Recent attempts at RNAi-mediated P-glycoprotein downregulation for reversal of multidrug resistance in cancer

Lipopolymers as the Basis of Non-Viral Delivery of Therapeutic siRNA Nanoparticles in a Leukemia (MOLM-13) Model

Small interfering RNA (siRNA) therapy in acute myeloid leukemia (AML) is a promising strategy as the siRNA molecule can specifically target proteins involved in abnormal cell proliferation. The development of a clinically applicable method for delivering siRNA molecules is imperative due to the challenges involved in effectively delivering the siRNA into cells. We investigated the delivery of siRNA to AML MOLM-13 cells with the use of two lipid-substituted polyethyleneimines (PEIs), a commercially available reagent (Prime-Fect) and a recently reported reagent with improved lipid substitution (PEI1.2k-PHPA-Lin9). The siRNAs utilized in this study were targeting the oncogenes FLT3 and KMT2A::MLLT3. Both lipopolymers gave similar-size siRNA complexes (210-220 nm) with positive ζ-potentials (+17 to +25 mV). While the binding efficiency of both lipopolymers to siRNA were similar, PEI1.2k-PHPA-Lin9 complexes were more resistant to heparin-induced dissociation. The quantitative analysis of gene silencing performed by qPCR as well as immunostaining/flow cytometry indicated significant reduction in both FLT3 expression and FLT3 protein after specific siRNA delivery. The desired inhibition of cell growth was attained with both FLT3 and KMT2A::MLLT3 siRNAs, and the combination provided more potent effects in both cell growth and colony formation assays. Induction of apoptosis was confirmed after specific siRNA treatments using the Annexin V assay. Using Luc(+) MOLM-13 cells, the growth of the xenografted cells was shown to be retarded with Prime-Fect-delivered FLT3 siRNA, unlike the siRNA delivered with PEI1.2k-PHPA-Lin9. These results demonstrate the potential of designed lipopolymers in implementing RNAi (via delivery of siRNA) for inhibition of leukemia growth and provide evidence for the feasibility of targeting different oncogenes using siRNA-mediated therapy.

Bioengineered Nanomaterials for siRNA Therapy of Chemoresistant Cancers

Chemoresistance remains a long-standing challenge after cancer treatment. Over the last two decades, RNA interference (RNAi) has emerged as a gene therapy modality to sensitize cancer cells to chemotherapy. However, the use of RNAi, specifically small-interfering RNA (siRNA), is hindered by biological barriers that limit its intracellular delivery. Nanoparticles can overcome these barriers by protecting siRNA in physiological environments and facilitating its delivery to cancer cells. In this review, we discuss the development of nanomaterials for siRNA delivery in cancer therapy, current challenges, and future perspectives for their implementation to overcome cancer chemoresistance.

Phe-Phe-Based Macroscopic Supramolecular Hydrogel Construction Strategies and Biomedical Applications

Since the phenylalanine (Phe) dipeptide moiety is referred to as an essential structure for building amyloid-β peptide from Alzheimer's disease, its wonderful assembly ability to form nanofibers has been extensively studied. Cross-linked Phe-Phe-based peptide nanofibers can construct networks, thus encapsulating the drugs to form supramolecular hydrogels. Recently, scientists have proposed a variety of Phe-Phe-based macroscopic supramolecular hydrogels and used them in biomedical applications. Therefore, we summarize the construction strategies of Phe-Phe-based macroscopic supramolecular hydrogels and list their represented biomedical applications (e.g., wound healing, eye protection, cancer therapy, etc.) since the birth of Phe-Phe-based supramolecular hydrogels. In addition, we present the perspectives and challenges of Phe-Phe-based macroscopic peptide hydrogels.

Non-viral vectors combined delivery of siRNA and anti-cancer drugs to reverse tumor multidrug resistance

Multidrug resistance (MDR) of tumors is one of the main reasons for the failure of chemotherapy. Multidrug resistance refers to the cross-resistance of tumor cells to multiple antitumor drugs with different structures and mechanisms of action. Current strategies to reverse multidrug resistance in tumors include MDR inhibitors and RNAi technology. siRNA is a small molecule RNA that is widely used in RNAi technology and has the characteristics of being prepared in large quantities and chemically modified. However, siRNA is susceptible to degradation in vivo. The effect of siRNA therapy alone is not ideal, so siRNA and anticancer drugs are administered in combination to reverse the MDR of tumors. Non-viral vectors are now commonly used to deliver siRNA and anticancer drugs to tumor sites. This article will review the progress of siRNA and chemotherapeutic drug delivery systems and their mechanisms for reversing multidrug resistance.

Synthesis and evaluation of WK-X-34 derivatives as P-glycoprotein (P-gp/ABCB1) inhibitors for reversing multidrug resistance

The emergence of multidrug resistance (MDR) in malignant tumors is one of the leading threats encountered currently by many chemotherapeutic agents. A proposed strategy to overcome MDR is to disable the efflux function of P-glycoprotein (P-gp/ABCB1), a critical member of the ABC transporter family that significantly increases the efflux of various anticancer drugs from tumor cells. In this study, structural modification of a third-generation P-gp inhibitor WK-X-34 based on bioisosteric and fragment-growing strategies led to the discovery of the adamantane derivative PID-9, which exhibited the best MDR reversal activity (IC50 = 0.1338 μM, RF = 78.6) in this series, exceeding those of the reported P-gp inhibitors verapamil and WK-X-34. In addition, compared with WK-X-34, PID-9 showed decreased toxicity to cells. Furthermore, the mechanism studies revealed that the reversal activity of adamantane derivatives PID-5, PID-7, and PID-9 stemmed from the inhibition of P-gp efflux. These results indicated that compound PID-9 is the most effective P-gp inhibitor among them with low toxicity and high MDR reversal activity, which provided a fundamental structural reference for further discovery of novel, effective, and non-toxic P-gp inhibitors.

Engineering extracellular vesicles mimetics for targeted chemotherapy of drug-resistant ovary cancer

Aim: To develop nanocarriers for targeting the delivery of chemotherapeutics to overcome multidrug-resistant ovarian cancer. Materials & methods: Doxorubicin-loaded nanovesicles were obtained through serial extrusion, followed by loading of P-glycoprotein siRNA and folic acid. The targeting ability and anticancer efficacy of the nanovesicles were evaluated. Results: The doxorubicin-loaded nanovesicles showed a high production yield. The presence of P-glycoprotein siRNA and folic acid resulted in reversed drug resistance and tumor targeting. This nanoplatform tremendously inhibited the viability of multidrug-resistant ovarian cancer cells, which was able to target tumor tissue and suppress tumor growth without adverse effects. Conclusion: These bioengineered nanovesicles could serve as novel extracellular vesicles mimetics for chemotherapeutics delivery to overcome multidrug resistance.

