Nanotechnology applied to overcome tumor drug resistance

Journal of Controlled Release 162 (2012) 45–55

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Review

Nanotechnology applied to overcome tumor drug resistance Zibin Gao ⁎, Linan Zhang, Yongjun Sun Department of Pharmacy, College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, 70 Yuhua East Road, Shijiazhuang 050018, P. R. China

a r t i c l e

i n f o

Article history: Received 27 March 2012 Accepted 31 May 2012 Available online 12 June 2012 Keywords: Emerging multidrug resistance Nanotechnology Drug delivery system Chemotherapy Tumor

a b s t r a c t Emerging multidrug resistance (MDR) to chemotherapy is a major obstacle in successfully treating malignant diseases. Nanotechnology provides an innovative and promising alternative strategy compared to conventional small molecule chemotherapeutics to circumvent MDR. This review focuses on recent literature examples of nanotechnology applications to overcome MDR. The advantages and limitations of various nanotechnologies are discussed as well as possible approaches to overcome the limitations. Developing a practical nanotechnology-based drug delivery system requires further studies of the tumor microenvironment, the mechanisms of MDR to chemotherapy, the optimal dosage regimen of anticancer drugs and/or siRNA, the transport kinetics of nanocarriers in tumor stroma and the pharmacokinetics of drug-loaded nanocarriers within MDR tumor cells. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanotechnology for enhancing the accumulation and internalization of drugs 2.1. Passive tumor targeting . . . . . . . . . . . . . . . . . . . . . . 2.2. “Stealth” nanotechnology to increase circulation time . . . . . . . . 2.3. “Stealth” endocytosis . . . . . . . . . . . . . . . . . . . . . . . 2.4. Active tumor localization and active tumor cell internalization . . . . 3. Nanotechnology for stimuli sensitive intercellular release . . . . . . . . . . 3.1. pH-sensitive intercellular release . . . . . . . . . . . . . . . . . . 3.2. External stimuli sensitive . . . . . . . . . . . . . . . . . . . . . 4. Nanotechnology for simultaneous targeting delivery of different agents . . . 5. Nanotechnology for targeting gene delivery . . . . . . . . . . . . . . . . 6. Nanotechnology for targeting to energy metabolism . . . . . . . . . . . . 7. ‘Pharmacologically active’ nanocarrier materials for overcoming MDR . . . . 8. Challenges and perspectives . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Chemotherapy combined with radiotherapy and surgical resection is the current standard approach to cancer treatment. A major obstacle to successfully treating malignant diseases is the emergence of multidrug resistance (MDR) to chemotherapy, whereby cancer cells become ⁎ Corresponding author at: Department of Pharmacy, Hebei University of Science and Technology, 70 Yuhua East Road, Shijiazhuang 050018, P. R. China. Tel.: + 86 311 88632427; fax: + 86 311 88623771. E-mail address: [email protected] (Z. Gao). 0168-3659/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jconrel.2012.05.051

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resistant to the cytotoxic effects of various structurally and mechanistically unrelated chemotherapeutic agents [1]. This phenomenon contributes to treatment failure in over 90% of patients with metastatic disease [2]. Tumor MDR can be intrinsic or acquired through exposure to chemotherapeutic agents [3]. The selection pressures within a tumor microenvironment and inherent high expression of the ATP-binding cassette (ABC) transporters by tumor cells may result in the development of intrinsic MDR. An acquired resistance in cancers may come from a drug stimulus, which leads to the overexpression of ABC transporters and subsequent efflux of anticancer drugs from the cancer cell cytoplasm [4].

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The most extensively studied mechanism of MDR is the overexpression of cell surface pumps, such as the ATP-binding cassette superfamily. These pumps can successfully purge a wide spectrum of chemotherapeutic agents from cells [5], thereby decreasing their intracellular accumulation. However, the ABC superfamily is only one of several modalities that cause MDR. The major mechanisms of tumor drug resistance can be grouped into at least five categories: decreased drug influx, increased drug efflux, DNA repair activation, detoxification, and inactivation of apoptosis pathways with simultaneous activation of anti-apoptotic cellular defense modalities [6]. The MDR phenotype is usually the synergistic result of a combination of MDR mechanisms [7]. Thus, inhibiting only one contributor to cellular resistance is usually insufficient to overcome all mechanisms of cancer-cell resistance to chemotherapy [8]. Therefore, designing an advanced multifunctional delivery system should be a priority to reverse MDR in cancer chemotherapy. Nanotechnology provides an innovative and promising alternative to conventional small-molecule chemotherapeutics, circumventing MDR by encapsulating, attaching, and conjugating drugs or therapeutic biological products to nanocarriers. Nanocarriers can include small molecules such as lipids or polymer nanoparticles that target the therapeutic payload to tumors or tumor cells. Simultaneously, multifunctional drug-loaded nanocarriers can also enhance particle penetration of physiological barriers and protect the labile drugs or therapeutic biological products. Furthermore, nanocarriers assist with solubilizing hydrophobic drugs, decrease drug clearance, deliver multiple therapeutic payloads, regulate sustained drug release, target drug delivery, and release drugs in a stimulus-triggered manner. These advantages endow nanotechnology with the capacity to address complicated and combined mechanisms of MDR and make nanotechnology a valuable candidate approach to overcoming MDR. Examples of nanocarriers applied to address MDR include the following: nanoparticles, liposomes, micelles, dendrimers, quantum dots, magnetic nanoparticles, nanogolds, minicells and polymer-drug conjugates. These nanoparticles are presented in Fig. 1. This review focuses on recently published literature that applied nanotechnology to overcoming MDR to chemotherapy and discusses the limitations and advantages of various nanotechnologies as well as alternative approaches to overcome them. Because there are a plethora of publications on this topic, instead of presenting characteristics of nanocarriers individually, we have classified nanotechnologies into several categories according to their mechanism of treating tumor drug resistance. The first class of nanotechnologies enhances the accumulation and internalization of drugs within tumors. These nanotechnologies include passive tumor targeting, “stealth” nanotechnology to increase circulation time, “stealth” endocytosis, active tumor localization and active tumor cell internalization. There is also nanotechnology for stimuli sensitive intercellular release, such as pH-sensitive intercellular release and external stimuli sensitive drug release. Additionally, there is nanotechnology for simultaneously targeting delivery of different agents (including genes) and for targeting energy metabolism. Finally, we have identified ‘pharmacologically active’ nanocarrier materials for overcoming MDR. In addressing each topic, we have provided representative examples to highlight the importance and illustrate their mechanism of action. 2. Nanotechnology for enhancing the accumulation and internalization of drugs within tumors Drugs that are encapsulated in nanocarriers do not have the same pharmacokinetic profiles as free drugs. The nanomaterial and the physical and chemical properties of the nanocarriers dictate the drug's biodistribution and pharmacokinetics upon systemic administration. This approach provides a convenient strategy for altering the drug biodistribution and enhances drug delivery to the target site. It is easy to customize the properties of nanocarriers to achieve a desired

pharmacokinetic profile for a carrier system. For example, the size, charge, density, and surface modification of a nanocarrier system can be manipulated to enhance tumor accumulation and cell internalization of drugs. 2.1. Passive tumor targeting The enhanced permeability and retention (EPR) effect in solid tumors was first introduced by Matsumura and Maeda [18]. The EPR effect can enhance the diffusion of macromolecules or nanomedicines into the tumor via leaky tumor vasculature. The EPR effect causes increased retention of macromolecules within tumor tissues due to the poor tumor lymphatic drainage. EPR is a universal phenomenon in solid tumors (with the exception of hypovascular tumors, such as prostate cancer or pancreatic cancer). The result of EPR is a relatively higher accumulation of nanocarriers at the site of a tumor. This accumulation creates passive tumor targeting. One of the key factors in passive tumor targeting is matching the particle size with the pore size of the tumor vasculature. Although the fenestrations associated with abnormal tumor vasculature can range from 10 nm to 1000 nm, the cell gap junctions depend on the tumor type, malignancy, and stage of disease [19–21]. The EPR effect can also function in bacteria (e.g., Lactobacillus sp. and Salmonella typhimurium) that are larger than 1000 nm [22,23]. Although particles smaller than 10 nm can be manufactured, they can cross the basement membranes in the glomeruli of kidneys and be quickly cleared, which leads to a short blood half-life [24]. A particle size of 10–100 nm might be optimal for in vivo targeting delivery based on EPR effects [25]. However, further knowledge is needed regarding the pore size of tumor vasculature and the relationship between the biodistribution of nanocarriers and particle size. Unfortunately, studies on the degree of tumor vascular permeability, and more precisely the pore size of tumor vasculature, lag behind the development of nanomedicines and hinder the targeted delivery of antitumor drugs by nanotechnology based on the EPR effect of solid tumors. Most articles in this field were published before 2000, and few systematic investigations have been reported in recent years. Furthermore, the data cited in the some publications, and even data within the same study, are sometimes incongruous. For example, one of the most frequently cited articles published in 1998 [19] was cited by several subsequent studies [26–28]. However, each publication used a different value for the pore size of tumor vasculature. This inconsistency might mislead readers. This example highlights how little is known about the tumor microenvironment. The EPR effect could be enhanced by properly exploiting the peculiarities of the tumor microenvironment. The tumor vasculature lacks a smooth-muscle layer, which plays a vital role in regulating blood pressure and flow volume. Maeda's group found that inducing hypertension augmented the EPR effect and enhanced the delivery of macromolecular drugs [29]. The amount of drug delivered to healthy organs such as the intestines, kidney, liver, and bone marrow was reduced because the extravasation of drug-loaded nanocarriers was suppressed by vasoconstriction and tightening of endothelial gap junctions. One result of the reduced extravasation was decreased drug toxicity in the healthy organs. Wayne et al. [30] demonstrated that exogenous vascular endothelial growth factor (VEGF), which is a vascular permeability factor, was useful in augmenting the transvascular delivery of larger antineoplastic agents such as gene targeting vectors and encapsulated nanodrug carriers (typical range, 100 – 300 nm) to tumors [31]. 2.2. “Stealth” nanotechnology to increase circulation time Although the EPR effect helps retain nanoparticles in the tumor, nanocarriers must first remain in circulation long enough to initially reach the site of a tumor. Once in the blood stream, nanoparticles can

