Stimulus-responsive targeted nanomicelles for ... - Future Medicine

Review For reprint orders, please contact: [email protected]

Stimulus-responsive targeted nanomicelles for effective cancer therapy Emerging nanotechnology has already developed various innovative nanomedicines. Nanomicelles, self-assemblies of block copolymers, are promising nanomedicines for targeted drug delivery and imaging. Stimulus-responsive targeted nanomicelles are designed to release drugs based on stimuli such as pH, temperature, redox potential, magnetism and ultrasound. This article will focus on recent advancements in the design of stimulus-responsive targeted nanomicelles loaded with anticancer drugs to fulfill the challenges associated with cancer cells (e.g., multidrug resistance) for the effective treatment of cancer. The significant toxicity issues and a possible future perspective associated with nanomicelles are also discussed here. KEYWORDS: cancer n nanomicelles n stimulus functions n targeted drug delivery n toxicity

Developments in novel drug-delivery systems and nanotechnology-based concepts have facilitated the targeting of specific molecular targets [1] . Nanomedicines are promising candidates, developed for extended and targeted drug delivery, that can change the future of various therapies [1–6] . Recently, various targeted/nontargeted nanomedicines (polymeric nanoparticles, solid lipid nanoparticles, nanomicelles, dendrimers, liposomes, gold nanoparticles and magnetic nano particles) and multifunctional nanomedicines capable of combining targeted drug delivery and imaging (nanomicelles, dendrimers and magnetic nanoparticles) have been reviewed by various researchers [7,8] . Among these nanomedicines, nanomicelles have emerged as one of the most useful modalities for effective drug delivery [9] . Nanomicelles, self-assemblies of block copolymers, have a fairly narrow size distribution in the range of 10 to 100 nm and are characterized by their unique core–shell architecture, in which an inner core loaded with hydrophobic drug is surrounded by an outer shell of hydrophilic layer, such as poly(ethylene glycol) (PEG). Nanomicelles have some advantages over other nanomedicines (e.g., liposomes, polymerosomes and polymeric nanoparticles), such as a simple preparation method, efficient drug loading, stability and controlled drug release, which can be modified by the physicochemical properties of the block copolymers [10–12] . Furthermore, nanomicelles have an advantage over conventional surfactant micelles in that they have better thermodynamic stability

in physiological solution, as indicated by their low critical micellar concentration, which makes them stable and prevents their rapid dissociation in vivo [9] . The critical association concentration of 10 ‑6 –10 ‑7 M for nanomicelles after dilution (during intravenous adminstration) suggested slower dissociation kinetics, around 1000‑fold lower than that of ordinary surfactant micelles. This allows the nanomicelles to circulate in the bloodstream until they reach their molecular targets [13,14] . Nanomicelles are safe alternatives for intravenous administration of poorly water-soluble drugs. The drugs can be partitioned in the hydrophobic core of nanomicelles and the outer hydrophilic layer forms a stable dispersion in aqueous media, which can then be administered intravenously (Figur e 1A) . The distribution of drug-loaded nanomicelles in the body is determined mainly by surface properties. Additionally, their individual particle size is less than 100 nm in diameter, which provides obvious benefits over liposomes. It makes them ideal drug-delivery carriers because they avoid renal exclusion and the reticuloendothelial system. In addition, it provides them with enhanced endothelial vascular permeability in the vicinity of solid tumors [12] . Nanomicelles have several advantages in comparison to long-circulating liposomes, such as controlled drug release, cell permeability and fewer adverse effects (e.g., hypersensitivity reaction) [15] . It has been reported that long-circulating liposomes induce some side effects such as hand-foot syndrome

10.2217/NNM.09.44 © 2009 Future Medicine Ltd

Nanomedicine (2009) 4(6), 657–667

Madaswamy S Muthu1†, Chellappa V Rajesh2, Amit Mishra2 & Sanjay Singh2 Author for correspondence: Department of Pharmacology, Institute of Medical Sciences, Banaras Hindu University, Varanasi – 221005, India Tel.: +91 930 518 0921; Fax: +91 542 236 7568; [email protected] 2 Department of Pharmaceutics, Institute of Technology, Banaras Hindu University, Varanasi – 221005, India †