NOD2 silencing promotes cell apoptosis and inhibits drug resistance in chronic lymphocytic leukemia by inhibiting the NF-κB signaling pathway

Recent years have witnessed increasing studies on the effect of epigenetic silencing of genes in the progression of chronic lymphocytic leukemia (CLL). This study investigates whether the nucleotide binding oligomerization domain containing 2 (NOD2) participates in the cell apoptosis and drug resistance of CLL cells. Cells were treated with adriamycin (ADR), etoposide, aclacinomycin and daunorubicin. After treatment, drug resistance and cell proliferation were examined to detect the inhibitory effect of ADR on cell proliferation; flow cytometry to identify ADR accumulation, the cell cycle distribution and apoptosis after transfection, and rhodamine 123 accumulation and efflux tests to assess P-glycoprotein (P-gp) function. NOD2 silencing or inhibition of the nuclear factor kappa-B (NF-κB) signaling pathway suppressed the multidrug resistance level in CLL, the inhibition rate, and cell proliferation caused by ADR at concentrations of approximately 0.25-1.5 μmol/L. Greater accumulation of ADR was observed in the CLL-AAT cell line than in the CLL-AAT/A02 cell line, but NOD2 silencing or inhibition of the NF-κB signaling pathway further increased the accumulation of ADR drugs in the CLL-AAT cell line and inhibited the drug efflux pump function of P-gp. Additionally, NOD2 silencing or NF-κB signaling pathway inhibition increased the apoptotic rate. The results of this study indicate that NOD2 promotes cell apoptosis and reduces the drug resistance of CLL by inhibiting the NF-κB signaling pathway.

CD44-Targeted Nanocarrier for Cancer Therapy

Cluster of differentiation 44 (CD44) is a cell surface glycoprotein overexpressed in varieties of solid tumors including pancreatic, breast, ovary, brain, and lung cancers. It is a multi-structural glycoprotein of the cell surface which is majorly involved in cell proliferation, cell-to-cell interaction, cellular migration, inflammation, and generation of immune responses. Numerous studies focus on the development of nanocarriers for active targeting of the CD44 receptor to improve efficacy of targeting chemotherapy and achieve precise chemotherapy by defining the release, uptake, and accumulation of therapeutic agents. The CD44 receptor has a selective binding affinity towards hyaluronic and chondroitin sulfate (CS). Taking this into consideration, this review focused on the role of CD44 in cancer and its therapy using several nanocarriers such as polymeric/non-polymeric nanoparticles, dendrimer, micelles, carbon nanotubes, nanogels, nanoemulsions etc., for targeted delivery of several chemotherapeutic molecules and nucleic acid. This review also illuminates the role of hyaluronic acid (HA) in cancer therapy, interaction of HA with CD44, and various approaches to target CD44-overexpressed neoplastic cells.

Therapeutic delivery of siRNA with polymeric carriers to down-regulate STAT5A expression in high-risk B-cell acute lymphoblastic leukemia (B-ALL)

Overexpression and persistent activation of STAT5 play an important role in the development and progression of acute lymphoblastic leukemia (ALL), the most common pediatric cancer. Small interfering RNA (siRNA)-mediated downregulation of STAT5 represents a promising therapeutic approach for ALL to overcome the limitations of current treatment modalities such as high relapse rates and poor prognosis. However, to effectively transport siRNA molecules to target cells, development of potent carriers is of utmost importance to surpass hurdles of delivery. In this study, we investigated the use of lipopolymers as non-viral delivery systems derived from low molecular weight polyethylenimines (PEI) substituted with lauric acid (Lau), linoleic acid (LA) and stearic acid (StA) to deliver siRNA molecules to ALL cell lines and primary samples. Among the lipid-substituted polymers explored, Lau- and LA-substituted PEI displayed excellent siRNA delivery to SUP-B15 and RS4;11 cells. STAT5A gene expression was downregulated (36-92%) in SUP-B15 and (32%) in RS4;11 cells using the polymeric delivery systems, which consequently reduced cell growth and inhibited the formation of colonies in ALL cells. With regard to ALL primary cells, siRNA-mediated STAT5A gene silencing was observed in four of eight patient cells using our leading polymeric delivery system, 1.2PEI-Lau8, accompanied by the significant reduction in colony formation in three of eight patients. In both BCR-ABL positive and negative groups, three of five patients demonstrated marked cell growth inhibition in both MTT and trypan blue exclusion assays using 1.2PEI-Lau8/siRNA complexes in comparison with their control siRNA groups. Three patient samples did not show any positive results with our delivery systems. Differential therapeutic responses to siRNA therapy observed in different patients could result from variable genetic profiles and patient-to-patient variability in delivery. This study supports the potential of siRNA therapy and the designed lipopolymers as a delivery system in ALL therapy.

NIR-II light triggered nitric oxide release nanoplatform combined chemo-photothermal therapy for overcoming multidrug resistant cancer

The overexpression of P-glycoprotein (P-gp) in multidrug resistance (MDR) cancer cells increases the efflux of anticancer drugs thereby causing the failure of clinical chemotherapy. To address this obstacle, in this study, we rationally designed a near-infrared (NIR) light-responsive nitric oxide (NO) delivery nanoplatform for targeting the MDR tumors based on core-shell structured nanocomposites. The mesoporous silica shell provided abundant sites for modification of the NO donor, N-diazeniumdiolate, and tumor-targeting molecule, folic acid (FA), and enabled high encapsulation capacity for doxorubicin (DOX) loading. Under NIR light irradiation, the generation of NO gas can efficiently augment chemotherapeutic effects via the inhibition of P-gp expression. Simultaneously, the photothermal conversion agents of the Cu_2-xSe core produce a large amount of heat for photothermal therapy (PTT). Finally, this combinational gas/chemo/PTT not only displays a superior and synergistic effect for overcoming MDR cancer, but also provides an efficient strategy to construct a multifunctional nano-drug delivery system with diversified therapeutic modalities.

Cancer Stem Cells and Combination Therapies to Eradicate Them

Cancer stem cells (CSCs) show self-renewal ability and multipotential differentiation, like normal stem or progenitor cells, and which proliferate uncontrollably and can escape the effects of drugs and phagocytosis by immune cells. Traditional monotherapies, such as surgical resection, radiotherapy and chemotherapy, cannot eradicate CSCs, however, combination therapy may be more effective at eliminating CSCs. The present review summarizes the characteristics of CSCs and several promising combination therapies to eradicate them.

