Overcoming multidrug resistance with nanomedicines

Introduction: Cancer remains the leading cause of death worldwide. Numerous therapeutic strategies that include smart biological treatments toward specific cellular pathways are being developed. Yet, inherent and acquired multidrug resistance (MDR) to chemotherapeutic drugs remains the major obstacle in effective cancer treatments.

Areas covered: Herein, we focused on an implementation of nanoscale drug delivery strategies (nanomedicines) to treat tumors that resist MDR. Specifically, we briefly discuss the MDR phenomenon and provide structural and functional characterization of key proteins that account for MDR. We next describe the strategies to target tumors using nanoparticles and provide a mechanistic overview of how changes in the influx:efflux ratio result in overcoming MDR.

Expert opinion: Various strategies have been applied in preclinical and clinical settings to overcome cancer MDR. Among them are the use of chemosensitizers that aim to sensitize the cancer cells to chemotherapeutic treatment and the use of nanomedicines as delivery vehicles that can increase the influx of drugs into cancer cells. These strategies can enhance the therapeutic response in resistant tumors by bypassing efflux pumps or by increasing the nominal amounts of therapeutic payloads into the cancer cells at a given time point.     

Revisiting the role of nanoparticles as modulators of drug resistance and metabolism in cancer

Introduction: Drug resistance is the major obstacle impeding the efficacy of chemotherapeutic agents. Although numerous drug delivery techniques have been developed to combat drug resistance, their limitations of non-specific targeting and inconsistent bioavailability has led to the search of novel delivering strategies, such as nanoparticles.

Areas covered: Nanoparticles for anti-cancer drug delivery are microscopic preparations encapsulating a chemotherapeutic and a chemosensitizer into a rationally designed drug delivery vehicle. Nano-strategies directed against multi-drug resistance (MDR) can be categorized into those inhibiting the drug efflux pumps, those effective against the cellular factors of drug resistance, and the combinational based strategies. Here, we review the most recent literature to reposition nanoparticles as chemotherapeutics and inhibitors of MDR.

Expert Opinion: Novelty in anti-cancer drug delivery has led to the formulation of chemotherapeutics and MDR inhibitors as nano-preparations, which are multi-functional and have better tumor cell-targeting effects. Their characteristics of size and surface attachments make them readily diffusible through the tumor vasculature and increase their retention time as well. With a better understanding of the molecular mechanisms of drug resistance, more potent and multi-targeted nano-preparations can be formulated in the near future.     

鲍曼不动杆菌抗生素主动外排泵转运系统与外排泵抑制剂

鲍曼不动杆菌(Acinetobacter baumannii)是医院内感染的重要病原菌,其耐药率呈现上升的趋势。该菌存在多种耐药机制,其中药物主动外排转运系统或外排泵介导的抗菌药物主动外排与多重耐药(multi-dmg resistance,MDR)密切相关,是近年来研究细菌多重耐药机制的重点。外排泵抑制剂(efflux pump inhibitors,EPIs)的研发有助于克服细菌主动外排机制导致的多重耐药性,为逆转细菌的多重耐药提供了一条新思路。本文就近年来鲍曼不动杆菌的药物主动外排转运系统与外排泵抑制剂的研究进展进行综述。     

The (patho)physiological functions of the MRP family

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

Nanoparticle delivery of anticancer drugs overcomes multidrug resistance in breast cancer

Breast cancer is a serious threat to women's health, because multidrug resistance (MDR) has hampered treatment and prognosis. Nanodelivery of anticancer agents is a new technology to be exploited in the treatment of patients, because it bypasses multispecific drug efflux transporters such as P-glycoprotein (ABCB1), multidrug resistance protein-1 (MRP1, ABCC1) and breast cancer resistance protein (BCRP, ABCG2). Drugs can be delivered to tumor tissue by passive and active tumor targeting strategies, which may reduce or reverse drug resistance. This review will mainly focus on MDR-associated proteins, as well as various nanoparticle formulations developed to overcome MDR in breast cancer.     

Recent advances in anti-multidrug resistance for nano-drug delivery system

Chemotherapy for tumors occasionally results in drug resistance, which is the major reason for the treatment failure. Higher drug doses could improve the therapeutic effect, but higher toxicity limits the further treatment. For overcoming drug resistance, functional nano-drug delivery system (NDDS) has been explored to sensitize the anticancer drugs and decrease its side effects, which are applied in combating multidrug resistance (MDR) via a variety of mechanisms including bypassing drug efflux, controlling drug release, and disturbing metabolism. This review starts with a brief report on the major MDR causes. Furthermore, we searched the papers from NDDS and introduced the recent advances in sensitizing the chemotherapeutic drugs against MDR tumors. Finally, we concluded that the NDDS was based on several mechanisms, and we looked forward to the future in this field.     