1

ISSN 1743-5889

657

Review

Muthu, Rajesh, Mishra & Singh

Anticancer drug

Hydrophilic group

Acid-sensitive bond

Targeting ligand

Acid-sensitive hydrophobic core

Hydrophobic core

Redox potential (disulfide bond)

Thermal-responsive hydrophilic group

Figure 1. Different targeted nanomicelles. (A) Passive targeted nanomicelles; (B) active targeted nanomicelles; (C) pH-responsive passive targeted nanomicelles; (D) pH-responsive active targeted nanomicelles; (E) temperature-responsive active targeted nanomicelles; and (F) multistimulus/redox potential responsive passive targeted nanomicelles.

and infusion-related reactions that require pretreatment before their adminstration [16–18] . Indeed, some clinical trial reports suggested that anticancer drug-loaded nanomicelles composed of biodegradable block copolymers may not cause any significant adverse effects which were observed with long-circulating liposomes owing to their lipid compositions [19] . Although the development of nanomicelles extends into all therapeutic classes of drugs, the development of effective treatment modalities for cancer remains a significant therapeutic need [20,21] . Currently, several passive targeted nano micelles containing anticancer drugs (e.g., doxorubicin, paclitaxel and cisplatin) are already under preclinical and clinical investigations [22] . Some nanomicelles have reduced the adverse effects of anticancer drugs: mainly a reduction in nephrotoxicity [23] and pulmonary toxicity [24] . Additionally, paclitaxel-loaded nanomicelles enhanced the radiosensitizing activity of the drug [25] . In some studies, the potential of nanomicelles for cancer therapy was enhanced by conjugating targeting ligands to the nanomicelles surface for active targeting [26] . Yoo and Park have formulated anticancer drug-loaded targeted nanomicelles (PEG–poly[lactide‑co‑glycolide]) 658

Nanomedicine (2009) 4(6)

with a folic acid (targeting ligand) attachment, encapsulating water-insoluble drug doxorubicin inside the hydrophobic core of the nanomicelles (Figure 1B) [27] . Recently, further improvements were made in the long-circulating and targeted nanomicelles for effective cancer therapy by the addition of various stimulus-responsive functions such as pH, temperature, redox potential, magnetism and ultrasound. The addition of these stimulus-responsive functions into the nanomicelles could beneficially modify the properties of the anticancer drug in nanomicelles, for example providing enhanced or controlled drug release, improving cellular uptake, controlling the intracellular drug fate or allowing for some physical activity on the surroundings of the target site (i.e., cancer cells) [28,29] . Information regarding the development of stimulus-responsive targeted nanomicelles for effective treatment of cancer has not been well classified and documented. However, there are many reviews on stimulus-responsive particles [29,30] . The literature also indicates that limited attention has been given to long-term toxicity investigations of nanomicelles developed for cancer therapy. Some toxicological reports have suggested that adminstration of these nanomicelles leads to serious effects in biological systems [31] . future science group

Stimulus-responsive targeted nanomicelles for effective cancer therapy

This review, therefore, was conducted with the view to summarize the recent developments in the area of stimulus-responsive targeted nanomicelles loaded with anticancer drugs for effective treatment of cancer, as well as the issues regarding their safety and toxicity. Design of stimulus-responsive targeted nanomicelles for effective cancer therapy