ROS and GSH-responsive S-nitrosoglutathione functionalized polymeric nanoparticles to overcome multidrug resistance in cancer

Multidrug resistance of cancer cells is one of the major obstacle for chemotherapeutic efficiency. Nitric oxide (NO) has raised the potential to overcome multidrug resistance (MDR) with low side effects. Herein, we report a reactive oxygen species (ROS) and glutathione (GSH) responsive nanoparticle for the delivery of NO prodrug such as S-nitrosoglutathione (GSNO), which was chemically conjugated to an amphiphilic block copolymer. The GSNO functionalized nanoparticles show high NO loading capacity, good stability and sustained NO release with specific GSH activated NO-releasing kinetics. Such GSNO functionalized nanoparticles delivered doxorubicin (DOX) in a ROS triggered manner and increased the intracellular accumulation of DOX. However, in normal healthy cells, showing physiological concentrations of ROS, these nanoparticles presented good biocompatibility. The present work indicated that these multifunctional nanoparticles can serve as effective co-delivery platforms of NO and DOX to selectively kill chemo-resistant cancer cells through increasing chemo-sensitivity. STATEMENT OF SIGNIFICANCE: In this work, we constructed nitric oxide donor (S-nitrosoglutathione, GSNO) functionalized amphiphilic copolymer (PEG-PPS-GSNO) to deliver doxorubicin (DOX). The developed PEG-PPS-GSNO@DOX nanoparticles presented high NO capacity, ROS triggered DOX release and GSH triggered NO release. Thus NO reversed the chemo-resistance in HepG2/ADR cells increasing intrcellular accumulation of DOX. Furthermore, these PEG-PPS-GSNO@DOX nanoparticles exhibited biocompatibility to healthy cells and toxicity to cancer cells, due to elevated ROS.

15-LOX-1 has diverse roles in the resensitization of resistant cancer cell lines to doxorubicin

Lipoxygenases (LOXs) are a family of enzymes that can oxygenate polyunsaturated fatty acids. As a member of the family, 15-lipoxygenase-1 (15-LOX-1) specifically metabolizes arachidonic acid and linoleic acid. 15-LOX-1 can affect physiological and pathophysiological events via regulation of the protein-lipid interactome, alterations in intracellular redox state and production of lipid metabolites that are involved in the induction and resolution of inflammation. Although several studies have shown that 15-LOX-1 has an antitumorigenic role in many different cancer models, including breast cancer, the role of the protein in cancer drug resistance has not been established yet. In this study, we, for the first time, aimed to show the potential role of 15-LOX-1 in acquired doxorubicin (DOX) resistance in MCF7 and HeLa cancer cell lines. Our results show that ALOX15 was transcriptionally downregulated in DOX-resistant cells compared with their drug-sensitive counterparts. Moreover, overexpression of ALOX15 in the drug-resistant cells resulted in resensitization of those cells to DOX in a cell-dependent manner. 15-LOX-1 expression could induce apoptosis by activating PPARγ and enhance the accumulation of DOX in drug-resistant MCF7 cells by altering cellular motility properties, and membrane dynamics. However, HeLa DOX cells did not show any of these effects but were susceptible to cell death when treated with 13(S)-HODE. These results underline the role and importance of 15-LOX-1 in cancer drug resistance, and points to novel mechanisms as a therapeutic approach to overcome cancer drug resistance.

Photostable and Biocompatible Fluorescent Silicon Nanoparticles for Imaging-Guided Co-Delivery of siRNA and Doxorubicin to Drug-Resistant Cancer Cells

The development of effective and safe vehicles to deliver small interfering RNA (siRNA) and chemotherapeutics remains a major challenge in RNA interference-based combination therapy with chemotherapeutics, which has emerged as a powerful platform to treat drug-resistant cancer cells. Herein, we describe the development of novel all-in-one fluorescent silicon nanoparticles (SiNPs)-based nanomedicine platform for imaging-guided co-delivery of siRNA and doxorubicin (DOX). This approach enhanced therapeutic efficacy in multidrug-resistant breast cancer cells (i.e., MCF-7/ADR cells). Typically, the SiNP-based nanocarriers enhanced the stability of siRNA in a biological environment (i.e., medium or RNase A) and imparted the responsive release behavior of siRNA, resulting in approximately 80% down-regulation of P-glycoprotein expression. Co-delivery of P-glycoprotein siRNA and DOX led to > 35-fold decrease in the half maximal inhibitory concentration of DOX in comparison with free DOX, indicating the pronounced therapeutic efficiency of the resultant nanocomposites for drug-resistant breast cancer cells. The intracellular time-dependent release behaviors of siRNA and DOX were revealed through tracking the strong and stable fluorescence of SiNPs. These data provide valuable information for designing effective RNA interference-based co-delivery carriers.

The effects of ultrasound exposure on P-glycoprotein-mediated multidrug resistance in vitro and in vivo

Background

Multidrug resistance (MDR) is often responsible for the failure of chemotherapy treatment, and current strategies for cancer MDR are not adequately satisfying as to their efficacy and safety. In this study, we sought to determine the anti-MDR effects of ultrasound (US) irradiation and its underlying mechanisms against drug-resistance.

Methods

MDR variant MCF-7/ADR cell lines and endothelial cell lines were used to determine the appropriate ultrasound intensity for in vitro experiments. MCF-7/ADR cell and HEPG2/ADM cells were used to assess the anti-MDR effect of US irradiation. Intracellular adriamycin (ADM) accumulation, Cell viability, cell proliferation and cell apoptosis were evaluated after ADM + US treatment or ADM treatment alone. MCF-7/ADR xenograft mice were used to investigate the appropriate ultrasound intensity for in vivo experiments and its effect on the long-term prognosis. Underlining mechanisms by which ultrasound exposure reversing MDR phenotype were investigated both in vitro and in vivo.