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Terminal moiety: Fig. 1. Examples of nanocarriers for treating multidrug resistance. Drug; A: nanoparticles, PEGylated nanoparticles and liganded PEGylated nanoparticles; figure modified based on the Fig. 1from Ref. [9]; B: liposomes and non-ionic surfactant inserting liposome; figure modified based on the Fig. 1 from Ref. [10]; C: polymeric micelles and pH-triggered multifunctionalized micelles; figure modified based on the Fig. 1 from Ref. [11]; D: MPA-CdTe QDs and QDs-DNR; Figure modified based on the Fig. 1 from Ref. [12]; E: Nanogolds and AuPEG-SS-DOX nanoconjugates; Figure modified based on the Fig. 1 from Ref. [13]; F: polymer–drug conjugates; figure modified from Ref. [14,15]; G: dendrimer and PEG-Glu (ADR) Dendrimer; Figure modified based on the Fig. 2 from Ref. [16]; H: minicell; figure reproduced from Ref. [17]. The illustrations were prepared based on the data, schemes and figures provided in the respective references with permission from the publishers.

be recognized by the host immune system. Nanoparticles are cleared by the reticuloendothelial system (RES), which includes the kidney, liver, spleen, and lymph nodes. Strategies such as modifying the nanoparticles with hydrophilic polymers/surfactants or formulating them with biodegradable copolymers with hydrophilic characteristics (e.g., polyethylene glycol (PEG), block copolymers of poly(ethylene oxide) (PEO), polyoxamer, poloxamine, and Tween 80) have been used to enhance particle circulation. These modifications can achieve passive targeted delivery by inhibiting phagocytosis by mononuclear phagocytes [32] and reducing the uptake of the nanocarriers by the RES [33]. Hydrophilic polymers shield particles from phagocytosis by forming a tightly bound network of water. This hydrophilic surface prevents the particle recognition and interaction required for uptake and clearance by macrophages. As the molecular weight of PEG increases, the systemic half-life of a nanoparticle system increases. The half-life increases by approximately 972 min when the molecular mass of the

PEG chains increases from 6 to 50 kDa [34]. Interestingly, the structure of PEG can also determine its shielding effects. When the polymer density increases, the space between each polymer and its degrees of freedom are reduced. The shape of the PEG molecule is then switched from a “mushroom” to a “brush” configuration [35]. The PEG molecules with brush-like and intermediate configurations reduce phagocytosis and complement activation. Conversely, the mushroom-like PEG structures potently activated complement and favored phagocytosis [36,37]. The pharmacokinetic profiles of PEG-modified nanocarrier systems are also greatly affected by the route of administration. The preferred route of administration is vascular injection [38]. Nanoparticles with a zeta potential above ±30 mV have been shown to be stable in suspension because the surface charge can prevent aggregation of the particles. A positive nanoparticle surface charge can also promote binding between nanocarriers and tumor cells. For example, cationic liposomes usually show a higher extent

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of cellular uptake than neutral or anionic liposomes. However, cationic carrier systems are removed by the kidneys faster than anionic and neutral systems. The cationic carriers could have a strong electrostatic interaction with negatively charged serum proteins that would result in aggregation [39]. The aggregation of cationic carriers causes a less efficient delivery and unpredictable pharmacokinetic properties in vivo. The addition of a PEG shell to the surface of particles can shield the positive charges and prevent the binding of plasma proteins and the consequent capture of circulating nanoparticles by macrophages [40,41]. Conversely, PEGylation inhibits nanoparticle (NP) binding to the cell surface and decreases the endocytosis of NPs in part by decreasing the positive surface charge. Thus, PEGylation may generate a so-called ‘PEG dilemma’. This negative effect of PEGylation can be eventually eliminated by introducing cleavable or bioreversible surface shielding. Most strategies take advantage of the unique intracellular microenvironments of tumor cells, such as low pH and specific endosomal enzymes [42,43]. Interestingly, the shielding effect of PEG was used to develop a pH-sensitive pop-up polymeric micelle system (PHSM pop-upTAT) (Fig. 2) for the targeting treatment of MDR [44]. The TAT (transactivator of transcription) peptide is a non-specific cell penetrating peptide (CPP) derived from the human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2). Despite a complex and incompletely understood endocytic mechanism, TAT peptide has been used extensively to increase the cellular internalization of otherwise minimally endocytosed nanoparticles [45]. One of the main obstacles in using TAT is its lack of selectivity. However, with the use of a PEG shield and pH-sensitive targeting delivery, this hurdle could be overcome. In a normal in vivo environment (pH ~ 7.4), TAT was shielded within the hydrophilic PEG corona by interfacial hydrophobic interactions between non-ionized polyHis and the PLA micelle core. In a slightly more acidic environment (pH 6.5–7.0) such as the tumor's extracellular pH (pHe), the polyHis (2 kDa) became charged prior to the ionization of the longer polyHis (5 kDa) that was blended with PLA in the micellar core. This ionization exposed TAT on the micellar surface and enhanced the cellular uptake of the PHSM pop-upTAT delivery system within the tumor. 2.3. “Stealth” endocytosis Free drugs are often internalized by diffusion across the cellar membrane and the drug efflux pumps on the membrane can sense free drug molecules while they cross the cellular membrane, and prevent drugs from entering the cytoplasm or making them spatially vulnerable to P-gp capture and efflux. However, nanocarriers are taken up by non-specific endocytosis and cross the cellular membrane in an ‘invisible’ form that prevents the drugs from being recognized by efflux pumps. This method of endocytosis results in a higher intracellular accumulation [46]. We call this type of endocytosis process “stealth endocytosis” (Fig. 3). The particles are internalized in endosomes that release drugs near the peri-nuclear region (or deep inside the cytoplasm) away from membrane ABC transporters [47]. All of these attributes help nanocarriers to bypass efflux dominated by ABC transporters [14]. 2.4. Active tumor localization and active tumor cell internalization Cancer cells and normal cells have subtle differences in their biochemical and molecular machinery. Active targeting that exploits these differences has been extensively explored in experimental and clinical research. However, the recent clinical results of drug-based molecular targeting were somewhat disappointing and only achieved a 4–5% response rate [48]. Attaching ligands such as monoclonal antibodies to drugs to form immunoconjugates for selective targeting may adversely affect the pharmacological action and the in vivo fate

Fig. 2. Schematic of the super pH sensitive pop-up polymeric micelle system. Figure modified based on the Fig. 1 from Ref. [44] with permission from the publishers.

of the drug. Another explanation for the disappointing results is the possibility that the active drug (or proteins, enzymes, or nucleic acids) was inactivated or modified when exposed to endogenous free radicals such as superoxide anion radical, hydrogen peroxide, NO, ONOO −, and hypochlorite in cancer tissues [48]. Using the EPR effect, drug-loaded NP delivery systems with bound tumor-selective ligands conferred greater selectivity [49] without changing the cargo's pharmacological action. This system protected the active drugs from being inactivated or modified by endogenous radicals. This so-called active targeting is different from “passive targeting”, which uses only the EPR effect. However, active targeting cannot be completely separated from passive targeting because active targeting occurs only after passive accumulation of the particles within tumors [50]. Although including ligands in various types of nanoparticle delivery systems results in increased efficacy [10,51], the biodistribution and pharmacokinetics involved in this enhancement have only recently begun to be addressed. The prevailing theory has been that the presence of ligands in the nanocarriers helps direct the carriers to the tumor and significantly augments nanocarrier accumulation in a tumor. However, Bartlett et al. [52] and Kirpotin et al. [53] demonstrated that enhanced cellular internalization rather than increased tumor accumulation is responsible for the antitumoral efficacy of active targeting nanocarriers. Conversely, Wu et al. [54] and other researchers [55,56] demonstrated that although a targeting ligand did not direct a nanocomplex to the tumor, its presence did play a significant role in tumor localization. Therefore, it is necessary to develop a novel theory to reconcile these differing conclusions [57]. The equilibrium concentration of nanocarriers in the tumor stroma results from the balance of many factors, such as blood pressure, the interstitial fluid pressure in the tumor stoma, the osmotic pressure in the vasculature and tumor stoma, the concentration gradient of nanocarriers, the internalization of nanocarriers by tumor cells, and other issues that can influence the EPR effect. These factors can be divided into two simpler categories: those that enhance tumor localization, such as the internalization of nanocarriers by tumor cells, and those that weaken the accumulation of nanocarriers in tumors, such as the high interstitial fluid pressure of tumor stoma. The particle localization within a tumor results from the dynamic balance of these two types of issues (Fig. 4). The high interstitial fluid pressure prevents the penetration of nanocarriers through the tumor tissue and “active targeting” delivery of drugs to the tumor cell. Therefore, active targeting nanoparticles have as much difficulty as passive targeting nanoparticles in localizing to the tumor in vivo. If the localized nanocarriers can be internalized quickly by the tumor cell or other cells in the tumor stroma, then the nanocarriers can be continuously leaked into the tumor microenvironment. In this case, active

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transferrin (Tf) [64], or antibodies [65]. In most cases, these ligand– receptor interactions resulted in efficient uptake of the nanocarriers into tumor cells by receptor-mediated endocytosis and were able to enhance the reversal of MDR. 3. Nanotechnology for stimuli sensitive intercellular release Although there are benefits to active targeting, such as enhanced efficacy, active targeting strategies can result in a high accumulation of the nanocarrier in non-diseased cells due to basal expression of antigens, carbohydrates, and receptors. An alternative approach to achieving localized drug release is using stimuli-responsive formulations. The concept of stimulus-responsive gate keeping was introduced as a means of regulating and controlling drug release by stimuli, including pH and physical forces (e.g., magnetic fields, ultrasound, hyperthermia or light). This technology may help focus and trigger the activation of nanosystems to overcome MDR. 3.1. pH-sensitive intercellular release