Prolonged half-life & targetability For successful cancer targeting using nano micelles, prolonged half-life of nanomicelles is the primary requirement. The prolonged halflife of nanomicelles is observed due to their stealth property; they avoid reticuloendothelial system recognition by a self-hydrophilic surface structure [32] . It has been demonstrated that nanomicelles composed of PEG-blockpoly(d,l,‑lactide) (PEG‑b‑PDLLA) showed a prolonged circulation (t½ of 18 h) after intra venous administration, and significant levels of the nanomicelles were observed in the circulation up to 24 h [33] . Generally, targeting of long-circulating nanomicelles is achieved by loading anticancer drugs that passively reach (passive targeting) the cancer cells. By passive targeting nanomicelles can preferentially accumulate in the vicinity of the tumor mass upon intravenous administration by a process known as the enhanced permeability and retention effect. This enhanced permeability and retention effect is observed in the tumor site owing to abnormal vasculature (hypervascularization, aberrant vascular architecture with large intracellular gap junctions, extensive production of vascular permeability factors stimulating extravasation within tumor site and lack of lymphatic drainage) [34] . Advances in the molecular pharmacology, synthetic polymer chemistry and nanotechno logy fields have led to the development from passive to active targeted nanomicelles. Active targeting of an anticancer drug is achieved by conjugating nanomicelles to a cancer cell-specific targeting ligand. A number of specific interactions, such as ligand–receptor and antibody– antigen binding are utilized in the development of cancer-targeted nanomicelles. These specific interactions result in preferential accumulation of nanomicelles into cancer cells [5] . Cancer targeting ligands Current cancer chemotherapies have demonstrated dose-related adverse effects owing to the uncontrolled biodistribution of anticancer future science group

Review

drugs. Targeted nanomedicines are emerging as one of the promising approaches in anticancer treatment and have major advantages [5] . During active targeting, the nanomedicines can deliver the anticancer drug specifically to cancer cells with the help of different targeting ligands attached to the nanomedicines. These ligands will bind to specific receptors on the cancer cells. The different targeting ligands, such as antibodies (Herceptin®, Mabthera® and Erbitux ® are antibodies that recognize HER2/neu, CD20 and EGF receptors, respectively), small molecules (folic acid, whose receptor is expressed on the surface of cancer cells) or peptides (amino acid sequence [Arg–Gly–Asp] that binds to tumor avb3 integrin) are attached covalently to the surface of nanomedicines for active targeting [35–39] . The expression of folic acid receptors is observed in approximately 89% of human ovarian cancer and in approximately 20–50% of solid cancer originating from the kidney, lung tumor, breast, bladder and pancreas [40] . Therefore, folic acid was mostly utilized as an active targeting ligand for targeted nanomicelles incorporated with stimulus-responsive functions.

Stimulus response Highly effective/selective drug delivery for cancer therapy using nanomicelles can be achieved by intracellular, environmentally selective drug release via a stimulus response that takes control of the biodistribution of anticancer drugs, along with cancer cell targeting. The different stimulus-responsive functions included are pH, temperature, redox potential, magnetism and ultrasound. At present, the development of block copolymers allowed the preparation of nanomicelles for cancer therapy with smart functions (cancer cell targetability as well as stimulus response). Thus, stimulus-responsive targeted nanomicelles (smart carriers) become a potent carrier system for the effective and safer delivery of anticancer drugs. Avoidance of drug leakage from the anticancer drug-loaded nanomicelles will also increase the selectivity and efficiency of drug delivery to the cancer cells, leading to a maximum therapeutic effect with fewer adverse effects [28] . Multidrug resistance in cancer therapy A major cause for the failure in cancer therapy is multidrug resistance (MDR) [41] . MDR in cancer refers to a state of resilience against structurally and functionally unrelated drugs [42] . MDR may be inherent or developed during chemotherapy [42] www.futuremedicine.com

659

Review


and leads to toxic adverse effects and poor anticancer therapy [43] . The major mechanisms of MDR are divided into five categories: increased drug efflux, decreased drug influx, DNA repair activation, detoxification and inhibition of apoptotic pathways [44] . A recent study shows that overexpression of P‑glycoprotein is the primary mechanism for MDR [43] . In addition, a review by Gottesman and colleagues suggested some mechanisms for targeting P‑glycoproteinassociated MDR, such as evading P‑glycoprotein by the usage of non-P‑glycoprotein substrates and usage of P‑glycoprotein inhibitors to block drug efflux [45] . Recently, various passive and active targeted nanomedicines and multifunctional nanomedicines have been developed to enhance anticancer drug delivery and to overcome MDR through various mechanisms [44] . The following mechanisms are utilized in the design of different nanomedicines to overcome MDR in cancer therapy: Co-administration of a MDR modulator or drug efflux modulator to reduce the drug efflux (e.g., doxorubicin and drug efflux modulator [verapamil]-loaded liposomes have been developed to overcome MDR in cancer. Results show the increased cytotoxic effect of doxorubicin/ verapamil-loaded liposomes in comparison to free doxorubicin adminstration) [46] ;