Results

Combination of ADM and 0.74 W/cm² US irradiation enhanced ADM intracellular concentration and nuclear accumulation in MCF-7/ADR and HEPG2/ADM cells, compared to those treated with ADM alone. Enhanced cellular ADM uptake and nuclei localization was associated with increased cytotoxicity of ADM to ADM-resistant cells, lower ADM-resistant cell viability and proliferative cell ratio, and higher apoptotic cell ratio. More importantly, US exposure increased the effectiveness of ADM to inhibit tumor growth in MCF-7/ADR xenograft mice. Mechanistically, US exposure promoted ADM accumulation in MDR cells mainly through down-regulation of P-glycoprotein (P-gp), which is dependent on US-induced intracellular reactive oxygen species (ROS) production. US-induced oxidative stress promoted miR-200c-3p and miR-34a-3p expression by forming miR-200c/34a/ZEB1 double-negative feedback loop. Finally, US-induced miR-200c/34a overexpression decreased P-gp expression and reversed MDR phenotype.

Conclusion

US irradiation could reverse MDR phenotype by activating ROS-ZEB1-miR200c/34a-P-gp signal pathway. Our findings offer a new and promising strategy for sensitizing cells to combat MDR and to improve the therapeutic index of chemotherapy.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0900-6) contains supplementary material, which is available to authorized users.

Nanotechnology for Cancer Therapy Based on Chemotherapy

Chemotherapy has been widely applied in clinics. However, the therapeutic potential of chemotherapy against cancer is seriously dissatisfactory due to the nonspecific drug distribution, multidrug resistance (MDR) and the heterogeneity of cancer. Therefore, combinational therapy based on chemotherapy mediated by nanotechnology, has been the trend in clinical research at present, which can result in a remarkably increased therapeutic efficiency with few side effects to normal tissues. Moreover, to achieve the accurate pre-diagnosis and real-time monitoring for tumor, the research of nano-theranostics, which integrates diagnosis with treatment process, is a promising field in cancer treatment. In this review, the recent studies on combinational therapy based on chemotherapy will be systematically discussed. Furthermore, as a current trend in cancer treatment, advance in theranostic nanoparticles based on chemotherapy will be exemplified briefly. Finally, the present challenges and improvement tips will be presented in combination therapy and nano-theranostics.

New tetrahydroisoquinoline-based P-glycoprotein modulators: decoration of the biphenyl core gives selective ligands

P-glycoprotein (P-gp, MDR1) is a membrane transporter expressed in several regions of our body. It plays a crucial defense role as it mediates the efflux of hundreds of potentially toxic substances. However, P-gp is one of the main causes of failure in cancer chemotherapy, as a number of chemotherapeutic agents are P-gp substrates. Another interesting implication concerns the correlation between P-gp expression impairment and the onset of several central nervous system pathologies such as Alzheimer's and Parkinson's diseases. In view of these considerations, in the present study, a new series of P-gp modulators have been designed, synthesized and evaluated for their activity towards P-gp and two other sister proteins (BCRP and MRP1). The compounds, structurally correlated to the potent but non-selective P-gp inhibitor MC70 [4'-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-ylmethyl)biphenyl-4-ol], proved fairly selective towards P-gp, with a potency in the micromolar range. Compounds 5a, 5d and 12d proved capable of restoring doxorubicin toxicity in resistant cancer cells.

Inhibit or Evade Multidrug Resistance P-Glycoprotein in Cancer Treatment

Multidrug resistance (MDR) is a major cause of failure in cancer chemotherapy. P-glycoprotein (P-gp), a promiscuous drug efflux pump, has been extensively studied for its association with MDR due to overexpression in cancer cells. Several P-gp inhibitors or modulators have been investigated in clinical trials in hope of circumventing MDR, with only limited success. Alternative strategies are actively pursued, such as the modification of existing drugs, development of new drugs, or combination of novel drug delivery agents to evade P-gp-dependent efflux. Despite the importance and numerous studies, these efforts have mostly been undertaken without a priori knowledge of how drugs interact with P-gp at the molecular level. This review highlights and discusses progress toward and challenges impeding drug development for inhibiting or evading P-gp in the context of our improved understanding of the structural basis and mechanism of P-gp-mediated MDR.

A polycation coated liposome as efficient siRNA carrier to overcome multidrug resistance

Multidrug resistance (MDR) is one of the important factors that impede effective chemotherapy against cancer. Codelivery of MDR1 siRNA (silencing ABCB1 gene) and anticancer drug can greatly inhibit tumor proliferation. Here in this work, we synthesized poly(diallyldimethylammonium chloride) (PDADMAC) coated liposome formula as siMDR1 carrier (AL-PDAD-RNA) and applied it to reverse doxorubicin resistance of OVCAR8/ADR cells. The AL-PDAD-RNA can load siRNA effectively and release siRNA under physiological conditions, leading to improved tumor inhibition than free DOX without siRNA treatment. Meanwhile, the gene silencing effect of AL-PDAD-RNA was shown to be comparable to that of commercial transfection agent lipofectamine, but with less toxicity. The main novelty of this work is to offer a new type of siRNA carrier (PDADMAC coated liposome, AL-PDAD), which is simple-structured, highly-effective and non-toxic. Therefore, we anticipate that PDADMAC-coated liposomes would be very promising in the application of other siRNA delivery or even plasmid delivery.

In vitro studies of phospholipid-modified PAMAM-siMDR1 complexes for the reversal of multidrug resistance in human breast cancer cells

The application of RNAi therapeutics is promising in combating several major human diseases including malignant tumors. However, this approach is limited due to its delivery barriers. In this study, we designed a new carrier system loaded with a functional siRNA targeting MDR1 gene to reverse multi-drug resistance (MDR) in human breast cancer MCF-7/ADR cells. Phospholipid-modified PAMAM-siMDR1 complexes were designed on the external decoration of polyamidoamine (PAMAM) with phospholipid (PL) and the electrostatical interaction between PAMAM and siMDR1 to form hybrid nanocomplexes (PL-dendriplexes). Compared with siMDR1 and dendriplexes (PAMAM-siMDR1), this delivery system represented higher gene silencing efficiency, enhanced cellular uptake of siMDR1, decreased p-gp expression, raised cellular accumulation of doxorubicin (DOX) and inhibited the tumor cell migration. Moreover, the siMDR1 loaded PL-dendriplexes worked synergistically with paclitaxel (PTX) for treating MDR, leading to increased cell apoptosis and cell phase regulation. Overall, this study shows that the PL-dendriplexes hold great promise in reversing the drug-resistance in MCF-7/ADR cells.