Fig. 3. A model of the internalization process for free drugs (A and B) and drugs in nanocarriers in normal tumor cells (A and C) and MDR tumor cells (B and D). Free drugs are often internalized by diffusion across the cell membrane. In normal tumor cells, only a small number of drugs can diffuse out of the cell and most reach their target site, such as the nucleus (A). However, the internalization of free drugs in MDR tumor cells activates the ABC transporters such as P-gp on the cell membrane. The transporters capture and pump drugs out of the MDR tumor cells before the drugs reach the cytoplasm. The result is that very little drug reaches its target site (B). The drugs encased in nanocarriers are internalized by tumor cells in a “stealth” endocytosis process (D) that cannot be sensed by ABC transporters on the MDR cell membrane. Thus, the drugs can be transported close to the peri-nuclear region (or deep inside the cytoplasm). The cellular transport reduces the possibility of the drug being pumped out of the MDR tumor cell. Therefore, nanotechnology could be a potential candidate to overcome MDR, although nanocarriers could also weaken the efficiency of drugs on normal tumor cells (C). Figure modified based on the Fig. 2 from Ref. [14] with permission from the publishers.

endocytosis, the internalization of targeting nanocarriers by tumor cells by various active endocytic pathways [58] plays an important role. The rapid saturation phenomenon that occurs in nonspecific endocytosis is less likely during active endocytosis because receptors are quickly recycled to the cell membrane. Active endocytosis is a pump of nanocarrier leakage that results in accumulation of nanocarriers in the tumor. The active endocytosis of nanocarriers into tumor cells causes efficient killing of these cells, which can lower the transport resistance around the blood vessels and enable nanocarriers to be transported deeper into the tumor tissue. The increased transport results in further enhanced particle localization (Fig. 4A). However, if the “functionalized nanocarriers” cannot be transported efficiently and quickly, they will accumulate near the leaky blood vessels after extravasation rather than migrating deep into the tumors. This road block effect could obstruct the additional accumulation of the carriers at tumor sites [59]. However, at the distal end of the “embolus”, active internalization of nanocarriers still occurs. Combining these two processes will enhance internalization without increasing the accumulation of nanocarriers in the tumor (Fig. 4B). In other words, active endocytosis may be crucial for determining the concentration or tumor localization of nanocarriers in tumor tissues. Although additional studies are needed to validate this theory, active endocytosis of nanocarriers results in a dramatic increase in drug concentration. It also has a vital role in reversing MDR by allowing enough drug to accumulate in the cytoplasm to be effective in both pump-dependent and pump-independent MDR [26,60]. Active nanocarrier endocytosis by MDR tumor cells has been accomplished by including a ligand, such as the RGD peptide [11,41], epidermal growth factor receptor (EGFR) [61], biotin [10,62], folate [26,63],

In the late stage of NP internalization, the endocytosis vesicle fuses with endosomes. The V-ATPase in the endosomal membrane causes an influx of protons that results in a continuous pH drop as endosomes mature from early endosomes to late endosomes. The pH decreases even further when late endosomes fuse with lysosomes [66]. Several groups have recently published work that uses an acidic pH-activated mechanism to overcome efflux-dependent MDR by disrupting the endosomal membrane and releasing NPs cargo drugs into the cytoplasm in a burst-like manner [13,67,68]. Most studies focused on endosome-targeted release of nanocarriers were promising. However, the exact pH gradients in the extracellular, endosomal, and lysosomal environments, especially the gradients between the extracellular compartment and other compartments are still not fully understood. It is possible that the nanocarriers designed to target endosomes (pH up to 6.5 [69]) could release drugs in the acidic extracellular tumor stroma (pH as low as 5.7 [70]) and fail to reverse the MDR. However, based on the literature from the last two decades there is a clear distinction in the pH value between the extracellular environment (no less than 5.7) [70] and lysosomal environments (no more than 5.4) [66]. In other words, we can exploit the pH difference between the extracellular environment and lysosomal environments to develop a precise pHresponsive drug release nanocarrier system to target lysosomal instead of endosomal compartments to avoid undesired drug release in the tumor stroma. Therefore, it is exceptionally important to design the pKb of polymers to be sensitive to the lysosomal pH to achieve precise pHsensitive drug release. These nanocarriers should be stable in slightly acidic conditions such as the extracellular tumor stroma. However, they should be sensitive to structural transformations and solubility changes in the more acidic lysosomal compartments to trigger drug release. In some cases, a drug can escape from the endosome and the drug still fails to reach the target of certain subcellular organelles even after being released from nanoparticles in the endosome or lysosome. The pH-responsive polymers can disturb endosomal membranes by proton absorption and/or membrane interactions [26,71]. The proton absorption involves osmotic swelling and rupture of the membranes. The membrane interaction creates defects such as pores or channels in lipid membranes. For example, in acidic late endosomes/lysosomes, doxorubicin (DOX) was quickly protonated and became water soluble and positively charged. Hydrophilic, charged DOX could not diffuse through the endosomal/lysosomal membrane and became trapped there. This is the same mechanism that resistant cancer cells used to sequester cytosolic DOX and served as the basis of the proposed protonation, sequestration and secretion (PSS) model. The PSS model accounts for the sensitivity of some tumor cells and resistance of MDR cells to weakly basic chemotherapeutic drugs [72]. In this case, DOXloaded nanoparticles are needed to escape from the endosome/lysosome

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by disrupting their membranes in an intact form instead of only disintegrating in the endosome/lysosome [73]. 3.2. External stimuli sensitive Hyperthermia is a cancer therapy that relies on the localized heating of tumors between 41 °C and 46 °C. Many groups have synthesized cancer-targeting nanosystems that can mediate hyperthermia, including carbon nanotubes and gold- and magnetic-based nanocarriers. Carbon nanotubes are unique materials that absorb infrared (IR) radiation, especially between 700 and 1100 nm, where body tissues are most transparent. The absorption of IR promotes molecular oscillation that leads to efficient heating of the surrounding environment. Single-walled carbon nanotubes (SWNTs) conjugated with an antiCD133 monoclonal antibody (anti-CD133) (Fig. 5) that were irradiated with near-infrared laser light were used as a thermal-coupling agent to effectively target and destroy chemotherapeutic resistant glioblastoma cancer stem-like cells (GBM CSCs) in vitro and in vivo [74]. Although both gold- and magnetic-based nanocarriers have been successfully modified (folic acid (FA) [75], conjugated with antiHER2 [76]) to target hyperthermic therapy directly to cancer cells rather than to the whole tissue, these two types of nanostructures have not yet been fully exploited to overcome MDR. However,

magnetic targeting (MT) was developed with a type of super highmagnetization nanocarriers (SHMNCs) comprised by a magnetic Fe3O4 (SHMNPs) core and an aqueous stable self-doped poly [N-(1one-butyric acid)] aniline (SPAnH) shell. The complex has a high drug loading capacity for DOX [77]. SHMNPs were delivered by magnetic targeting (MT) therapy to MGH-U1 bladder cancer cells and avoided the P-gp pump-based drug resistance. The result was an increased intracellular DOX concentration (23%) compared to freeDOX. The high R2 relaxivity of SHMNCs made it a more effective chemotherapeutic MT carrier and an excellent MRI contrast agent, which allowed for the assessment of the distribution and concentration of DOX in various tissues and organs. In the last few years, photodynamic therapy (PDT) has also been successfully applied to overcome tumor MDR. PDT combines photosensitizers and specific light sources to treat various diseases. After activating the photosensitizer with a specific wavelength of light, the photosensitizer transfers energy to molecular oxygen to generate reactive oxygen species (ROS) and cause target cell death. Recently, nanotechnology-based photosensitizer delivery systems were able to reduce skin photosensitivity significantly. Skin photosensitivity is a major side effect of PDT. A new generation of photosensitizer in DPc/m-mediated PDT permits deeper light penetration and provides passive targeting and lower skin phototoxicity for treating cancer [78]. The dual selectivity is provided by the NP's ability to localize to tumors.

Fig. 4. Schematic diagram of the equilibrium concentration of nanocarriers in tumor stroma. The rapid saturation phenomenon that occurs in nonspecific endocytosis is less likely during active endocytosis because receptors are quickly recycled to the cell membrane. Active endocytosis will result in the accumulation of nanocarriers in the tumor (A). The active endocytosis of nanocarriers into tumor cells causes efficient killing of these cells, which can lower the transport resistance around the blood vessels and enable nanocarriers to be transported deeper into the tumor tissue. The result is further enhanced particle localization (A). However, if the “functionalized nanocarriers” cannot be transported efficiently and quickly they will accumulate near leaky blood vessels after extravasation, rather than migrating deep into the tumors. This roadblock effect could obstruct the additional accumulation of the carriers at tumor site. However, at the distal end of the “embolus”, active internalization of nanocarriers still occurs. Combining these two processes will enhance internalization without increasing the accumulation of nanocarriers in the tumor (B).

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Fig. 5. Schematic of SWNTs suspended by chitosan under ultrasonication (US) and then conjugated with anti-CD133-PE that had been activated by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in N-hydroxysuccinimide (NHS). Figure reproduced from Ref. [74] with permission from the publishers.

