its accumulation. This mechanism efficiently traps DOX, preventing its cytotoxic interaction with nuclear DNA. This review discusses these effects and highlights a novel mechanism by which redox-active and protonatable Pgp substrates can utilize lysosomal Pgp to gain access to this compartment, resulting in catastrophic lysosomal membrane permeabilization and cell death. Hence, a key MDR mechanism that utilizes Pgp (the "gun") to sequester protonatable drug substrates safely within lysosomes can be "turned on" MDR cancer cells to destroy them from within.

Multiple myeloma and persistence of drug resistance in the age of novel drugs (Review)

Multiple myeloma (MM) is a mature B cell neoplasm that results in multi-organ failure. The median age of onset, diverse clinical manifestations, heterogeneous survival rate, clonal evolution, intrinsic and acquired drug resistance have impact on the therapeutic management of the disease. Specifically, the emergence of multidrug resistance (MDR) during the course of treatment contributes significantly to treatment failure. The introduction of the immunomodulatory agents and proteasome inhibitors has seen an increase in overall patient survival, however, for the majority of patients, relapse remains inevitable with evidence that these agents, like the conventional chemotherapeutics are also subject to the development of MDR. Clinical management of patients with MM is currently compromised by lack of a suitable procedure to monitor the development of clinical drug resistance in individual patients. The current MM prognostic measures fail to pick the clonotypic tumor cells overexpressing drug efflux pumps, and invasive biopsy is insufficient in detecting sporadic tumors in the skeletal system. This review summarizes the challenges associated with treating the complex disease spectrum of myeloma, with an emphasis on the role of deleterious multidrug resistant clones orchestrating relapse.

Design of folic acid conjugated chitosan nano-cur–bioenhancers to attenuate the hormone-refractory metastatic prostate carcinoma by augmenting oral bioavailability

Folic acid conjugated nano-cur–bioenhancer as a functional P-glycoprotein inhibitor enhancing oral bioavailability of curcumin.

pH-responsive polymeric micelles based on poly(2-ethyl-2-oxazoline)-poly(D,L-lactide) for tumor-targeting and controlled delivery of doxorubicin and P-glycoprotein inhibitor

The combination of a chemotherapeutic drug with a P-glycoprotein (P-gp) inhibitor has emerged as a promising strategy for treating multidrug resistance (MDR) cancer. To ensure that two drugs can be co-delivered to the tumor region and quickly released in tumor cells, tumor-targeted and pH-sensitive polymeric micelles were designed and prepared by combining cationic ring-opening polymerization of 2-ethyl-2-oxazoline (EOz) with anionic ring-opening polymerization of D,L-lactide (LA), and then encapsulating doxorubicin (DOX) and D-alpha-tocopheryl polyethylene glycol 1000 succinate (TPGS1000) into the micelles self-assembled by poly(2-ethyl-2-oxazoline)-poly(D,L-lactide) (PEOz-PLA) and DSPE-PEG-folate. PEOz-PLA exhibited a low critical micelle concentration and negligible cytotoxicity. The micelles enabled the rapid release of DOX when pH decreased from 7.4 to 5.0. The targeting ability of the micelles was demonstrated by in vitro flow cytometry in KBv cells and in vivo real time near-infrared fluorescence imaging in KBv tumor-bearing nude mice. The efficiency of MDR reversion for the micelles was testified by enhancement of intracellular DOX accumulation and cytotoxicity. The efficient drug delivery by the micelles was attributed to synergistic effects of folate-mediated targeting, pH-triggered drug release and TPGS1000-aroused P-gp inhibition. Therefore, the designed multifunctional polymeric micelles may have significant promise for therapeutic application of MDR cancer.