Simultaneous delivery of anti-miR21 with doxorubicin prodrug by mimetic lipoprotein nanoparticles for synergistic effect against drug resistance in cancer cells

The development of drug resistance in cancer cells is one of the major obstacles to achieving effective chemotherapy. We hypothesized that the combination of a doxorubicin (Dox) prodrug and microRNA (miR)21 inhibitor might show synergistic antitumor effects on drug-resistant breast cancer cells. In this study, we aimed to develop new high-density lipoprotein-mimicking nanoparticles (HMNs) for coencapsulation and codelivery of this potential combination. Dox was coupled with a nuclear localization signal (NLS) peptide to construct a prodrug (NLS-Dox), thereby electrostatically condensing miR21 inhibitor (anti-miR21) to form cationic complexes. The HMNs were formulated by shielding these complexes with anionic lipids and Apo AI proteins. We have characterized that the coloaded HMNs had uniformly dispersed distribution, favorable negatively charged surface, and high coencapsulation efficiency. The HMN formulation effectively codelivered NLS-Dox and anti-miR21 into Dox-resistant breast cancer MCF7/ADR cells and wild-type MCF7 cells via a high-density-lipoprotein receptor-mediated pathway, which facilitated the escape of Pgp drug efflux. The coloaded HMNs consisting of NLS-Dox/anti-miR21 demonstrated greater cytotoxicity with enhanced intracellular accumulation in resistant MCF7/ADR cells compared with free Dox solution. The reversal of drug resistance by coloaded HMNs might be attributed to the suppression of miR21 expression and the related antiapoptosis network. Furthermore, the codelivery of anti-miR21 and NLS-Dox by HMNs showed synergistic antiproliferative effects in MCF7/ADR-bearing nude mice, and was more effective in tumor inhibition than other drug formulations. These data suggested that codelivery of anti-miR21 and chemotherapeutic agents by HMNs might be a promising strategy for antitumor therapy, and could restore the drug sensitivity of cancer cells, alter intracellular drug distribution, and ultimately enhance chemotherapeutic effects.

Near-Infrared-Triggered Azobenzene-Liposome/Upconversion Nanoparticle Hybrid Vesicles for Remotely Controlled Drug Delivery to Overcome Cancer Multidrug Resistance

Overcoming multidrug resistance is achieved by developing a novel drugdelivery-system paradigm based on azobenzene liposome and phosphatidylcholine-modified upconversion nanoparticle (UCNP) hybrid vesicles for controlled drug release using a nearinfrared (NIR) laser. Upon 980 nm light irradiation, the reversible photoisomerization of the azobenzene derivatives by simultaneous UV and visible light emitted from the UCNPs makes it possible to realize NIR-triggered release of the chemotherapeutic drug doxorubicin.

Tackling breast cancer chemoresistance with nano-formulated siRNA

Breast cancer is the leading cancer diagnosed in women and the second leading cause of cancer-related deaths in women. Current limitations to standard chemotherapy in the clinic are extensively researched, including problems arising from repeated treatments with the same drugs. The phenomenon that cancer cells become resistant toward certain chemo drugs is called chemotherapy resistance. In this review, we are focusing on nanoformulation of siRNA for the fight against breast cancer chemoresistance.

Delivery strategies and potential targets for siRNA in major cancer types

Small interfering RNA (siRNA) has gained attention as a potential therapeutic reagent due to its ability to inhibit specific genes in many genetic diseases. For many years, studies of siRNA have progressively advanced toward novel treatment strategies against cancer. Cancer is caused by various mutations in hundreds of genes including both proto-oncogenes and tumor suppressor genes. In order to develop siRNAs as therapeutic agents for cancer treatment, delivery strategies for siRNA must be carefully designed and potential gene targets carefully selected for optimal anti-cancer effects. In this review, various modifications and delivery strategies for siRNA delivery are discussed. In addition, we present current thinking on target gene selection in major tumor types.

Pharmacokinetics and biodistribution of recently-developed siRNA nanomedicines

Small interfering RNA (siRNA) is a promising drug candidate, expected to have broad therapeutic potentials toward various diseases including viral infections and cancer. With recent advances in bioconjugate chemistry and carrier technology, several siRNA-based drugs have advanced to clinical trials. However, most cases address local applications or diseases in the filtering organs, reflecting remaining challenges in systemic delivery of siRNA. The difficulty in siRNA delivery is in large part due to poor circulation stability and unfavorable pharmacokinetics and biodistribution profiles of siRNA. This review describes the pharmacokinetics and biodistribution of siRNA nanomedicines, focusing on those reported in the past 5years, and their pharmacological effects in selected disease models such as hepatocellular carcinoma, liver infections, and respiratory diseases. The examples discussed here will provide an insight into the current status of the art and unmet needs in siRNA delivery.

Binary-copolymer system base on low-density lipoprotein-coupled N-succinyl chitosan lipoic acid micelles for co-delivery MDR1 siRNA and paclitaxel, enhances antitumor effects via reducing drug

The development of effective and stable carriers of small interfering RNA (siRNA) is important for treating cancer with multidrug resistance (MDR). We developed a new gene and drug co-delivery system and checked its characteristics. Low-density lipoprotein (LDL) was coupled with N-succinyl chitosan (NSC) Lipoic acid (LA) micelles and co-delivered MDR1 siRNA and paclitaxel (PTX-siRNA/LDL-NSC-LA) to enhance antitumor effects by silencing the MDR gene of tumors (Li et al., Adv Mater 2014;26:8217-8224). In our study, we developed a new type of containing paclitaxel-loaded micelles and siRNA-loaded LDL nanoparticle. This "binary polymer" is pH and reduction dual-sensitive core-crosslinked micelles. PTX-siRNA/LDL-NSC-LA had an average particle size of (171.6 ± 6.42) nm, entrapment efficiency of (93.92 ± 1.06) %, and drug-loading amount of (12.35% ± 0.87) %. In vitro, MCF-7 cells, high expressed LDL receptor, were more sensitive to this delivery system than to taxol^® and cell activity was inhibited significantly. Fluorescence microscopy showed that PTX-siRNA/LDL-NSC-LA was uptaken very conveniently and played a key role in antitumor activity. PTX-siRNA/LDL-NSC-LA protected the siRNA from degradation by macrophage phagocytosis and evidently down-regulated the level of mdr1 mRNA as well as the expression of P-gp. We tested the target ability of PTX-siRNA/LDL-NSC-LA in vivo in tumor-bearing nude mice. Results showed that this system could directly deliver siRNA and PTX to cancer cells. Thus, new co-delivering siRNA and antitumor drugs should be explored for solving MDR in cancer. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1114-1125, 2017.