Nanoparticles conjugated with ligands were especially effective at localizing to tumors [79]. A ligand such as an anti-P-gp antibody (anti-P-gp) [65] could have the spatial control required to significantly reduce the systemic toxicity associated with classical PDT therapy. Although UV [80] and visible wavelengths [78] are used in PDT therapy both are readily absorbed by the skin and present some limitations. Infra-red, near infra-red lights [65], and short laser pulses [81] were adopted to improve the therapeutic effect and lower the side effects of PDT therapy. Lukianova-Hleb et al. [81] reported an interesting method called plasmonic nanobubble-enhanced endosomal escape (PNBEE) for fast, selective, guided intracellular drug delivery by irradiating with short laser pulses through a self-assembly by cancer cells of separately targeted gold nanoparticles and encapsulated drug (DOX). The colocalized with DOX plasmonic nanobubbles optically generated in drug-resistant cancer cells released the drug into the cytoplasm. This delivery method increased the therapeutic efficacy by 31-fold, and reduced the drug dose by 20-fold. Additionally, the treatment time was reduced by 3-fold, and the non-specific toxicity decreased by 10-fold compared to standard treatments. To increase the effectiveness of reversal of MCF-7/ADR cells, Shieh et al. combined photochemical internalization (PCI, a specific arm of PDT) with chemotherapy (DOX), nanoparticle delivery, and inhibition of P-gp [82]. The results showed that the combination therapy was more cytotoxic to drug-resistant breast cancer cells than each of the individual agents. Ultrasound is another physical technique that can enhance drug delivery for treating MDR [83–85]. Ultrasound can induce thermal effects, enhance extravasation of drug-encapsulated micelles into tumors, enhance drug diffusion through the tumor interstitium, release drug from micelles within the tumor, and enhance the intracellular uptake of both released and encapsulated drug at the irradiation site [86]. Howard et al. used a micellar paclitaxel delivery system combined with an ultrasonically triggered release to treat a drugresistant breast cancer tumor cell line [87]. Their results showed that sonication increased the drug uptake from micellar paclitaxel by more than 20-fold compared to non-sonicated samples and inhibited cellular proliferation by nearly 90%. One important advantage of ultrasound is that it can noninvasively penetrate deep into the interior of the body. Additionally, it can be focused to control drug release from nanocarriers. These properties make ultrasound a promising strategy for future MDR treatments [88]. 4. Nanotechnology for simultaneous targeting delivery of different agents Members of the ABC superfamily, including P-glycoprotein (P-gp/ ABCB1), multidrug resistance proteins (MRPs/ABCC), and breast cancer resistance protein (BCRP/ABCG2), function as ATP-driven drug efflux transporters. A number of MDR reversal agents with P-gp inhibitory activity, also known as “chemosensitizers”, were identified [4,89]. However, their unacceptable side effects (e.g., cyclosporine-A, verapamil) include unpredicted pharmacokinetic interaction with anticancer drugs and other transport proteins (e.g., PSC833, VX-710).

Their long-term safety (e.g., tariquidar) still needs to be elucidated. Another concern with these inhibitors is they may increase the chemotherapeutic side effects by blocking physiological anticancer drug efflux from normal cells [90] because P-gp plays an important role in regulating endogenous and xenobiotic compounds in the body. For example, there are high levels of P-gp in the small intestine [91] and on the luminal side of the blood brain barrier [92] and the blood-testis barrier [93]. Therefore, it is important to limit the exposure of normal cells and tissues to the efflux inhibitor. The differences in physio-chemical properties of the anticancer drug and efflux inhibitor may result in differences in the pharmacokinetics and tumor accumulation of the two agents. These two obstacles can be bypassed using a targeted nanocarrier delivery system to co-encapsulate the anticancer drug and chemosensitizer [94,95]. The functionalization with ligands [10,64,96] and the pH-responsive technique [97] (Fig. 6) could also be utilized to enhance the reversal of MDR. An interesting nanoassembly technique called “sonication assisted layer-by-layer” (SLBL) was introduced by Daniele et al. [98]. The technique is based on powerful ultrasonication and interaction of opposite charges. The SLBL techniques could be applied to combining two drugs into one nanocapsule and locating paclitaxel in the core and lapatinib on the shell periphery for synergistic medical treatment of tumor MDR [98]. 5. Nanotechnology for targeting gene delivery RNA interference (RNAi) gene silencing technology has become a powerful tool in target validation, especially in gene-specific therapeutics. RNAi is a post-transcriptional gene silencing mechanism that is mediated by small interfering RNAs (siRNAs) of 21–25 nucleotides (nt). The double-stranded RNA molecules are incorporated into the RNA-induced silencing complex (RISC), where they induce degradation of target mRNAs in a sequence-specific manner. In MDR cancer therapy, siRNA has been used to down-regulate MDR-related proteins by silencing MDR-1 [99], MDP1, and BCL2 [8]. There are additional specific and powerful methods to inhibit P-gp expression, involving MDR1 gene silencing triggered by siRNA rather than by chemical function inhibitors. However, because a siRNA is a negatively charged and water-soluble macromolecule, its application in vivo faces many barriers such as ribonuclease (RNase) degradation, elimination, poor permeability, and endosomal trapping. The adequate protection and delivery of labile therapeutics limits the clinical application of these agents. Recent research has focused on developing safe and efficient non-viral siRNA delivery carriers because of biosafety issues with using viral vectors in humans. Nanocarriers, including minicells [61], are useful for protecting labile therapeutics (e.g., DNA [60], RNAi [41], and antisense oligonucleotides [63,100]) from degradation and site-specific drug targeting to reverse MDR. Various nanocarrier platforms such as polymeric nanoparticles, cationic liposomes, and lipoplexes have been shown to increase the stability of labile therapeutics. Theoretically, chemotherapeutic drugs such as paclitaxel (PTX) should be administered after a proper period of time of MDR-1 gene silencing to provide sufficient time for substantial down-regulation

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of P-gp expression. Numerous studies have demonstrated the validity of this hypothesis [41,61,67]. For example, Yadav et al. prepared PEOPbAE and PEO-PCL nanoparticles with encapsulated MDR-1 siRNA and PTX, respectively [67]. The PEO-PbAE and PEO-PCL nanocarriers localized to the tumor because of the EPR effect and the long circulation time from the particles' PEO shells. The particles increased the drug concentration inside the tumor cells by non-specific endocytosis, which was followed by either pH-responsive triggered release (for PbAE) or sustained release (for PCL) of the encapsulated payload. PbAE also helped to stabilize negatively charged nucleic acids in the siRNA upon cellular internalization. Similar to other studies [41,61], PTX was administered after 24 h of MDR-1 gene silencing to provide sufficient time for substantial down-regulation of P-gp expression. The results showed that MDR-1 gene silencing by siRNA significantly enhanced the chemotherapeutic effects in drug-resistant tumor cells. However, simultaneous delivery of anticancer drug and P-gp-targeted siRNAs with liposomes or nanoparticles also reported satisfactory results for MDR cancer therapy [8]. This approach is easier because it uses a single dosage protocol and can ensure that both cargos are delivered into the same MDR cancer cell. However, more studies are needed to compare the effect of co-delivery versus sequential treatment for drug-resistant tumor therapy. Most of the in vivo results in these studies show that drug-resistant tumor growth was incompletely inhibited. The complicated mechanisms of multi-drug resistance are an important factor in these results, although improper dosage regimens and dose rates of anticancer drugs or siRNA may have also played a part. Many widespread types of human MDR cancers (lung, breast, colon and ovarian cancer) activate both pump and non-pump MDR resistance in response to treatment with an anticancer drug. Consequently, to enhance the efficacy of the treatment both of the intracellular molecular mechanisms should be simultaneously inhibited. Both of the proteins that are key players in pump resistance (mainly P-glycoprotein) and anti-apoptotic cellular defense (mainly BCL2 protein) must be inhibited. All cell-death inducer(s) and suppressor(s) for both types of cellular-drug resistance should be delivered simultaneously inside the cancer cell. Additionally, the active components should be released with a comparable schedule. Such spatio‐temporal synchronization requires one complex system that simultaneously encapsulates all the aforementioned active components [8]. Maha et al. introduced a cationic liposomal carrier system to deliver DOX simultaneously with two species of siRNA targeting MRP1 and Bcl-2 [8]. The cytotoxicity of liposomes containing DOX and the two types of siRNA targeted against MRP1 and BCL2 mRNA were almost 4.5-fold higher than that of free DOX. These liposomes were also 4.1fold more cytotoxic than liposomal DOX and 1.8‐ to 2.7-fold more cytotoxic than a liposomal DOX formulation containing only one type of siRNA. Additionally, the liposomes were more than 1.5 times more cytotoxic than a mixture of both liposomal siRNAs and free DOX. However, similar attempts in another research group showed negative results [101]. Therefore, further studies are needed to optimize this combination therapy due to the complexity of preparing and manufacturing of these technologies.

6. Nanotechnology for targeting to energy metabolism An alternative to current complicated strategies for treating MDR is inhibiting intracellular ATP levels by inhibiting mitochondrial function in MDR cells with a mitochondrial targeted nanotechnology [11,97] that can significantly reduce the drug efflux capability of P-gp. Dequalinium (DQA)-functionalized nanocarriers were developed to target mitochondria [102,103]. Wang et al. [103] developed mitochondrial targeted resveratrol liposomes by conjugating dequalinium polyethylene glycol-distearoylphosphatidylethanolamine (DQA-PEG2000DSPE) to the surface of liposomes. By targeting mitochondria, this technology overcame drug resistance by inducing apoptosis of both non-resistant and resistant cancer cells and exhibited significant antitumor efficacy in xenografted resistant A549/cDDP cancers in nude mice. Pluronics have been reported to directly inhibit drug efflux pumps and other cellular detoxification mechanisms. Pluronics exert their effect by incorporating themselves into cellular membranes and then translocating into cells, thereby affecting various cellular functions such as mitochondrial respiration, ATP synthesis, activity of drug efflux transporters, apoptotic signal transduction, and gene expression. Recent findings suggest that the block copolymer reduced ATP production in drug-resistant cells without affecting ATP production in drug-sensitive cells. As a result, Pluronics could sensitize MDR tumors to various anticancer agents [104,105] and have been successfully used in developing nanocarriers to overcome tumor MDR [106–108]. 7. ‘Pharmacologically active’ nanocarrier materials for overcoming MDR The findings of ‘pharmacologically active’ nanocarrier materials will be another promising direction in overcoming MDR. In addition to Pluronics, some other surfactants such as CTAB [97,109], Tween 80 [110], Triton X-100 and Nonidet P-40 [111] all have the potential to become anti-cancer drugs and/or chemosensitizers for overcoming the MDR in cancer. The N-(2-hydroxypropyl) methacrylamide (HPMA) copolymer has also been demonstrated to be effective in overcoming MDR. HPMA copolymers were initially developed as plasma expanders [112,113]. Jindrich Kopecek and colleagues started using these highly hydrophilic, non-immunogenic, non-toxic, long-circulating synthetic macromolecules as carriers for low molecular weight drugs in the mid 1970s. A HPMA copolymer conjugate containing doxorubicin designed 30 years ago became the first synthetic polymer-based anticancer conjugate to enter clinical trial beginning in 1994. There are a growing array of HPMA copolymer-based systems involving combination therapies [114–116] incorporating targeting ligands [117–122] that have a more complex architecture for more effective drugs or therapeutic biological product delivery in the treatment of cancer [123–128]. For example, a conjugate of 2-pyrrolinodoxorubicin (p-DOX) with HPMA copolymer had the higher potential to bypass MDR and exhibited less cardiotoxicity at the therapeutic dose than free p-DOX [15]. In addition to functioning as only an ‘inactive’ nanocarrier of antitumor drugs, HPMA could enhance the cell death-inducing ability of conjugated anticancer drugs