Co-delivery of doxorubicin and P-gp inhibitor by a reduction-sensitive liposome to overcome multidrug resistance, enhance anti-tumor efficiency and reduce toxicity

To overcome multidrug resistance (MDR) in cancer chemotherapy with high efficiency and safety, a reduction-sensitive liposome (CL-R8-LP), which was co-modified with reduction-sensitive cleavable PEG and octaarginine (R8) to increase the tumor accumulation, cellular uptake and lysosome escape, was applied to co-encapsulate doxorubicin (DOX) and a P-glycoprotein (P-gp) inhibitor of verapamil (VER) in this study. The encapsulation efficiency (EE) of DOX and VER in the binary-drug loaded CL-R8-LP (DOX + VER) was about 95 and 70% (w/w), respectively. The uptake efficiencies, the cytotoxicity, and the apoptosis and necrosis-inducing efficiency of CL-R8-LP (DOX + VER) were much higher than those of DOX and the other control liposomes in MCF-7/ADR cells or tumor spheroids. Besides, CL-R8-LP (DOX + VER) was proven to be uptaken into MCF-7/ADR cells by clathrin-mediated and macropinocytosis-mediated endocytosis, followed by efficient lysosomal escape. In vivo, CL-R8-LP (DOX + VER) effectively inhibited the growth of MCF-7/ADR tumor and reduce the toxicity of DOX and VER, which could be ascribed to increased accumulation of drugs in drug-resistant tumor cells and reduced distribution in normal tissues. In summary, the co-delivery of chemotherapeutics and P-gp inhibitors by our reduction-sensitive liposome was a promising approach to overcome MDR, improve anti-tumor effect and reduce the toxicity of chemotherapy.

Expression of P-glycoprotein in human retinoblastoma and its clinical significance

Retinoblastoma is the most common malignant intraocular tumor of childhood. Drug resistance and relapses are major problems with chemotherapy, which is regarded as the mainstay of globe preserving treatment in retinoblastoma. P-glycoprotein (P-gp) expression has been reported to be associated with chemoresistance and poor prognosis in various malignancies. We analyzed P-gp expression in retinoblastoma specimens, enucleated either primarily or after neoadjuvant chemotherapy by immunohistochemistry and immunoblotting, and correlated with the histopathological findings. Variables were statistically analyzed by Fischer's exact and chi-square tests. Tumor tissues were collected from enucleated eyes of 24 children. Fifteen of these were primarily enucleated (group I), and nine (group II) had received chemotherapy prior to enucleation. P-gp was expressed in 4/15 (26.7 %) eyes in group I and in 5/9 (55.6 %) eyes in group II. P-gp was highly expressed in group II as compared to group I. There was no correlation between P-gp expression and tumor differentiation, invasion, or laterality. In conclusion, there was markedly high expression of P-gp in eyes with retinoblastoma enucleated after chemotherapy. This may possibly play a role in chemoresistance or it may be that chemotherapy might have induced high expression. These findings may have important implications for the treatment of retinoblastoma patients but need further prospective investigations in a larger patient population.

Biomimetic RNA-silencing nanocomplexes: overcoming multidrug resistance in cancer cells

RNA interference (RNAi) is an RNA-dependent gene silencing approach controlled by an RNA-induced silencing complex (RISC). Herein, we present a synthetic RISC-mimic nanocomplex, which can actively cleave its target RNA in a sequence-specific manner. With high enzymatic stability and efficient self-delivery to target cells, the designed nanocomplex can selectively and potently induce gene silencing without cytokine activation. These nanocomplexes, which target multidrug resistance, are not only able to bypass the P-glycoprotein (Pgp) transporter, due to their nano-size effect, but also effectively suppress Pgp expression, thus resulting in successful restoration of drug sensitivity of OVCAR8/ADR cells to Pgp-transportable cytotoxic agents. This nanocomplex approach has the potential for both functional genomics and cancer therapy.

Nano-sized polymers and liposomes designed to deliver combination therapy for cancer

The standard of care for cancer patients comprises more than one therapeutic agent. Treatment is complex since several drugs, administered by different routes, need to be coordinated, taking into consideration their side effects and mechanisms of resistance. Drug delivery systems (DDS), such as polymers and liposomes, are designed to improve the pharmacokinetics and efficacy of bioactive agents (drugs, proteins or oligonucleotides), while reducing systemic toxicity. Using DDS for co-delivery of several agents holds great potential since it targets simultaneously synergistic therapeutic agents increasing their selective accumulation at the tumor site and enhancing their activity allowing administration of lower doses of each agent, thus reducing their side effects. Taken together, implementation of smart DDS will hopefully result in increased patient's compliance and better outcome. This review will focus on the latest developments of combination therapy for cancer using DDS.