n

The co-adminstration of pro-apoptotic modulators may overcome MDR cancer. In this mechanism, lowering the apoptotic threshold of MDR cells leads to more effective anticancer drugs (e.g., co-adminstration of ceramide [a pro-apoptotic modulator] with paclitaxelloaded poly[ethylene oxide] –poly[ ecaprolactone]-based nanoparticles results in increased cytotoxic effect at low doses of paclitaxel in an MDR human ovarian cancer cell line) [47] ;

n

The lower pH associated with MDR cells is utilized in different ways; some researchers altered the intracellular pH while others worked on the use of pH-sensitive constituents to control the release of anticancer drugs. (e.g., a nanomedicine containing paclitaxel-loaded poly[ethylene oxide]–poly[b‑amino ester]based nanoparticles [soluble below pH 6.5] was shown to increase intracellular and intratumor levels of paclitaxel in MDR cancer, and also showed a higher cytotoxic effect and decreased tumor volume in comparison to paclitaxel solution) [48] ;

n

660


In one study, researchers used high-frequency ultrasound to enhance the release of doxorubicin from the nanomedicine in MDR and drug-sensitive ovarian cancer cell lines. This study showed another promising mechanism for targeting and treating MDR cancer [49] .

n

The designs of nanomicelles are well developed, but with limited mechanisms (incorporating pH-responsive [50] or ultrasound-responsive [49] functions) for targeting MDR cancer. Therefore, future studies should be focused on the development of nanomicelles by incorporating the remaining MDR mechanisms developed with other nanomedicines (e.g., liposomes). Different stimulus-responsive targeted nanomicelles

pH-responsive nanomicelles Cancer cells have lower pH values (as low as 5.7) than normal cells (pH 7.4), owing to the glycolysis metabolism of cancer cells. Endosomal/lysosomal changes are also associated with low pH values of around 5.0–5.5 in the cancer cells. Therefore, the changes in pH values met by the nanomicelles upon intravenous injection will achieve a stimulus-responsive drug release [51] . Bae and colleagues have developed pHresponsive nanomicelles in which doxorubicin is attached to the side chain of the core-forming segment via an acid-labile hydrazone bond (Figure 1C) [52] . The results showed significant drug release only in lower pH conditions (i.e., cancer cells). Additionally, in vivo studies of the doxorubicinloaded pH-responsive nanomicelles showed drug circulation in the blood for a longer duration owing to minimal drug leakage and prolonged half-life of the nanomicelles. This results in highly selective drug accumulation and anticancer activity in C‑26-bearing mice [53] . Furthermore, the surface of doxorubicin-loaded pH-sensitive nanomicelles was modified with folic acid (targeting ligand) for active targeting (Figure 1D) . This study has developed the concept of stimulus-responsive nanomicelles with cancer targetability. The cytotoxic assay (against KB cells) of pH-responsive nanomicelles (doxorubicin loaded) conjugated with folic acid showed better cytotoxicity in comparison to nontargeted pH-responsive nanomicelles [54] . In some studies, poly(l‑histidine) composition is also used as a pH-responsive material for the preparation of pH-responsive nanomicelles [55] . In one study, anticancer drug-loaded stimulusresponsive (pH sensitive) nanomicelles were developed for the effective therapy of MDR cancer. future science group