逆转肿瘤耐药性的策略及相关小分子药物的研究进展

多药耐药(multidrug resistance, MDR)是肿瘤化疗失败最常见而又最难解决的问题。研究 MDR 机制,开发具有克服 MDR 作用的新型化疗药物是抗肿瘤药物研究的一个热点领域。肿瘤细胞耐药是一个复杂的、动态的体系,可以发生在细胞膜或细胞质水平,也可发生在细胞解毒系统、DNA 修复系统以及药物作用靶点的改变上。该文在总结逆转肿瘤耐药性策略的基础上,对 MDR 逆转剂、作用于细胞解毒系统以及作用于 Bcl-2 家族、DNMT和 p53 等新靶点的小分子药物的研究进展进行综述。     

Imaging drug resistance with radiolabeled molecules

A major obstacle to successful cancer chemotherapy is drug resistance. Multidrug resistance (MDR) is often seen with chemotherapeutic agents such as anthracycline derivatives, vinca alkaloids and taxanes. Multiple aspects of cellular biochemistry have been implicated in the MDR process. Cellular mechanisms of resistance are due to the presence of efflux pumps, P-glycoprotein (P-gp) and multiple resistance-associated protein (MRP), which belong to the ATP-binding cassette (ABC) family of transporters. Another form of drug resistance is involved in the chemotherapy of cancers with alkylating agents such as nitrosourea derivatives and nitrogen mustards. The cytotoxicity of these agents is primarily due to alkylation of the DNA guanine residues at their O6-position, which leads, via a cascade of events, to DNA strand breaks. The DNA repair protein, alkylguanine-DNA alkyl transferase (AGT) removes the alkyl groups from the lesions stoichiometrically to a cysteine in its active site. This process is irreversible and results in the degradation of the protein and its recovery is entirely from de novo synthesis. Noninvasive methodologies for monitoring the transport activity of these efflux pumps and determining tumor content of AGT could serve as critical tools for optimizing chemotherapeutic protocols on a patient-specific basis and gaining an understanding of the dynamics of resistance in living patients. In this review, we will describe the efforts made to date to synthesize radioactive probes of chemotherapy resistance and their use to quantitate these transporters and DNA repair protein by radionuclide imaging.     

Overcoming drug resistance by regulating nuclear receptors

Drug resistance involves multiple mechanisms. Multidrug resistance (MDR) is the leading cause of treatment failure in cancer therapy. Elevated levels of MDR proteins [members of the ATP-binding cassette (ABC) transporter family] increase cellular efflux and decrease the effectiveness of chemotherapeutic agents. As a salvage approach to overcome drug resistance, inhibitors of MDR proteins have been developed, but have had limited success mainly due to undesired toxicities. Nuclear receptors (NRs), including pregnane X receptor (PXR), regulate the expression of proteins (including MDR proteins) involved in drug metabolism and drug clearance, suggesting that it is possible to overcome drug resistance by regulating NR. This review discusses the progress in the development of MDR inhibitors, with a focus on MDR1 inhibitors. Recent development of PXR antagonists to pharmacologically modulate PXR is also reviewed. The review proposes that selectively preventing the elevation of MDR levels by regulating NRs rather than non-selectively inhibiting the MDR activity by using MDR inhibitors can be a less toxic approach to overcome drug resistance during cancer therapy.     

Overcoming cellular multidrug resistance using classical nanomedicine formulations

Over the past few decades, many different types of nanomedicines have been evaluated, both in vitro and in vivo. In general, nanomedicines are designed to improve the in vivo properties of low-molecular-weight (chemo-) therapeutic drugs, i.e. their biodistribution and the target site accumulation, and to thereby improve the balance between their efficacy and toxicity. A significant number of studies have also addressed the in vitro properties of nanomedicines, showing e.g. their ability to overcome cellular multidrug resistance (MDR). Particularly promising results in this regard have been reported for 'pharmacologically active' carrier materials, such as Pluronics, which are able to directly inhibit drug efflux pumps and other cellular detoxification mechanisms. In the present report, we have set out to evaluate the ability of classical (and pharmacologically inactive) carrier materials to overcome MDR. To this end, four different drug-sensitive and drug-resistant cancer cell lines were treated with increasing concentrations of free doxorubicin, of polymer-bound doxorubicin, of micellar doxorubicin and of liposomal doxorubicin, and resistance indices (IC(50) in resistant cells/IC(50) in sensitive cells) were determined. In addition, the cellular uptake of the four formulations was evaluated using fluorescence microscopy. It was found that the carrier materials did manage to overcome MDR to some extent, but that the overall benefit was quite small; only for polymer-bound doxorubicin in A431 cells, a significant (4-fold) reduction in the resistance index was observed. These findings indicate that the ability of classical nanomedicines to overcome cellular MDR should not be overestimated.     