The synergistic therapeutic effect of temozolomide and hyperbaric oxygen on glioma U251 cell lines is accompanied by alterations in vascular endothelial growth factor and multidrug resistance-associated protein-1 levels

OBJECTIVE

Temozolomide (TMZ) is an oral alkylating agent widely used in the treatment of refractory glioma. Its efficacy is limited, however, by poor cancer cell penetration and drug resistance. The present study, therefore, aimed to investigate whether hyperbaric oxygen (HBO) may facilitate drug delivery and enhance the anticancer effect of TMZ.

METHODS

Cultured glioma U251 cells were treated with HBO, TMZ, or TMZ + HBO, or were untreated (controls). Rates of growth inhibition, cell death and apoptosis were investigated using the 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, propidium iodide staining and flow cytometry, respectively. Protein levels of vascular endothelial growth factor (VEGF) and multidrug resistance-associated protein-1 (MRP-1) were evaluated by enzyme-linked immunosorbent assay.

RESULTS

Compared with TMZ or HBO alone, combined treatment with both therapies synergistically inhibited growth and induced apoptosis and death of cultured glioma U251 cells, which was accompanied by a significant decrease in levels of VEGF and MRP-1.

CONCLUSIONS

TMZ and HBO synergistically induced the apoptosis of glioma cells, possibly through reduced vascularization and inhibition of drug resistance. The combination of TMZ and HBO may be a powerful treatment for malignant glioma.

Star-shape copolymer of lysine-linked di-tocopherol polyethylene glycol 2000 succinate for doxorubicin delivery with reversal of multidrug resistance

A star-shape copolymer of nanostructure-forming material, P-glycoprotein (P-gp) reversible inhibitor and anticancer enhancer, lysine-linked di-tocopherol polyethylene glycol 2000 succinate (PLV(2K)), was synthesized to overcome multidrug resistance (MDR) in cancer chemotherapy. The critical micellar concentration of PLV(2K) was as low as 1.14 μg/mL, which can endow nanoassemblies good physical stability. Doxorubicin (DOX) was encapsulated into the hydrophobic core of PLV(2K) (PLV(2K)-DOX), with encapsulation efficiency as high as 94.5% and a particle size of 16.4 nm. DOX released from PLV(2K)-DOX nanomicelles was pH-dependent, which ensures micelles stable in blood circulation and releases DOX within tumor cells. Facilitated by the cytotoxicity and uncompetitive P-gp ATPase inhibition by PLV(2K), PLV(2K)-DOX showed greater cytotoxicity compared with DOX solution with increased intracellular accumulation in resistant MCF-7/Adr cells. PLV(2K)-DOX nanomicelles were uptaken into MCF-7/Adr cells via macropinocytosis and caveolae-mediated endocytosis, which further facilitate escapement of P-gp efflux. The anticancer efficacy in vivo was evaluated in 4T1-bearing mice and inhibition of tumor by PLV(2K)-DOX was more effective than TPGS-DOX and DOX solution. In summary, PLV(2K) copolymer has striking functions such as uncompetitive P-gp ATPase reversible inhibitor and anticancer efficacy, and could be a promising nanocarrier in improving the chemotherapy of hydrophobic anticancer drugs.