These nanomicelles were formulated from two block copolymers poly(l‑histidine)‑b‑PEGfolic acid and poly(l‑lactide)‑b‑PEG-folic acid to achieve dissolution in an acidic environment (i.e., cancer cells) and folic acid receptor targeting. Results showed the absence of any significant cytotoxicity in wild-type breast cancer cells (MCF‑7) at pH 7.4 in comparison to free doxorubicin solution, and there was a dramatic increase in cytotoxicity in MDR breast cancer cells at pH 6.8 (20% cell viability) compared with free doxorubicin solution (85% cell viability). The in vivo studies yielded similar results, as assessed by tumor volume measurements. The pH-sensitive folic acid-modified nanomicelles are superior to nanomicelles without folic acid in both cellular and animal models. It is suggested that the addition of active targeting ligands in the design of pH-responsive targeted nanomicelles may be used for the effective treatment of MDR cancer [50] . Furthermore, pH-responsive function is incorporated along with redox potential responsive function in the design of multistimulus-responsive nanomicelles [56] . In some designs of multifunctional nanomicelles, pH-responsive function is also commonly incorporated as a stimulus function [57] .

Temperature-responsive nanomicelles Technologies that permit site-specific elevation of temperature have led to the development of temperature-sensitive nanomicelles. The polymer of choice is poly(N‑isopropylacrylamide) (pNIPAM), which has a lower critical solution temperature (LCST) of 32°C. These polymers are soluble below their LCST and precipitate when the temperature increases above the LCST, allowing for site-specific drug release from the nanomicelles [58] . Chung and colleagues prepared temperature-sensitive nanomicelles using poly(butyl methacrylate) (PBMA) as a hydrophobic core while pNIPAM was used as the thermosensitive shell (hydrophilic group) [59] . It was suggested that pNIPAM-b-PBMA-based nanomicelles loaded with doxorubicin released 15% of the drug after 15 h at 30°C, in comparison to 90% drug release at 37°C. The cytotoxicity experiments indicate less than 5% cell death at 29°C, but 65% cell death at 37°C, owing to the temperature-dependent drug release. Additionally, temperature-sensitive nanomicelles for active targeting were also synthesized by a reversible addition fragmentation chain transfer polymerization method using N,N‑dimethyl acrylamide (for hydrophilic future science group

Review

exterior shells) and NIPAM (for hydrophobic cores). Folic acid residues were efficiently conjugated with the a-azido chain of hydrophilic exterior shells for active targeting (Figure 1E) . In vitro drug release studies of folic acid–pN,N,dimethyl acrylamide–pNIPAM-based nanomicelles showed temperature-responsive dissociation and drug release [60] .

Redox potential responsive nanomicelles The intracellular concentration of glutathione (i.e., redox potential) in cancer cells is 100‑fold higher than the normal extracellular level of glutathione. This stimulus has been successfully used for specific anticancer drug delivery [61] . Drugs can be loaded into nanomicelles, whose structure is maintained under normal conditions by disulfide bonds. These bonds are reduced to thiol groups, owing to the higher glutathione level in the cancer cells, leading to drug release [62] . Recently, redox potential responsive nanomicelles were prepared along with pH- and temperature-responsive functions as multistimuli-responsive nanomicelles [56] . In this study, triple stimulus-responsive nanomicelle assembly was reported that responded to changes in temperature, pH and redox potential. The block copolymer design constitutes an acid-sensitive tetrahydropyran-protected 2‑hydroxyethyl methacrylate as the hydrophobic part and a temperature-sensitive pNIPAM as the hydrophilic part with an intervening disulfide bond (redox potential response) (Figure 1F) . It has been reported that the stimulus-responsive degradation of these nanomicelles can be achieved under the following conditions: above the LCST, the hydrophilic temperature-responsive block of nanomicelles is converted to a hydrophobic one, rendering the polymer insoluble in water and hence no assembly; lowering the pH transforms the acid-sensitive hydrophobic block of nanomicelles to a hydrophilic one, resulting in the dissolution of the assembly; and a reducing environment affords the reduction of the disulfide bond and hence disruption of the nanomicelle assembly [56] . However, preparation of redox potential responsive nanomicelles (anticancer drug loaded) and their anticancer efficacy were not described by any researchers. Magnetically responsive nanomicelles Magnetite (Fe3O4) and maghemite (g‑Fe2O3) are the most commonly used magnetic nanoparticles (size of 4–10 nm) for targeted drug delivery, www.futuremedicine.com