Fig. 6. Schematic of the pH-responsive nano multi-drug delivery systems (nano-MDDSs) including a poorly water-soluble drug and surfactant chemosensitizer (cetyl trimethyl ammonium bromide, CTAB) achieved by an in situ drug co-loading strategy. CTAB was chosen as not only a structure-directing agent for constructing a drugs@micelles@MSNs (mesoporous silica nanoparticles) nano-MDDS but also a chemosensitizer for overcoming MDR and enhancing the drug efficacies. Figure reproduced from Ref. [97] with permission from the publishers.

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and overcome existing MDR. Additionally, this approach could be used to prevent the development of MDR in cancer cells by improving the internalization and intracellular trafficking of HPMA-drug complex [15], inhibiting drug detoxification inside cells, and affecting cell death induced by apoptosis and necrosis through modulation of cellular signaling pathways [129–133]. Please refer to Professor Minko's review [134] on this subject for additional reference. 8. Challenges and perspectives MDR still presents a major challenge for treating cancers. Although nanotechnology has made promising progress in overcoming MDR, the following issues should be further studied and considered: a better understanding of the selective pressure in the tumor microenvironment, such as understanding the vasculature leakiness; the exact pH range in endosomes and lysosomes as well as in the extracellular environment; the negative effects of PEGylation and ways to overcome them; the transport dynamics of nanocarriers and factors that can improve their transport within the tumor stroma; the exact mechanisms of active delivery of functionalized nanocarriers; the fate of nanocarriers in MDR tumor cells; the pharmacokinetics of drugs delivered by nanocarriers in MDR tumor cells; the optimal dosage regimen and dose rate of anticancer drugs or/and siRNA; and the need to simultaneously inhibit both pump and non-pump mechanisms to overcome MDR. Nanotechnologies for overcoming MDR tend to become more complicated because of the complicated mechanisms of multi-drug resistance. Although these delivery systems have made significant contributions to reversing MDR, the complications of manufacturing the nanocarriers must be addressed. Now that many successful nanotechnologies have been published, researchers need to consider the complexity of synthesizing the nanocarriers and their efficacy when designing an MDR drug delivery system. Acknowledgements The authors acknowledge support from the Natural Science Foundation of China (NSFC, 30801444), the Natural Science Foundation of Hebei Province (C2009000693, H2012208020), the Hebei University of Science and Technology Discipline Construction Office and the State Key Laboratory Breeding Base—Hebei Key Laboratory of Molecular Chemistry For Drug. References [1] B. Desoize, J. Jardillier, Multicellular resistance: a paradigm for clinical resistance? Crit. Rev. Oncol. Hematol. 36 (2000) 193–207. [2] Y.A. Luqmani, Mechanisms of drug resistance in cancer chemotherapy, Med. Princ. Pract. 14 (Suppl. 1) (2005) 35–48. [3] A.L. Harris, D. Hochhauser, Mechanisms of multidrug resistance in cancer treatment, Acta Oncol. 31 (1992) 205–213. [4] G. Szakacs, J.K. Paterson, J.A. Ludwig, C. Booth-Genthe, M.M. Gottesman, Targeting multidrug resistance in cancer, Nat. Rev. Drug Discov. 5 (2006) 219–234. [5] M.M. Gottesman, Mechanisms of cancer drug resistance, Annu. Rev. Med. 53 (2002) 615–627. [6] B.C. Baguley, Multidrug resistance in cancer, Methods Mol. Biol. 596 (2010) 1–14. [7] R.I. Pakunlu, Y. Wang, W. Tsao, V. Pozharov, T.J. Cook, T. Minko, Enhancement of the efficacy of chemotherapy for lung cancer by simultaneous suppression of multidrug resistance and antiapoptotic cellular defense: novel multicomponent delivery system, Cancer Res. 64 (2004) 6214–6224. [8] M. Saad, O.B. Garbuzenko, T. Minko, Co-delivery of siRNA and an anticancer drug for treatment of multidrug-resistant cancer, Nanomedicine (Lond.) 3 (2008) 761–776. [9] L. Milane, Z.F. Duan, M. Amiji, Pharmacokinetics and biodistribution of lonidamine/paclitaxel loaded. EGFR-targeted nanoparticles in an orthotopic animal model of multi-drug resistant breast cancer, Nanomedicine 7 (2011) 435–444. [10] Y. Patil, T. Sadhukha, L. Ma, J. Panyam, Nanoparticle-mediated simultaneous and targeted delivery of paclitaxel and tariquidar overcomes tumor drug resistance, J. Control. Release 136 (2009) 21–29.

53

[11] X.B. Xiong, Z. Ma, R. Lai, A. Lavasanifar, The therapeutic response to multifunctional polymeric nano-conjugates in the targeted cellular and subcellular delivery of doxorubicin, Biomaterials 31 (2010) 757–768. [12] Y. Zhou, L. Shi, Q. Li, H. Jiang, G. Lv, J. Zhao, C. Wu, M. Selke, X. Wang, Imaging and inhibition of multi-drug resistance in cancer cells via specific association with negatively charged CdTe quantum dots, Biomaterials 31 (2010) 4958–4963. [13] Y.J. Gu, J. Cheng, C.W. Man, W.T. Wong, S.H. Cheng, Gold-doxorubicin nanoconjugates for overcoming multidrug resistance, Nanomedicine 8 (2012) 204–211. [14] S. Kunjachan, A. Blauz, D. Mockel, B. Theek, F. Kiessling, T. Etrych, K. Ulbrich, L.V. Bloois, G. Storm, G. Bartosz, B. Rychlik, T. Lammers, Overcoming cellular multidrug resistance using classical nanomedicine formulations, Eur. J. Pharm. Sci. 45 (2012) 421–428. [15] M. Studenovsky, K. Ulbrich, M. Ibrahimova, B. Rihova, Polymer conjugates of the highly potent cytostatic drug 2-pyrrolinodoxorubicin, Eur. J. Pharm. Sci. 42 (2011) 156–163. [16] K. Kono, C. Kojima, N. Hayashi, E. Nishisaka, K. Kiura, S. Watarai, A. Harada, Preparation and cytotoxic activity of poly(ethylene glycol)-modified poly(amidoamine) dendrimers bearing adriamycin, Biomaterials 29 (2008) 1664–1675. [17] J.A. MacDiarmid, H. Brahmbhatt, Minicells: versatile vectors for targeted drug or si/shRNA cancer therapy, Curr. Opin. Biotechnol. 22 (2011) 909–916. [18] Y. Matsumura, H. Maeda, A new concept for macromolecular therapeutics in cancer chemotherapy: mechanism of tumoritropic accumulation of proteins and the antitumor agent smancs, Cancer Res. 46 (1986) 6387–6392. [19] S.K. Hobbs, W.L. Monsky, F. Yuan, W.G. Roberts, L. Griffith, V.P. Torchilin, R.K. Jain, Regulation of transport pathways in tumor vessels: role of tumor type and microenvironment, Proc. Natl. Acad. Sci. U. S. A. 95 (1998) 4607–4612. [20] D.R. Siwak, A.M. Tari, G. Lopez-Berestein, The potential of drug-carrying immunoliposomes as anticancer agents (Commentary re:), in: J.W. Park, et al., (Eds.), Anti-HER2 immunoliposomes: enhanced efficacy due to targeted deliveryClin. Cancer Res. 8 (2002) 1172–1181Clin. Cancer Res. 8 (2002) 955–956. [21] H. Hashizume, P. Baluk, S. Morikawa, J.W. McLean, G. Thurston, S. Roberge, R.K. Jain, D.M. McDonald, Openings between defective endothelial cells explain tumor vessel leakiness, Am. J. Pathol. 156 (2000) 1363–1380. [22] M. Zhao, M. Yang, X.M. Li, P. Jiang, E. Baranov, S. Li, M. Xu, S. Penman, R.M. Hoffman, Tumor-targeting bacterial therapy with amino acid auxotrophs of GFP-expressing Salmonella typhimurium, Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 755–760. [23] R.M. Hoffman, Tumor-targeting amino acid auxotrophic Salmonella typhimurium, Amino Acids 37 (2009) 509–521. [24] H.S. Choi, W. Liu, F. Liu, K. Nasr, P. Misra, M.G. Bawendi, J.V. Frangioni, Design considerations for tumour-targeted nanoparticles, Nat. Nanotechnol. 5 (2010) 42–47. [25] A.K. Gupta, M. Gupta, Synthesis and surface engineering of iron oxide nanoparticles for biomedical applications, Biomaterials 26 (2005) 3995–4021. [26] E.S. Lee, K. Na, Y.H. Bae, Doxorubicin loaded pH-sensitive polymeric micelles for reversal of resistant MCF-7 tumor, J Control. Release 103 (2005) 405–418. [27] Y.L. Lin, Y.K. Liu, N.M. Tsai, J.H. Hsieh, C.H. Chen, C.M. Lin, K.W. Liao, A Lipo-PEG-PEI complex for encapsulating curcumin that enhances its antitumor effects on curcumin-sensitive and curcumin-resistance cells, Nanomedicine 8 (2012) 318–327. [28] V. Torchilin, Tumor delivery of macromolecular drugs based on the EPR effect, Adv. Drug Deliv. Rev. 63 (2011) 131–135. [29] C.J. Li, Y. Miyamoto, Y. Kojima, H. Maeda, Augmentation of tumour delivery of macromolecular drugs with reduced bone marrow delivery by elevating blood pressure, Br. J. Cancer 67 (1993) 975–980. [30] A. Nagamitsu, T. Inuzuka, K. Greish, H. Maeda, SMANCS dynamic therapy for various advanced solid tumors and promising clinical effects: enhanced drug delivery by hydrodynamic modulation with vascular mediators, particularly angiotensin II, during arterial infusion, Drug Deliv. Syst. 22–5 (2007) 510–521. [31] W.L. Monsky, D. Fukumura, T. Gohongi, M. Ancukiewcz, H.A. Weich, V.P. Torchilin, F. Yuan, R.K. Jain, Augmentation of transvascular transport of macromolecules and nanoparticles in tumors using vascular endothelial growth factor, Cancer Res. 59 (1999) 4129–4135. [32] D. Bazile, C. Prud'homme, M.T. Bassoullet, M. Marlard, G. Spenlehauer, M. Veillard, Stealth Me.PEG-PLA nanoparticles avoid uptake by the mononuclear phagocytes system, J. Pharm. Sci. 84 (1995) 493–498. [33] A. Gabizon, D. Papahadjopoulos, Liposome formulations with prolonged circulation time in blood and enhanced uptake by tumors, Proc. Natl. Acad. Sci. U. S. A. 85 (1988) 6949–6953. [34] M. Hamidi, P. Rafiei, A. Azadi, Designing PEGylated therapeutic molecules: advantages in ADMET properties, Expert Opin. Drug Discov. 3 (2008) 1293–1307. [35] D.E. Owens 3rd, N.A. Peppas, Opsonization, biodistribution, and pharmacokinetics of polymeric nanoparticles, Int. J. Pharm. 307 (2006) 93–102. [36] D. Bhadra, S. Bhadra, P. Jain, N.K. Jain, Pegnology: a review of PEG-ylated systems, Pharmazie 57 (2002) 5–29. [37] J.C. Olivier, Drug transport to brain with targeted nanoparticles, NeuroRx 2 (2005) 108–119. [38] M. Hamidi, A. Azadi, P. Rafiei, Hydrogel nanoparticles in drug delivery, Adv. Drug Deliv. Rev. 60 (2008) 1638–1649. [39] S.D. Li, L. Huang, Targeted delivery of antisense oligodeoxynucleotide and small interference RNA into lung cancer cells, Mol. Pharm. 3 (2006) 579–588. [40] C.M. Galmarini, G. Warren, M.T. Senanayake, S.V. Vinogradov, Efficient overcoming of drug resistance to anticancer nucleoside analogs by nanodelivery of active phosphorylated drugs, Int. J. Pharm. 395 (2010) 281–289.