Interference of Frizzled 1 (FZD1) reverses multidrug resistance in breast cancer cells through the Wnt/β-catenin pathway

Multidrug resistance (MDR) represents a major obstacle in the successful treatment of breast cancer. The MDR1 gene is a direct target of the Wnt/β-catenin signaling pathway, which controls tumor development. Overexpression of P-glycoprotein, encoded by the MDR1 gene, is one of the most common causes of MDR. We found that the Frizzled 1 (FZD1) protein, which is an essential component of the Wnt/β-catenin pathway, is overexpressed in the multidrug resistant breast cancer cell subline MCF-7/ADM, coincident with MDR1/P-gp. FZD1 silencing induced down-regulation of MDR1/P-gp, restored sensitivity to four chemotherapy drugs, and significantly decreased cytoplasmic and nuclear β-catenin levels. FZD1 appears to mediate multidrug resistance by regulating the Wnt/β-catenin pathway.

Purification and biochemical characterization of a novel protein-tongue cancer chemotherapy resistance-associated protein1 (TCRP1)

Multidrug resistance is a major obstacle to successful treatment of oral squamous cell carcinoma (OSCC). Lately, we found a novel human gene named tongue cancer chemotherapy resistance-associated protein1 (TCRP1) in the tongue cancer multi-drug resistance cell line (Tca8113/PYM) established by us. In this study, we focus on recombinant expression, purification, and biochemical characterization of TCRP1. After molecular cloning and purification of the gene encoding the 24-kDa protein, a mouse polyclonal antibody against TCRP1 was prepared, and the specialty of the antibody was confirmed by Western blot. The cell proliferation was evaluated by MTS assay and DNA damage was determined by comet assay, the results indicated that this protein especially mediated the cell's resistance to cisplatin; it was associated with its role of providing protection against DNA damage. We also found that TCRP1 expression was increased in cisplatin-resistant carcinoma cell lines (Tca/PYM and A549/DDP), but not in cisplatin-sensitive MDR cell lines (MCF-7/5-Fu), compared with their parental counterparts by Western blot analysis. Immunofluorescence and immunohistochemical analysis showed TCRP1 is mainly expression in cytoplasmic, the Mann-Whitney U test exhibited that TCRP1 positive patients predicted the worst sensitive with cisplatin of OSCC patients. All these findings suggest that TCRP1 is a novel cisplatin-resistant protein which is mainly localized in the cytoplasm and can mediate cisplatin resistance against DNA damage; the expression level of TCRP1 in patients with OSCC may be useful as an indicator of therapeutic efficacy of the sensitivity to cisplatin.

The use of nanoparticle-mediated targeted gene silencing and drug delivery to overcome tumor drug resistance

Overexpression of drug efflux transporters such as P-glycoprotein (P-gp) enables cancer cells to develop resistance to multiple anticancer drugs. Functional inhibitors of P-gp have shown promising efficacy in early clinical trials, but their long-term safety is yet to be established. A novel approach to overcome drug resistance is to use siRNA-mediated RNA interference to silence the expression of the efflux transporter. Because P-gp plays an important role in the physiological regulation of endogenous and xenobiotic compounds in the body, it is important to deliver P-gp targeted siRNA and anticancer drug specifically to tumor cells. Further, for optimal synergy, both the drug and siRNA may need to be temporally colocalized in the tumor cells. In the current study, we investigated the effectiveness of simultaneous and targeted delivery of anticancer drug, paclitaxel, along with P-gp targeted siRNA, using poly(D,L-lactide-co-glycolide) nanoparticles to overcome tumor drug resistance. Nanoparticles were surface functionalized with biotin for active tumor targeting. Dual agent nanoparticles encapsulating the combination of paclitaxel and P-gp targeted siRNA showed significantly higher cytotoxicity in vitro than nanoparticles loaded with paclitaxel alone. Enhanced therapeutic efficacy of dual agent nanoparticles could be correlated with effective silencing of the MDR1 gene that encodes for P-gp and with increased accumulation of paclitaxel in drug-resistant tumor cells. In vivo studies in a mouse model of drug-resistant tumor demonstrated significantly greater inhibition of tumor growth following treatment with biotin-functionalized nanoparticles encapsulating both paclitaxel and P-gp targeted siRNA at a paclitaxel dose that was ineffective in the absence of gene silencing. These results suggest that that the combination of P-gp gene silencing and cytotoxic drug delivery using targeted nanoparticles can overcome tumor drug resistance.