661

Review


owing to their good chemical stability and biocompatibility. These inorganic magnetic nanomedicines can diffuse through biological membranes and interact closely with biomolecules [63] . Novel applications of magnetic nanomedicines are expected in the field of cancer thermal treatment, magnetic targeting, imaging and remotely triggered targeted drug release [64] . Recently, magnetically guided nanomicelles (magnetic nanomedicines incorporated) have been developed to target the cancer site under the influence of external magnets. If cancer drugs are attached or loaded into such nanomicelles then such a system will offer an increased therapeutic activity at lower doses with fewer toxic effects [65] . Kim and colleagues developed novel temperature-responsive magnetomicelles that consist of a magnetic core, Fe3O4-oleic acid, and an amphiphilic surface layer of temperature-responsive poly(oleic acid‑co‑N‑isopropylacrylamide) copolymer [66] . The surface polymeric layer of Fe3O4‑oleic acid‑g-poly(oleic acid‑co‑N‑isopropyl acrylamide) nanoparticles would self-assemble in aqueous media to form a nanomicellar structure attributed to the surface amphiphilic poly(oleic acid‑co‑N‑isopropylacrylamide) polymers. The in vitro release behavior of drug-loaded magnetomicelles was further investigated, which showed a temperature-responsive release behavior due to the temperature-responsive structural changes of the nanomicellar surface layer. Talelli and colleagues have recently prepared biodegradable and temperature-responsive nanomicelles by encapsulating hydrophobic oleic acid-coated magnetic nanoparticles (diameter: 5–10 nm) [67] . The nanomicelles were composed of amphiphilic, thermosensitive and biodegradable block copolymers of PEG‑b‑poly[N‑(2‑hydroxypropyl) methacrylamide] dilactate. These magnetically responsive nanomicelles may be highly suitable for magnetically guided anticancer drug delivery (Figure 2A) . Hong and colleagues examined the behavior of targeted (with folic acid used as the receptor targeting ligand) PEG‑b‑poly(e‑caprolactone)based nanomicelles loaded with magnetic nanoparticles and doxorubicin [68] . They used Prussian blue staining to examine uptake in an immortal hepatic cell line and confirmed targeting with MRI while evaluating nanomedicine toxicity with the MTT assay. Studies show that magnetic-responsive targeted nanomicelles were internalized more effectively than nontargeted nanomicelles, resulting in decreased cellular proliferation and increased contrast in MRI [68] . 662


Ultrasound-responsive nanomicelles Ultrasound can be carefully controlled and focused on the target site during drug delivery. Ultrasound consists of pressure waves (with frequencies of 20 kHz or greater) generated by piezoelectric transducers that change an applied voltage into mechanical movement [69] . Ultrasound is used to trigger drug release from nanomicelles through mechanisms that include local temperature increase, cavitation that increases the permeability of cell membranes and the production of highly reactive free radical species that can accelerate polymer degradation [70] . Other important advantages of ultrasound-responsive nanomicelles are their noninvasive character, their ability to penetrate deep into the interior of the body, and their ability to be focused and carefully controlled. Studies on ultrasound-responsive nanomicelles and their mechanisms for controlling anticancer drug release towards tumor site have become an important area of research in cancer therapy [71] . Pruitt and Pitt have developed ultrasoundsensitive micelles (Pluronic ® based) containing doxorubicin stabilized with PEG–phospholipid [72] . An in vivo study suggested that ultrasound-triggered release of doxorubicin from the micelles can delay the cancer growth significantly longer in comparison to micelles without ultrasound. In addition, several studies have reported on the effect of Pluronic surfactants in overcoming MDR. Venne and colleagues have also shown that the use of Pluronic-based ultrasound-responsive micelles were effective in overcoming MDR [49] . In vivo results on the effect of doxorubicin on MDR hamster ovarian and breast cancer cells observed a 290- and 700‑fold increase, respectively, on the cytotoxic action of doxorubicin in the presence of Pluronic L61. Gao and colleagues studied the effect of ultrasound on the biodistribution of a nanomicelle loaded with doxorubicin (Figure 2B) with a mixed micelle size of 12.9 nm (fluorescently labeled Pluronic micelles) in ovarian cancerbearing nu/nu mice [71] . The degree of nanomicelle accumulation in the cells of various organs was characterized by flow cytometry. Nanomicelles were formed in either pure Pluronic P‑105 solutions (unstabilized micelles) or mixtures of Pluronic P‑105 with PEG–diacylphospholipid (stabilized micelles). The data showed that a 30 s ultrasonic irradiation by 1 or 3 MHz ultrasound, applied locally to the tumor, significantly enhanced accumulation of future science group