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.     

Reversal of antifungal resistance mediated by ABC efflux pumps from Candida albicans functionally expressed in yeast

The enhanced efflux of antifungal drugs through ATP-binding cassette (ABC) transporters constitutes a major cause of clinical multidrug resistance (MDR). The inhibition of drug efflux pumps by specific compounds is considered to be a feasible strategy to overcome clinical antifungal resistance. Therefore, several blockers of mammalian and yeast ABC drug pumps, including FK506, propafenones, as well as the antifungal drug terbinafine were tested for their capacity to reverse CDR-mediated azole resistance in bakers yeast and in clinical isolates of Candida albicans. We have functionally expressed the C. albicans Cdr1p and Cdr2p transporters in hypersensitive Saccharomyces cerevisiae recipient strains lacking several endogenous ABC pumps. Cdr1p and Cdr2p were functional in yeast, as they conferred pronounced drug resistance to known antifungal drugs, including azoles and terbinafine. We employ two functional assays to demonstrate that ABC pump inhibitors reverse CDR-mediated antifungal resistance, thereby restoring drug susceptibility of yeast cells and resistant clinical isolates. Our results suggest that reversal of antifungal resistance can be achieved through ABC pump-dependent and independent mechanisms.     

真菌多药耐药外排机制的研究进展

真菌多药耐药性是指真菌细胞对结构不同、作用靶点不同的药物同时具有耐药性的现象,是导致临床抗真菌治疗失败的重要原因之一。本文综述了酿酒酵母、条件致病真菌白假丝酵母、光滑假丝酵母和烟曲霉中多药耐药相关转运蛋白、药物外排机制以及基因表达调控网络的研究进展,旨在为深入了解真菌多药耐药性的机制、探讨克服多药耐药性的策略和提高抗真菌药物的药效提供参考。     

Overcoming chemotherapy resistance via simultaneous drug-efflux circumvention and mitochondrial targeting

Multidrug resistance (MDR) has been considered as a huge challenge to the effective chemotherapy. Therefore, it is necessary to develop new strategies to effectively overcome MDR. Here, based on the previous research of N-(2-hydroxypropyl)methacrylamide (HPMA) polymer–drug conjugates, we designed an effective system that combined drug-efflux circumvention and mitochondria targeting of anticancer drug doxorubicin (Dox). Briefly, Dox was modified with mitochondrial membrane penetrating peptide (MPP) and then attached to (HPMA) copolymers (P-M-Dox). Our study showed that macromolecular HPMA copolymers successfully bypassed drug efflux pumps and escorted Dox into resistant MCF-7/ADR cells via endocytic pathway. Subsequently, the mitochondria accumulation of drugs was significantly enhanced with 11.6-fold increase by MPP modification. The excellent mitochondria targeting then resulted in significant enhancement of reactive oxygen species (ROS) as well as reduction of adenosine triphosphate (ATP) production, which could further inhibit drug efflux and resistant cancer cell growth. By reversing Dox resistance, P-M-Dox achieved much better suppression in the growth of 3D MCF-7/ADR tumor spheroids compared with free Dox. Hence, our study provides a promising approach to treat drug-resistant cancer through simultaneous drug efflux circumvention and direct mitochondria delivery.     

DbMDR: a relational database for multidrug resistance genes as potential drug targets

DbMDR is non-redundant reference database of multidrug resistance (MDR) genes and their orthologs acting as potential drug targets. Drug resistance is a common phenomenon of pathogens, creating a serious problem of inactivation of drugs and antibiotics resulting in occurrence of diseases. Apart from other factors, the MDR genes present in pathogens are shown to be responsible for multidrug resistance. Much of the unorganized information on MDR genes is scattered across the literature and other web resources. Thus, consolidation of such knowledge about MDR genes into one database will make the drug discovery research more efficient. Mining of text for MDR genes has resulted into a large number of publications but in scattered and unorganized form. This information was compiled into a database, which enables a user not only to look at a particular MDR gene but also to find out putative homologs based on sequence similarity, conserved domains, and motifs in proteins encoded by MDR genes more efficiently. At present, DbMDR database contains 2843 MDR genes characterized experimentally as well as functionally annotated with cross-referencing search support. The DbMDR database (http://203.190.147.116/dbmdr/) is a comprehensive resource for comparative study focused on MDR genes and metabolic pathway efflux pumps and intended to provide a platform for researchers for further research in drug resistance.     