54

Z. Gao et al. / Journal of Controlled Release 162 (2012) 45–55

[41] J. Jiang, S.J. Yang, J.C. Wang, L.J. Yang, Z.Z. Xu, T. Yang, X.Y. Liu, Q. Zhang, Sequential treatment of drug-resistant tumors with RGD-modified liposomes containing siRNA or doxorubicin, Eur. J. Pharm. Biopharm. 76 (2010) 170–178. [42] J.A. Boomer, M.M. Qualls, H.D. Inerowicz, R.H. Haynes, V.S. Patri, J.M. Kim, D.H. Thompson, Cytoplasmic delivery of liposomal contents mediated by an acid-labile cholesterol-vinyl ether-PEG conjugate, Bioconjug. Chem. 20 (2009) 47–59. [43] H. Xu, K.Q. Wang, Y.H. Deng, W. Chen da, Effects of cleavable PEG-cholesterol derivatives on the accelerated blood clearance of PEGylated liposomes, Biomaterials 31 (2010) 4757–4763. [44] E.S. Lee, Z. Gao, D. Kim, K. Park, I.C. Kwon, Y.H. Bae, Super pH-sensitive multifunctional polymeric micelle for tumor pH(e) specific TAT exposure and multidrug resistance, J. Control. Release 129 (2008) 228–236. [45] J.G. Huang, T. Leshuk, F.X. Gu, Emerging nanomaterials for targeting subcellular organelles, Nano Today 6 (2011) 478–492. [46] M.E. Davis, Z.G. Chen, D.M. Shin, Nanoparticle therapeutics: an emerging treatment modality for cancer, Nat. Rev. Drug Discov. 7 (2008) 771–782. [47] F. Shen, S. Chu, A.K. Bence, B. Bailey, X. Xue, P.A. Erickson, M.H. Montrose, W.T. Beck, L.C. Erickson, Quantitation of doxorubicin uptake, efflux, and modulation of multidrug resistance (MDR) in MDR human cancer cells, J. Pharmacol. Exp. Ther. 324 (2008) 95–102. [48] H. Maeda, G.Y. Bharate, J. Daruwalla, Polymeric drugs for efficient tumor-targeted drug delivery based on EPR-effect, Eur. J. Pharm. Biopharm. 71 (2009) 409–419. [49] N. Dinauer, S. Balthasar, C. Weber, J. Kreuter, K. Langer, H. von Briesen, Selective targeting of antibody-conjugated nanoparticles to leukemic cells and primary T-lymphocytes, Biomaterials 26 (2005) 5898–5906. [50] J. Fang, H. Nakamura, H. Maeda, The EPR effect: unique features of tumor blood vessels for drug delivery, factors involved, and limitations and augmentation of the effect, Adv. Drug Deliv. Rev. 63 (2011) 136–151. [51] J.D. Byrne, T. Betancourt, L. Brannon-Peppas, Active targeting schemes for nanoparticle systems in cancer therapeutics, Adv. Drug Deliv. Rev. 60 (2008) 1615–1626. [52] M.G. Qaddoumi, H. Ueda, J. Yang, J. Davda, V. Labhasetwar, V.H. Lee, The characteristics and mechanisms of uptake of PLGA nanoparticles in rabbit conjunctival epithelial cell layers, Pharm. Res. 21 (2004) 641–648. [53] J. Panyam, S.K. Sahoo, S. Prabha, T. Bargar, V. Labhasetwar, Fluorescence and electron microscopy probes for cellular and tissue uptake of poly(D, L-lactide-co-glycolide) nanoparticles, Int. J. Pharm. 262 (2003) 1–11. [54] A.M. Wu, P.J. Yazaki, S. Tsai, K. Nguyen, A.L. Anderson, D.W. McCarthy, M.J. Welch, J.E. Shively, L.E. Williams, A.A. Raubitschek, J.Y. Wong, T. Toyokuni, M.E. Phelps, S.S. Gambhir, High-resolution microPET imaging of carcinoembryonic antigen-positive xenografts by using a copper-64-labeled engineered antibody fragment, Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 8495–8500. [55] J. Zhao, C.S. Liu, Y. Yuan, X.Y. Tao, X.Q. Shan, Y. Sheng, F. Wu, Preparation of hemoglobin-loaded nano-sized particles with porous structure as oxygen carriers, Biomaterials 28 (2007) 1414–1422. [56] N. Debotton, M. Parnes, J. Kadouche, S. Benita, Overcoming the formulation obstacles towards targeted chemotherapy: in vitro and in vivo evaluation of cytotoxic drug loaded immunonanoparticles, J. Control. Release 127 (2008) 219–230. [57] K.F. Pirollo, E.H. Chang, Does a targeting ligand influence nanoparticle tumor localization or uptake? Trends Biotechnol. 26 (2008) 552–558. [58] R. Duncan, Polymer conjugates as anticancer nanomedicines, Nat. Rev. Cancer 6 (2006) 688–701. [59] F. Yuan, M. Leunig, S.K. Huang, D.A. Berk, D. Papahadjopoulos, R.K. Jain, Microvascular permeability and interstitial penetration of sterically stabilized (stealth) liposomes in a human tumor xenograft, Cancer Res. 54 (1994) 3352–3356. [60] Z. Xu, Z. Zhang, Y. Chen, L. Chen, L. Lin, Y. Li, The characteristics and performance of a multifunctional nanoassembly system for the co-delivery of docetaxel and iSur-pDNA in a mouse hepatocellular carcinoma model, Biomaterials 31 (2010) 916–922. [61] J.A. MacDiarmid, N.B. Amaro-Mugridge, J. Madrid-Weiss, I. Sedliarou, S. Wetzel, K. Kochar, V.N. Brahmbhatt, L. Phillips, S.T. Pattison, C. Petti, B. Stillman, R.M. Graham, H. Brahmbhatt, Sequential treatment of drug-resistant tumors with targeted minicells containing siRNA or a cytotoxic drug, Nat. Biotechnol. 27 (2009) 643–651. [62] Y.B. Patil, S.K. Swaminathan, T. Sadhukha, L. Ma, J. Panyam, The use of nanoparticle-mediated targeted gene silencing and drug delivery to overcome tumor drug resistance, Biomaterials 31 (2010) 358–365. [63] J. Wang, X. Tao, Y. Zhang, D. Wei, Y. Ren, Reversion of multidrug resistance by tumor targeted delivery of antisense oligodeoxynucleotides in hydroxypropyl-chitosan nanoparticles, Biomaterials 31 (2010) 4426–4433. [64] J. Wu, Y. Lu, A. Lee, X. Pan, X. Yang, X. Zhao, R.J. Lee, Reversal of multidrug resistance by transferrin-conjugated liposomes co-encapsulating doxorubicin and verapamil, J. Pharm. Pharm. Sci. 10 (2007) 350–357. [65] R. Li, R. Wu, L. Zhao, M. Wu, L. Yang, H. Zou, P-glycoprotein antibody functionalized carbon nanotube overcomes the multidrug resistance of human leukemia cells, ACS Nano 4 (2010) 1399–1408. [66] D.J. Yamashiro, F.R. Maxfield, Regulation of endocytic processes by pH, Trends Pharmacol. Sci. 9 (1988) 190–193. [67] S. Yadav, L.E. van Vlerken, S.R. Little, M.M. Amiji, Evaluations of combination MDR-1 gene silencing and paclitaxel administration in biodegradable polymeric nanoparticle formulations to overcome multidrug resistance in cancer cells, Cancer Chemother. Pharmacol. 63 (2009) 711–722. [68] F.M. Kievit, F.Y. Wang, C. Fang, H. Mok, K. Wang, J.R. Silber, R.G. Ellenbogen, M. Zhang, Doxorubicin loaded iron oxide nanoparticles overcome multidrug resistance in cancer in vitro, J. Control. Release 152 (2011) 76–83.