The Wnt receptor FZD1 mediates chemoresistance in neuroblastoma through activation of the Wnt/beta-catenin pathway

The development of chemoresistance represents a major obstacle in the successful treatment of cancers such as neuroblastoma (NB), a particularly aggressive childhood solid tumour. The mechanisms underlying the chemoresistant phenotype in NB were addressed by gene expression profiling of two doxorubicin (DoxR)-resistant vs sensitive parental cell lines. Not surprisingly, the MDR1 gene was included in the identified upregulated genes, although the highest overexpressed transcript in both cell lines was the frizzled-1 Wnt receptor (FZD1) gene, an essential component of the Wnt/beta-catenin pathway. FZD1 upregulation in resistant variants was shown to mediate sustained activation of the Wnt/beta-catenin pathway as revealed by nuclear beta-catenin translocation and target genes transactivation. Interestingly, specific micro-adapted short hairpin RNA (shRNAmir)-mediated FZD1 silencing induced parallel strong decrease in the expression of MDR1, another beta-catenin target gene, revealing a complex, Wnt/beta-catenin-mediated implication of FZD1 in chemoresistance. The significant restoration of drug sensitivity in FZD1-silenced cells confirmed the FZD1-associated chemoresistance. RNA samples from 21 patient tumours (diagnosis and postchemotherapy), showed a highly significant FZD1 and/or MDR1 overexpression after treatment, underlining a role for FZD1-mediated Wnt/beta-catenin pathway in clinical chemoresistance. Our data represent the first implication of the Wnt/beta-catenin pathway in NB chemoresistance and identify potential new targets to treat aggressive and resistant NB.

MDR1 C3435T polymorphism in patients with breast cancer

BACKGROUND

The human multidrug-resistant gene (MDR1) encodes P-glycoprotein (Pgp), a membrane-bound efflux transporter conferring resistance to a number of natural cytotoxic drugs and potentially toxic xenobiotics. Single-nucleotide polymorphisms (SNPs) in MDR1 gene are associated with phenotypic variation in Pgp expression levels of tissue. SNPs may alter the physiological protective role of Pgp and, therefore, influence disease risk.

METHODS

In our study we identified the MDR1 C3435T polymorphism in breast cancer patients (n = 57) and healthy subjects (n = 50). DNA was extracted from peripheral blood samples by standard phenol/chloroform extraction method. Polymerase chain reaction-restriction fragment length polymorphism was used for the detection of C3435T single nucleotide polymorphism.

RESULTS

We obtained CC, CT and TT genotype frequencies in breast cancer patients as 12.3%, 57.9% and 29.8%, respectively. In the control group, frequencies of genotypes were found as 36% for CC, 46% for CT and 18% for TT. We observed difference in SNPs in MDR1 gene C3435T polymorphism between breast cancer patients and healthy controls (chi(2) = 8.66, df = 2, p = 0.013). The C allele frequency was found in 41.2% and the T allele frequency was found in 58.8%. C3435T MDR1 gene allele frequencies in breast cancer patients as compared to results in control group were as follows: [OR = 1.5 (95% CI: 1.09-1.96)]. In the patient group, T allele frequency was significantly higher than controls (p <0.01). Clinicopathological parameters of patients with breast cancer were compared for C3435T polymorphism. We did not find any significant difference between clinicopathological parameters and MDR1 phenotype of breast cancer patients. The progression-free survival rate in a subgroup analysis based on MDR1 genotypes with CC genotype was 71.4%, CT genotype was 75.7%, and TT genotype was 88.2%, respectively. This difference was not statistically significant (log rank p = 0.63).

CONCLUSIONS

Results of the present study demonstrated a 1.5-fold increased risk for development of breast cancer in T allele carriers.

[Expression of multidrug resistance (MDR)-associated proteins in solid tumors]

The causes of drug resistance in tumor cells vary widely. The present study aims to provide an update of multidrug resistance in tumor cells and, in particular, of multidrug resistance-associated proteins.

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.