Stimuli-Responsive Codelivery of Oligonucleotides and Drugs by Self-Assembled Peptide Nanoparticles

Ever more emerging combined treatments exploiting synergistic effects of drug combinations demand smart, responsive codelivery carriers to reveal their full potential. In this study, a multifunctional stimuli-responsive amphiphilic peptide was designed and synthesized to self-assemble into nanoparticles capable of co-bearing and -releasing hydrophobic drugs and antisense oligonucleotides for combined therapies. The rational design was based on a hydrophobic l-tryptophan-d-leucine repeating unit derived from a truncated sequence of gramicidin A (gT), to entrap hydrophobic cargo, which is combined with a hydrophilic moiety of histidines to provide electrostatic affinity to nucleotides. Stimuli-responsiveness was implemented by linking the hydrophobic and hydrophilic sequence through an artificial amino acid bearing a disulfide functional group (H3SSgT). Stimuli-responsive peptides self-assembled in spherical nanoparticles in sizes (100-200 nm) generally considered as preferable for drug delivery applications. Responsive peptide nanoparticles revealed notable nucleotide condensing abilities while maintaining the ability to load hydrophobic cargo. The disulfide cleavage site introduced in the peptide sequence induced responsiveness to physiological concentrations of reducing agent, serving to release the incorporated molecules. Furthermore, the peptide nanoparticles, singly loaded or coloaded with boron-dipyrromethene (BODIPY) and/or antisense oligonucleotides, were efficiently taken up by cells. Such amphiphilic peptides that led to noncytotoxic, reduction-responsive nanoparticles capable of codelivering hydrophobic and nucleic acid payloads simultaneously provide potential toward combined treatment strategies to exploit synergistic effects.

Influence of MDR1 methylation on the curative effect of interventional embolism chemotherapy for cervical cancer

Background

Multi-drug resistance (MDR) is the main cause of tumor failure to chemotherapy. This study aims to explore the influence of MDR1 methylation on curative effect of interventional embolism chemotherapy for cervical cancer.

Methods

Sixty-seven patients with cervical cancer receiving embolism chemotherapy were selected, and 45 normal cervical tissues were included as a control. Immunohistochemistry was used to detect the level of P-glycoprotein (P-gp) in cervical cancer, and to make an analysis compared with normal tissues. The methylation status of the MDR1 gene promoter region 16 CpG units was analyzed by using kilobase-specific cracking and matrix-assisted laser desorption ionization time of flight mass spectrometry.

Results

The results indicated that the positive expression rates of P-gp were 0% (0/45) in normal cervical tissue, and 61.19% (41/67) and 77.61% (52/67) before and after interventional embolism chemotherapy in cervical cancer tissues, respectively. There were significant differences compared with normal cervical tissues (χ²=4.2523, 0.0392). The positive expression rate of P-gp before chemotherapy was negatively correlated with efficacy of chemotherapy (r=−0.340, P=0.005). Methylation rate of 13 CpG units in normal tissues was significantly greater than cervical tissues (P<0.05). In cervical cancer tissue, methylation rate of six CpG units before interventional embolism chemotherapy was higher than after chemotherapy, but that of one CpG unit was lower than after chemotherapy (P<0.05). The methylation rate of one CpG unit with effective chemotherapy before chemotherapy was significantly higher than ineffective chemotherapy (P<0.05), and the other CpG units were similar (P>0.05).

Conclusion

P-gp expression level coded by MDR1, methylation status of partial MDR1 gene promoter regions CpG island, is closely related to the efficacy of interventional embolism chemotherapy for cervical cancer before the operation.

Supramolecular peptide amphiphile based nanocarrier for pH-triggered Dox release, overcoming drug resistance

Self-assembled peptide amphiphile–doxorubicin conjugates showed pH-triggered drug release and ability to combat the drug resistance in cancer cells.

How can nanomedicines overcome cellular-based anticancer drug resistance?

This review discusses the mechanisms of anticancer drug resistance according to its cellular level of action and outlines the nanomedicine-based strategies adopted to overcome it.

Mechanisms of Drug Resistance in Veterinary Oncology- A Review with an Emphasis on Canine Lymphoma

Drug resistance (DR) is the major limiting factor in the successful treatment of systemic neoplasia with cytotoxic chemotherapy. DR can be either intrinsic or acquired, and although the development and clinical implications are different, the underlying mechanisms are likely to be similar. Most causes for DR are pharmacodynamic in nature, result from adaptations within the tumor cell and include reduced drug uptake, increased drug efflux, changes in drug metabolism or drug target, increased capacity to repair drug-induced DNA damage or increased resistance to apoptosis. The role of active drug efflux transporters, and those of the ABC-transporter family in particular, have been studied extensively in human oncology and to a lesser extent in veterinary medicine. Methods reported to assess ABC-transporter status include detection of the actual protein (Western blot, immunohistochemistry), mRNA or ABC-transporter function. The three major ABC-transporters associated with DR in human oncology are ABCB1 or P-gp, ABCC1 or MRP1, and ABCG2 or BCRP, and have been demonstrated in canine cell lines, healthy dogs and dogs with cancer. Although this supports a causative role for these ABC-transporters in DR cytotoxic agents in the dog, the relative contribution to the clinical phenotype of DR in canine cancer remains an area of debate and requires further prospective studies.

Inhibition of P-glycoprotein Gene Expression and Function Enhances Triptolide-induced Hepatotoxicity in Mice

Triptolide (TP) is the major active principle of Tripterygium wilfordii Hook f. and very effective in treatment of autoimmune diseases. However, TP induced hepatotoxicity limited its clinical applications. Our previous study found that TP was a substrate of P-glycoprotein and its hepatobiliary clearance was markedly affected by P-gp modulation in sandwich-cultured rat hepatocytes. In this study, small interfering RNA (siRNA) and specific inhibitor tariquidar were used to investigate the impact of P-gp down regulation on TP-induced hepatotoxicity. The results showed that when the function of P-gp was inhibited by mdr1a-1 siRNA or tariquidar, the systemic and hepatic exposures of TP were significantly increased. The aggravated hepatotoxicity was evidenced with the remarkably lifted levels of serum biomarkers (ALT and AST) and pathological changes in liver. The other toxicological indicators (MDA, SOD and Bcl-2/Bax) were also significantly changed by P-gp inhibition. The data analysis showed that the increase of TP exposure in mice was quantitatively correlated to the enhanced hepatotoxicity, and the hepatic exposure was more relevant to the toxicity. P-gp mediated clearance played a significant role in TP detoxification. The risk of herb-drug interaction likely occurs when TP is concomitant with P-gp inhibitors or substrates in clinic.