[69] L. Jabr-Milane, L. van Vlerken, H. Devalapally, D. Shenoy, S. Komareddy, M. Bhavsar, M. Amiji, Multi-functional nanocarriers for targeted delivery of drugs and genes, J. Control. Release 130 (2008) 121–128. [70] K. Engin, D.B. Leeper, J.R. Cater, A.J. Thistlethwaite, L. Tupchong, J.D. McFarlane, Extracellular pH distribution in human tumours, Int. J. Hyperthermia 11 (1995) 211–216. [71] R. Chen, S. Khormaee, M.E. Eccleston, N.K. Slater, The role of hydrophobic amino acid grafts in the enhancement of membrane-disruptive activity of pH-responsive pseudo-peptides, Biomaterials 30 (2009) 1954–1961. [72] A.K. Larsen, A.E. Escargueil, A. Skladanowski, Resistance mechanisms associated with altered intracellular distribution of anticancer agents, Pharmacol. Ther. 85 (2000) 217–229. [73] Y. Shen, H. Tang, Y. Zhan, E.A. Van Kirk, W.J. Murdoch, Degradable poly(beta-amino ester) nanoparticles for cancer cytoplasmic drug delivery, Nanomedicine 5 (2009) 192–201. [74] C.H. Wang, S.H. Chiou, C.P. Chou, Y.C. Chen, Y.J. Huang, C.A. Peng, Photothermolysis of glioblastoma stem-like cells targeted by carbon nanotubes conjugated with CD133 monoclonal antibody, Nanomedicine 7 (2011) 69–79. [75] C. Fan, W. Gao, Z. Chen, H. Fan, M. Li, F. Deng, Z. Chen, Tumor selectivity of stealth multi-functionalized superparamagnetic iron oxide nanoparticles, Int. J. Pharm. 404 (2011) 180–190. [76] J. Chen, D. Wang, J. Xi, L. Au, A. Siekkinen, A. Warsen, Z.Y. Li, H. Zhang, Y. Xia, X. Li, Immuno gold nanocages with tailored optical properties for targeted photothermal destruction of cancer cells, Nano Lett. 7 (2007) 1318–1322. [77] M.Y. Hua, H.W. Yang, H.L. Liu, R.Y. Tsai, S.T. Pang, K.L. Chuang, Y.S. Chang, T.L. Hwang, Y.H. Chang, H.C. Chuang, C.K. Chuang, Superhigh-magnetization nanocarrier as a doxorubicin delivery platform for magnetic targeting therapy, Biomaterials 32 (2011) 8999–9010. [78] H.L. Lu, W.J. Syu, N. Nishiyama, K. Kataoka, P.S. Lai, Dendrimer phthalocyanine-encapsulated polymeric micelle-mediated photochemical internalization extends the efficacy of photodynamic therapy and overcomes drug-resistance in vivo, J. Control. Release 155 (2011) 458–464. [79] S. Febvay, D.M. Marini, A.M. Belcher, D.E. Clapham, Targeted cytosolic delivery of cell-impermeable compounds by nanoparticle-mediated, light-triggered endosome disruption, Nano Lett. 10 (2010) 2211–2219. [80] D. Guo, C. Wu, H. Jiang, Q. Li, X. Wang, B. Chen, Synergistic cytotoxic effect of different sized ZnO nanoparticles and daunorubicin against leukemia cancer cells under UV irradiation, J. Photochem. Photobiol. B 93 (2008) 119–126. [81] E.Y. Lukianova-Hleb, A. Belyanin, S. Kashinath, X. Wu, D.O. Lapotko, Plasmonic nanobubble-enhanced endosomal escape processes for selective and guided intracellular delivery of chemotherapy to drug-resistant cancer cells, Biomaterials 33 (2012) 1821–1826. [82] M.J. Shieh, C.Y. Hsu, L.Y. Huang, H.Y. Chen, F.H. Huang, P.S. Lai, Reversal of doxorubicin-resistance by multifunctional nanoparticles in MCF-7/ADR cells, J. Control. Release 152 (2011) 418–425. [83] N. Rapoport, Combined cancer therapy by micellar-encapsulated drug and ultrasound, Int. J. Pharm. 277 (2004) 155–162. [84] Y. Liu, K. Lillehei, W.N. Cobb, U. Christians, K.Y. Ng, Overcoming MDR by ultrasound-induced hyperthermia and P-glycoprotein modulation, Biochem. Biophys. Res. Commun. 289 (2001) 62–68. [85] A. Marin, H. Sun, G.A. Husseini, W.G. Pitt, D.A. Christensen, N.Y. Rapoport, Drug delivery in pluronic micelles: effect of high-frequency ultrasound on drug release from micelles and intracellular uptake, J. Control. Release 84 (2002) 39–47. [86] U. Kedar, P. Phutane, S. Shidhaye, V. Kadam, Advances in polymeric micelles for drug delivery and tumor targeting, Nanomedicine 6 (2010) 714–729. [87] B. Howard, Z. Gao, S.W. Lee, Ultrasound-enhanced chemotherapy of drug-resistant breast cancer tumors by micellar-encapsulated paclitaxel, Am. J. Drug Deliv. 4 (2006) 97–104. [88] L. Milane, S. Ganesh, S. Shah, Z.F. Duan, M. Amiji, Multi-modal strategies for overcoming tumor drug resistance: hypoxia, the Warburg effect, stem cells, and multifunctional nanotechnology, J. Control. Release 155 (2011) 237–247. [89] E. Teodori, S. Dei, S. Scapecchi, F. Gualtieri, The medicinal chemistry of multidrug resistance (MDR) reversing drugs, Farmaco 57 (2002) 385–415. [90] M. Hubensack, C. Muller, P. Hocherl, S. Fellner, T. Spruss, G. Bernhardt, A. Buschauer, Effect of the ABCB1 modulators elacridar and tariquidar on the distribution of paclitaxel in nude mice, J. Cancer Res. Clin. Oncol. 134 (2008) 597–607. [91] M. Bebawy, D.M. Sze, Targeting P-glycoprotein for effective oral anti-cancer chemotherapeutics, Curr. Cancer Drug Targets 8 (2008) 47–52. [92] K. Linnet, T.B. Ejsing, A review on the impact of P-glycoprotein on the penetration of drugs into the brain. Focus on psychotropic drugs, Eur. Neuropsychopharmacol. 18 (2008) 157–169. [93] L. van Zuylen, J. Verweij, K. Nooter, E. Brouwer, G. Stoter, A. Sparreboom, Role of intestinal P-glycoprotein in the plasma and fecal disposition of docetaxel in humans, Clin. Cancer Res. 6 (2000) 2598–2603. [94] X.R. Song, Z. Cai, Y. Zheng, G. He, F.Y. Cui, D.Q. Gong, S.X. Hou, S.J. Xiong, X.J. Lei, Y.Q. Wei, Reversion of multidrug resistance by co-encapsulation of vincristine and verapamil in PLGA nanoparticles, Eur. J. Pharm. Sci. 37 (2009) 300–305. [95] H. Devalapally, Z. Duan, M.V. Seiden, M.M. Amiji, Modulation of drug resistance in ovarian adenocarcinoma by enhancing intracellular ceramide using tamoxifen-loaded biodegradable polymeric nanoparticles, Clin. Cancer Res. 14 (2008) 3193–3203. [96] H.J. Cho, H.Y. Yoon, H. Koo, S.H. Ko, J.S. Shim, J.H. Lee, K. Kim, I.C. Kwon, D.D. Kim, Self-assembled nanoparticles based on hyaluronic acid-ceramide (HA-CE) and Pluronic(R) for tumor-targeted delivery of docetaxel, Biomaterials 32 (2011) 7181–7190.