Contribution of efflux pump activity to the delivery of pulmonary therapeutics

To date, there are few in vitro models of the human lung that have been used to characterize multidrug resistant (MDR) efflux pump activity. It is expected that the presence of these protein transporter molecules, such as P-glycoprotein (Pgp) and the multidrug resistance protein associated protein-1 (MRP1), might play a role in limiting drug absorption through the pulmonary epithelium, as has been reported for other epithelial drug delivery barriers such as the intestine and brain. To date, the exact role of the lung resistance related protein (LRP) in MDR is unclear. In this article, we have summarized the biochemistry, function and in vitro/in vivo modulation of Pgp and MRP1. These topics are discussed in light of pulmonary delivery of therapeutic agents, with particular emphasis being placed on the bronchial region of human airways.     

MDR1/P-glycoprotein (ABCB1) as target for RNA interference-mediated reversal of multidrug resistance

Resistance of tumor cells to multiple structurally unrelated cytotoxic drugs, multidrug resistance (MDR), is the major limitation to the successful chemotherapeutic treatment of disseminated neoplasms. The "classical" MDR phenotype is the result from decreased cellular drug accumulation mediated by the adenosine triphosphate binding cassette (ABC)-transporter MDR1/P-glycoprotein (MDR1/P-gp, ABCB1) encoded by the human MDR1 gene. Inhibition of the drug extrusion activity of MDR1/P-gp by low-molecular weight pharmacologically active compounds as a method to reverse MDR in patients suffering on malignant diseases has been studied capaciously, but the clinical results have generally been disappointing. Thus, experimental therapeutic strategies to reverse MDR are under extensive investigation. These strategies included gene therapeutic approaches with antisense oligonucleotides (ODNs), ribozymes, or DNAzymes and, most recently, the application of the RNA interference (RNAi) technology. RNAi is a physiological double stranded RNA-triggered mechanism resulting in gene-silencing in a sequence-specific manner. Transient RNAi can be attained by application of small interferring RNAs (siRNAs), whereas a stable RNAi-mediated gene-silencing can be achieved by transfection of mammalian cells with short hairpin RNA (shRNA) encoding expression cassettes localized on plasmid or viral vectors. Transient and stable RNAi strategies were applied to overcome MDR1/P-gp-mediated MDR in different in vitro models derived from various neoplastic tissue and will be come up for discussion.     

The role of structural factors in the kinetics of cellular uptake of pyrazoloacridines and pyrazolopyrimidoacridines: implications for overcoming multidrug resistance towards leukaemia K562/DOX cells

The appearance of multidrug resistance (MDR) of tumour cells to a wide array of antitumour drugs, structurally diverse and having different mechanisms of action, constitutes the major obstacle to the successful treatment of cancer. Our approach to search for non-cross resistant antitumour agents is based on the rational design of derivatives, which have a high kinetics of passive cellular uptake rendering their active efflux by MDR exporting pumps inefficient. Recently, two families of acridine cytotoxic agents were obtained, pyrazoloacridines (PACs) and pyrazolopyrimidoacridines (PPACs). The aim of this study was to examine molecular basis of the reported differences in retaining cytotoxic activity of these derivatives at cellular level against resistant erythroleukaemia K562/DOX (overexpressing P-glycoprotein) cell line. The study was performed using a spectrofluorometric method, which allows continuous monitoring of the uptake and efflux of fluorescent molecules by living cells. It was demonstrated that the presence of two additional rings, pyrazole and pyrimidine, fused to the acridine chromophore structure (PPAC) favoured more rapid cellular diffusion than the presence of only one additional pyrazole ring (PAC). The presence of hydrophobic substituent OCH3 markedly favoured the cellular uptake of pyrazoloacridines and pyrazolopyrimidoacridines while compounds having hydrophilic substituent OH exhibited very low kinetics of cellular uptake. In contrast, it was found that neither structure of the ring system nor the hydrophobic/hydrophilic character of examined substituents determined the rate of active efflux of these compounds by P-glycoprotein. Our data showed that a nearly linear relation exists between the resistance factor (RF) and lnV+ reflecting the impact of the cellular uptake rate (V+) on the ability of these compounds to overcome MDR.