Phospholipid-modified PEI-based nanocarriers for in vivo siRNA therapeutics against multidrug-resistant tumors

Multidrug resistance (MDR) mediated by P-glycoprotein overexpression in solid tumors is a major factor in the failure of many forms of chemotherapy. Here we evaluated phospholipid-modified, low-molecular-weight polyethylenimine (DOPE-PEI) nanocarriers for intravenous delivery of anti-P-pg siRNA to tumors with the final goal of modulating MDR in breast cancer. First, we studied the biodistribution of DOPE-PEI nanocarriers and the effect of PEG coating in a subcutaneous breast tumor model. Four hours postinjection, PEGylated carriers showed an 8% injected dose (ID) accumulation in solid tumor via the enhanced permeability and retention effect and 22% ID in serum due to a prolonged, PEG-mediated circulation. Second, we established the therapeutic efficacy and safety of DOPE-PEI/siRNA-mediated P-gp downregulation in combination with doxorubicin (Dox) chemotherapy in MCF-7/MDR xenografts. Weekly injection of siRNA nanopreparations and Dox for up to 5 weeks sensitized the tumors to otherwise non-effective doses of Dox and decreased the tumor volume by threefold vs controls. This therapeutic improvement in response to Dox was attributed to the significant, sequence-specific P-gp downregulation in excised tumors mediated by the DOPE-PEI formulations.

Cancer-targeted MDR-1 siRNA delivery using self-cross-linked glycol chitosan nanoparticles to overcome drug resistance

P-glycoprotein (Pgp) mediated multi-drug resistance (MDR) is a major cause of failure in chemotherapy. In this study, small interfering RNA (siRNA) for Pgp down-regulation was delivered to tumors to overcome MDR in cancer. To achieve an efficient siRNA delivery in vivo, self-polymerized 5'-end thiol-modified siRNA (poly-siRNA) was incorporated in tumor targeting glycol chitosan nanoparticles. Pgp-targeted poly-siRNA (psi-Pgp) and thiolated glycol chitosan polymers (tGC) formed stable nanoparticles (psi-Pgp-tGC NPs), and the resulting nanoparticles protected siRNA molecules from enzymatic degradation. The psi-Pgp-tGC NPs could release functional siRNA molecules after cellular delivery, and they were able to facilitate siRNA delivery to Adriamycin-resistant breast cancer cells (MCF-7/ADR). After intravenous administration, the psi-Pgp-tGC NPs accumulated in MCF-7/ADR tumors and down-regulated P-gp expression to sensitize cancer cells. Consequently, chemo-siRNA combination therapy significantly inhibited tumor growth without systemic toxicity. These psi-Pgp-tGC NPs showed great potential as a supplementary therapeutic agent for drug-resistant cancer.

Reversal of multidrug resistance in vitro by co-delivery of MDR1 targeting siRNA and doxorubicin using a novel cationic poly(lactide-co-glycolide) nanoformulation

Over expression of drug efflux transporters such as P-glycoprotein (P-gp) cumulatively leading to multidrug resistance (MDR) embodies a major hindrance for successful cancer therapy. A paradigm nanomedicinal approach involving an anticancer drug and modulators of drug resistance within one multifunctional nanocarrier-based delivery system represent an ideal modality for the treatment of MDR. In this regards, we have developed a cationic polymeric nanoparticulate system loaded with MDR1-siRNA and doxorubicin. Results indicated augmented synergistic effect of combinational nanoformulation in overcoming MDR in MCF-7/ADR cells. Therefore, the above regime could be a promising co-delivery system for effective therapy of drug resistant breast cancer.

Restoration of chemosensitivity by multifunctional micelles mediated by P-gp siRNA to reverse MDR

One of the main obstacles in tumor therapy is multiple drug resistance (MDR) and an underlying mechanism of MDR is up-regulation of the transmembrane ATP-binding cassette (ABC) transporter proteins, especially P-glycoprotein (P-gp). In the synergistic treatment of siRNA and anti-cancer drug doxorubicin, it is crucial that both the siRNA and doxorubicin are simultaneously delivered to the tumor cells and the siRNA can fleetly down-regulate P-g before doxorubicin inactivates the P-gp and is pumped out. Herein, a type of micelles comprising a polycationic PEI-CyD shell to condense the siRNA and hydrophobic core to package doxorubicin is reported. The structure of the polymer is determined by (1)H NMR, FT-IR, DSC, and XRD and the micelles are characterized by DLS, 2D-NOESY NMR, and TEM to study the self-assembly of the micelles with siRNA and drugs. In vitro studies demonstrate controlled release and temporal enhancement of the therapeutic efficacy of P-gp siRNA and doxorubicin. Release of siRNA down-regulates the mRNA and protein levels of P-gp in the MCF-7/ADR cell lines effectively and the accumulated doxorubicin facilitates apoptosis of the cells to reverse MDR. Moreover, in vivo research reveals that the siRNA and doxorubicin loaded micelles induce tumor cell apoptosis and inhibit the growth of MDR tumor. The western blotting and RT-PCR results illustrate that the synergistic treatment of siRNA and doxorubicin leads to efficient reduction of the P-gp expression as well as cell apoptotic induction in MDR tumors at a small dosage of 0.5 mg/kg. The micelles have large clinical potential in drug/RNAi synergistic treatment via restoration of the chemosensitivity in MDR cancer therapy.

Cell-penetrating, guanidinium-rich molecular transporters for overcoming efflux-mediated multidrug resistance

Multidrug resistance (MDR) is a major cause of chemotherapy failure in the clinic. Drugs that were once effective against naïve disease subsequently prove ineffective against recurrent disease, which often exhibits an MDR phenotype. MDR can be attributed to many factors; often dominating among these is the ability of a cell to suppress or block drug entry through upregulation of membrane-bound drug efflux pumps. Efflux pumps exhibit polyspecificity, recognizing and exporting many different types of drugs, especially those whose lipophilic nature contributes to residence in the membrane. We have developed a general strategy to overcome efflux-based resistance. This strategy involves conjugating a known drug that succumbs to efflux-mediated resistance to a cell-penetrating molecular transporter, specifically, the cell-penetrating peptide (CPP), d-octaarginine. The resultant conjugates are discrete single entities (not particle mixtures) and highly water-soluble. They rapidly enter cells, are not substrates for efflux pumps, and release the free drug only after cellular entry at a rate controlled by linker design and favored by target cell chemistry. This general strategy can be applied to many classes of drugs and allows for an exceptionally rapid advance to clinical testing, especially of drugs that succumb to resistance. The efficacy of this strategy has been successfully demonstrated with Taxol in cellular and animal models of resistant cancer and with ex vivo samples from patients with ovarian cancer. Next generation efforts in this area will involve the extension of this strategy to other chemotherapeutics and other MDR-susceptible diseases.