Z. Gao et al. / Journal of Controlled Release 162 (2012) 45–55 [97] Q. He, Y. Gao, L. Zhang, Z. Zhang, F. Gao, X. Ji, Y. Li, J. Shi, A pH-responsive mesoporous silica nanoparticles-based multi-drug delivery system for overcoming multi-drug resistance, Biomaterials 32 (2011) 7711–7720. [98] D. Vergara, C. Bellomo, X. Zhang, V. Vergaro, A. Tinelli, V. Lorusso, R. Rinaldi, Y.M. Lvov, S. Leporatti, M. Maffia, Lapatinib/Paclitaxel polyelectrolyte nanocapsules for overcoming multidrug resistance in ovarian cancer, Nanomedicine (2011) ( http://www.ncbi.nlm.nih.gov/pubmed/22100754). [99] C. Liu, G. Zhao, J. Liu, N. Ma, P. Chivukula, L. Perelman, K. Okada, Z. Chen, D. Gough, L. Yu, Novel biodegradable lipid nano complex for siRNA delivery significantly improving the chemosensitivity of human colon cancer stem cells to paclitaxel, J. Control. Release 140 (2009) 277–283. [100] R.I. Pakunlu, Y. Wang, M. Saad, J.J. Khandare, V. Starovoytov, T. Minko, In vitro and in vivo intracellular liposomal delivery of antisense oligonucleotides and anticancer drug, J. Control. Release 114 (2006) 153–162. [101] H. Meng, M. Liong, T. Xia, Z. Li, Z. Ji, J.I. Zink, A.E. Nel, Engineered design of mesoporous silica nanoparticles to deliver doxorubicin and P-glycoprotein siRNA to overcome drug resistance in a cancer cell line, ACS Nano 4 (2010) 4539–4550. [102] Y. Yu, Z.H. Wang, L. Zhang, H.J. Yao, Y. Zhang, R.J. Li, R.J. Ju, X.X. Wang, J. Zhou, N. Li, W.L. Lu, Mitochondrial targeting topotecan-loaded liposomes for treating drug-resistant breast cancer and inhibiting invasive metastases of melanoma, Biomaterials 33 (2012) 1808–1820. [103] X.X. Wang, Y.B. Li, H.J. Yao, R.J. Ju, Y. Zhang, R.J. Li, Y. Yu, L. Zhang, W.L. Lu, The use of mitochondrial targeting resveratrol liposomes modified with a dequalinium polyethylene glycol-distearoylphosphatidyl ethanolamine conjugate to induce apoptosis in resistant lung cancer cells, Biomaterials 32 (2011) 5673–5687. [104] E.V. Batrakova, A.V. Kabanov, Pluronic block copolymers: evolution of drug delivery concept from inert nanocarriers to biological response modifiers, J. Control. Release 130 (2008) 98–106. [105] E.V. Batrakova, S. Li, A.M. Brynskikh, A.K. Sharma, Y. Li, M. Boska, N. Gong, R.L. Mosley, V.Y. Alakhov, H.E. Gendelman, A.V. Kabanov, Effects of pluronic and doxorubicin on drug uptake, cellular metabolism, apoptosis and tumor inhibition in animal models of MDR cancers, J. Control. Release 143 (2010) 290–301. [106] Y. Zhang, L. Tang, L. Sun, J. Bao, C. Song, L. Huang, K. Liu, Y. Tian, G. Tian, Z. Li, H. Sun, L. Mei, A novel paclitaxel-loaded poly(epsilon-caprolactone)/Poloxamer 188 blend nanoparticle overcoming multidrug resistance for cancer treatment, Acta Biomater. 6 (2010) 2045–2052. [107] X. Ji, Y. Gao, L. Chen, Z. Zhang, Y. Deng, Y. Li, Nanohybrid systems of non-ionic surfactant inserting liposomes loading paclitaxel for reversal of multidrug resistance, Int. J. Pharm. 422 (2012) 390–397. [108] W. Zhang, Y. Shi, Y. Chen, J. Ye, X. Sha, X. Fang, Multifunctional pluronic P123/F127 mixed polymeric micelles loaded with paclitaxel for the treatment of multidrug resistant tumors, Biomaterials 32 (2011) 2894–2906. [109] Q. He, J. Shi, F. Chen, M. Zhu, L. Zhang, An anticancer drug delivery system based on surfactant-templated mesoporous silica nanoparticles, Biomaterials 31 (2010) 3335–3346. [110] H. Riehm, J.L. Biedler, Potentiation of drug effect by Tween 80 in Chinese hamster cells resistant to actinomycin D and daunomycin, Cancer Res. 32 (1972) 1195–1200. [111] D.W. Loe, F.J. Sharom, Interaction of multidrug-resistant Chinese hamster ovary cells with amphiphiles, Br. J. Cancer 68 (1993) 342–351. [112] J. Kopecek, H. Bazilova, Poly[N-(2-hydroxypropyl)methacrylamide]. I. Radical polymerization and copolymerization, Eur. Polym. 9 (1973) 7–14. [113] L. Sprincl, J. Exner, O. Sterba, J. Kopecek, New types of synthetic infusion solutions. III. Elimination and retention of poly-[N-(2-hydroxypropyl)methacrylamide] in a test organism, J. Biomed. Mater. Res. 10 (1976) 953–963. [114] E. Segal, H. Pan, L. Benayoun, P. Kopeckova, Y. Shaked, J. Kopecek, R. Satchi-Fainaro, Enhanced anti-tumor activity and safety profile of targeted nano-scaled HPMA copolymer-alendronate-TNP-470 conjugate in the treatment of bone malignances, Biomaterials 32 (2011) 4450–4463. [115] T. Etrycha, H. Krakovičováa, M. Šírováb, B. Říhováb, K. Ulbrich, HPMAcopolymerconjugates for dualtherapy, J. Control. Release 132 (2008) e63–e65.

55

[116] J. Hongrapipat, P. Kopeckova, S. Prakongpan, J. Kopecek, Enhanced antitumor activity of combinations of free and HPMA copolymer-bound drugs, Int. J. Pharm. 351 (2008) 259–270. [117] K. Greish, A. Ray, H. Bauer, N. Larson, A. Malugin, D. Pike, M. Haider, H. Ghandehari, Anticancer and antiangiogenic activity of HPMA copolymer-aminohexylgeldanamycin-RGDfK conjugates for prostate cancer therapy, J. Control. Release 151 (2011) 263–270. [118] Q. Xiang, Y. Yang, Z. Zhou, D. Zhou, Y. Jin, Z. Zhang, Y. Huang, Synthesis and in vitro anti-tumor activity of novel HPMA copolymer–drug conjugates with potential cell surface targeting property for carcinoma cells, Eur. J. Pharm. Biopharm. 80 (2012) 379–386. [119] Y. Yang, Z. Zhou, S. He, T. Fan, Y. Jin, X. Zhu, C. Chen, Z.R. Zhang, Y. Huang, Treatment of prostate carcinoma with (galectin-3)-targeted HPMA copolymer-(G3-C12)-5-Fluorouracil conjugates, Biomaterials 33 (2012) 2260–2271. [120] M.P. Borgman, T. Coleman, R.B. Kolhatkar, S. Geyser-Stoops, B.R. Line, H. Ghandehari, Tumor-targeted HPMA copolymer-(RGDfK)-(CHX-A''-DTPA) conjugates show increased kidney accumulation, J. Control. Release 132 (2008) 193–199. [121] A. Mitra, J. Mulholland, A. Nan, E. McNeill, H. Ghandehari, B.R. Line, Targeting tumor angiogenic vasculature using polymer-RGD conjugates, J. Control. Release 102 (2005) 191–201. [122] P.J. Julyan, L.W. Seymour, D.R. Ferry, S. Daryani, C.M. Boivin, J. Doran, M. David, D. Anderson, C. Christodoulou, A.M. Young, S. Hesslewood, D.J. Kerr, Preliminary clinical study of the distribution of HPMA copolymers bearing doxorubicin and galactosamine, J. Control. Release 57 (1999) 281–290. [123] R.N. Johnson, D.S. Chu, J. Shi, J.G. Schellinger, P.M. Carlson, S.H. Pun, HPMA-oligolysine copolymers for gene delivery: optimization of peptide length and polymer molecular weight, J. Control. Release 155 (2011) 303–311. [124] Z. Qin, W. Liu, L. Guo, X. Li, Preparation and characterization of guanidinated N-3-aminopropyl methacrylamide-N-2-hydroxypropyl methacrylamide copolymers as gene delivery carriers, J. Control. Release 152 (Suppl. 1) (2011) e168–e170. [125] J. Shi, R.N. Johnson, J.G. Schellinger, P.M. Carlson, S.H. Pun, Reducible HPMA-co-oligolysine copolymers for nucleic acid delivery, Int. J. Pharm. 427 (2012) 113–122. [126] T. Etrych, T. Mrkvan, B. Rihova, K. Ulbrich, Star-shaped immunoglobulin-containing HPMA-based conjugates with doxorubicin for cancer therapy, J. Control. Release 122 (2007) 31–38. [127] R. Duncan, Development of HPMA copolymer-anticancer conjugates: clinical experience and lessons learnt, Adv. Drug Deliv. Rev. 61 (2009) 1131–1148. [128] P.C. Griffiths, A. Paul, B. Apostolovic, H.A. Klok, E. de Luca, S.M. King, R.K. Heenan, Conformational consequences of cooperative binding of a coiled-coil peptide motif to poly(N-(2-hydroxypropyl) methacrylamide) HPMA copolymers, J. Control. Release 153 (2011) 173–179. [129] S. Zalipsky, M. Saad, R. Kiwan, E. Ber, N. Yu, T. Minko, Antitumor activity of new liposomal prodrug of mitomycin C in multidrug resistant solid tumor: insights of the mechanism of action, J. Drug Target 15 (2007) 518–530. [130] T. Minko, P. Kopeckova, V. Pozharov, J. Kopecek, HPMA copolymer bound adriamycin overcomes MDR1 gene encoded resistance in a human ovarian carcinoma cell line, J. Control. Release 54 (1998) 223–233. [131] T. Minko, P. Kopeckova, J. Kopecek, Chronic exposure to HPMA copolymer-bound adriamycin does not induce multidrug resistance in a human ovarian carcinoma cell line, J. Control. Release 59 (1999) 133–148. [132] M. Tijerina, K.D. Fowers, P. Kopeckova, J. Kopecek, Chronic exposure of human ovarian carcinoma cells to free or HPMA copolymer-bound mesochlorin e6 does not induce P-glycoprotein-mediated multidrug resistance, Biomaterials 21 (2000) 2203–2210. [133] T. Minko, P. Kopeckova, J. Kopecek, Preliminary evaluation of caspasesdependent apoptosis signaling pathways of free and HPMA copolymer-bound doxorubicin in human ovarian carcinoma cells, J. Control. Release 71 (2001) 227–237. [134] T. Minko, HPMA copolymers for modulating cellular signaling and overcoming multidrug resistance, Adv. Drug Deliv. Rev. 62 (2010) 192–202.