Reversal of ATP-binding cassette drug transporter activity to modulate chemoresistance: why has it failed to provide clinical benefit?

Enhanced drug extrusion from cells due to the overexpression of the ATP-binding cassette (ABC) drug transporters inhibits the cytotoxic effects of structurally diverse and mechanistically unrelated anticancer agents and is a major cause of multidrug resistance (MDR) of human malignancies. Multiple compounds can suppress the activity of these efflux transporters and sensitize resistant tumor cells, but despite promising preclinical and early clinical data, they have yet to find a role in oncologic practice. Based on the knowledge of the structure, function, and distribution of MDR-related ABC transporters and the results of their preclinical and clinical evaluation, we discuss probable reasons why these inhibitors have not improved the outcome of therapy for cancer patients. We also outline new MDR-reversing strategies that directly target ABC transporters or circumvent relevant signaling pathways.

Nanotechnology approaches for personalized treatment of multidrug resistant cancers

The efficacy of chemotherapy is substantially limited by the resistance of cancer cells to anticancer drugs that fluctuates significantly in different patients. Under identical chemotherapeutic protocols, some patients may receive relatively ineffective doses of anticancer agents while other individuals obtain excessive amounts of drugs that induce severe adverse side effects on healthy tissues. The current review is focused on an individualized selection of drugs and targets to suppress multidrug resistance. Such selection is based on the molecular characteristics of a tumor from an individual patient that can potentially improve the treatment outcome and bring us closer to an era of personalized medicine.

Reversal of multidrug resistance by stimuli-responsive drug delivery systems for therapy of tumor

Multidrug resistance (MDR) is a major obstacle to successful cancer therapy, especially for chemotherapy. The new drug delivery system (DDS) provides promising approaches to reverse MDR, for which the poor cellular uptake and insufficient intracellular drug release remain rate-limiting steps for reaching the drug concentration level within the therapeutic window. Stimulus-coupled drug delivery can control the drug-releasing pattern temporally and spatially, and improve the accumulation of chemotherapeutic agents at targeting sites. In this review, the applications of DDS which is responsive to different types of stimuli in MDR cancer therapy is introduced, and the design, construction, stimuli-sensitivity and the effect to reverse MDR of the stimuli-responsive DDS are discussed.

Nanopreparations to overcome multidrug resistance in cancer

Multidrug resistance is the most widely exploited phenomenon by which cancer eludes chemotherapy. Broad variety of factors, ranging from the cellular ones, such as over-expression of efflux transporters, defective apoptotic machineries, and altered molecular targets, to the physiological factors such as higher interstitial fluid pressure, low extracellular pH, and formation of irregular tumor vasculature are responsible for multidrug resistance. A combination of various undesirable factors associated with biological surroundings together with poor solubility and instability of many potential therapeutic small & large molecules within the biological systems and systemic toxicity of chemotherapeutic agents has necessitated the need for nano-preparations to optimize drug delivery. The physiology of solid tumors presents numerous challenges for successful therapy. However, it also offers unique opportunities for the use of nanotechnology. Nanoparticles, up to 400 nm in size, have shown great promise for carrying, protecting and delivering potential therapeutic molecules with diverse physiological properties. In this review, various factors responsible for the MDR and the use of nanotechnology to overcome the MDR, the use of spheroid culture as well as the current technique of producing microtumor tissues in vitro are discussed in detail.

Cellular robustness conferred by genetic crosstalk underlies resistance against chemotherapeutic drug doxorubicin in fission yeast

Doxorubicin is an anthracycline antibiotic that is among one of the most commonly used chemotherapeutic agents in the clinical setting. The usage of doxorubicin is faced with many problems including severe side effects and chemoresistance. To overcome these challenges, it is important to gain an understanding of the underlying molecular mechanisms with regards to the mode of action of doxorubicin. To facilitate this aim, we identified the genes that are required for doxorubicin resistance in the fission yeast Schizosaccharomyces pombe. We further demonstrated interplay between factors controlling various aspects of chromosome metabolism, mitochondrial respiration and membrane transport. In the nucleus we observed that the subunits of the Ino80, RSC, and SAGA complexes function in the similar epistatic group that shares significant overlap with the homologous recombination genes. However, these factors generally act in synergistic manner with the chromosome segregation regulator DASH complex proteins, possibly forming two major arms for regulating doxorubicin resistance in the nucleus. Simultaneous disruption of genes function in membrane efflux transport or the mitochondrial respiratory chain integrity in the mutants defective in either Ino80 or HR function resulted in cumulative upregulation of drug-specific growth defects, suggesting a rewiring of pathways that synergize only when the cells is exposed to the cytotoxic stress. Taken together, our work not only identified factors that are required for survival of the cells in the presence of doxorubicin but has further demonstrated that an extensive molecular crosstalk exists between these factors to robustly confer doxorubicin resistance.

Drug efflux transporters and multidrug resistance in acute leukemia: therapeutic impact and novel approaches to mediation

Multidrug resistance (MDR), which is mediated by multiple drug efflux ATP-binding cassette (ABC) transporters, is a critical issue in the treatment of acute leukemia, with permeability glycoprotein (P-gp), multidrug resistance-associated protein 1, and breast cancer resistance protein (i.e., ABCG2) consistently being shown to be key effectors of MDR in cell line studies. Studies have demonstrated that intrinsic MDR can arise as a result of specific gene expression profiles and that drug-induced overexpression of P-gp and other MDR proteins can result in acquired resistance, with multiple ABC transporters having been shown to be overexpressed in cell lines selected for resistance to multiple drugs used to treat acute leukemia. Furthermore, numerous anticancer drugs, including agents commonly used for the treatment of acute leukemia (e.g., doxorubicin, vincristine, mitoxantrone, and methotrexate), have been shown to be P-gp substrates or to be susceptible to efflux mediated by other MDR proteins, and multiple clinical studies have demonstrated associations between P-gp or other MDR protein expression and responses to therapy or survival rates in acute leukemia. Here we review the importance of MDR in cancer, with a focus on acute leukemia, and we highlight the need for rapid accurate assessment of MDR status for optimal treatment selection. We also address the latest research on overcoming MDR, from inhibition of P-gp and other MDR proteins through various approaches (including direct antagonism and gene silencing) to the design of novel agents or novel delivery systems for existing therapeutic agents, to evade cellular efflux.