Review

Pluronic in the tumor cells, which was observed for both intraperitoneal and intravenous injections and for unstabilized and stabilized nanomicelles. The results indicated the targeting effect of Pluronic-based ultrasound-responsive nanomicelles to the tumors. Stimulus-responsive multifunctional nanomicelles for targeted drug delivery Multifunctional targeted nanomicelles are designed to facilitate simultaneous active targeted drug delivery and imaging. The imaging agent or diagnostic is loaded within the hydrophobic core or linked covalently to the surface of the multifunctional nanomicelle, which is loaded with the drug and targeting ligand. These nanomicelles may circulate for prolonged periods in the blood, evading host defenses, and gradually release the drug by targeting and simultaneously facilitating in vivo imaging [73] . The development of multifunctional targeted nanomicelles in combination with stimulus response now represents an important field in targeted nanomedicine research, and could be useful for effective cancer therapy and diagnosis in a single platform. Nasongkla and colleagues developed novel stimulus-responsive multifunctional targeted nanomicellar platforms that incorporate the targeting ligand, anticancer drug, imaging agent and pH-responsive drug release behavior [74] . Doxorubicin and magnetic nanoparticles (imaging agent) were loaded into the cores of PEG–poly(l‑lactic acid) nanomicelles, with cRGD (that can target avb3 integrin) attached as a targeting ligand on the surface of the nanomicelles (Figur e 2C) . The stimulus-responsive multifunctional targeted nanomicelles demonstrated a higher uptake during the in vitro anticancer study. Recently, Lee and colleagues designed pHresponsive multifunctional nanomicelles that consisted of two block copolymers of poly(l‑lactic acid)‑b‑PEG‑b‑poly(l‑histidine)‑TAT and poly(l‑histidine)‑b‑PEG [57] . The nanomicelle surface hides TAT, which has the strong capability to translocate the micelle into cells during circulation, and exposes TAT at a slightly acidic tumor extracellular pH to facilitate the internalization process. High doxorubicin concentrations in the cytosol and the target site were observed owing to the selective and targeted effect of the nanomicelles, thus increasing the doxorubicin potency in various wild-type and MDR cell lines. future science group

pH-dependent drug release

Anticancer drug Hydrophilic group Targeting ligand Hydrophobic core Ultrasound-responsive hydrophilic group Magnetic nanoparticles

Figure 2. Different targeted nanomicelles. (A) Magnetically responsive passive targeted nanomicelles; (B) ultrasound-responsive passive targeted nanomicelles; and (C) pH-responsive multifunctional active targeted nanomicelles.

Toxicity & safety of nanomicelles The toxicity of targeted nanomicelles should be monitored during intravenous adminstration owing to their large surface area. The fate of nanomicelles and their constituents, particularly those which are not biodegradable, such as coating agents (e.g., PEG), should be investigated [75] . Poly(l-lactic acid) is commonly used in nanomicelle design. In one study, degradation products produced by poly(l-lactic acid)-based particles showed cytotoxicity. During the injection of these particles in the peritoneal cavity of mice, macrophages demonstrated signs of cell damage, cell death and cell lysis due to phagocytosis of a large number of poly(l‑lactic acid) particles [76] . Pluronic, which is used for surface modification and in the design of nanomicelles (e.g., temperature-responsive nanomicelles), inhibits P‑glycoprotein efflux pumps expressed in polarized endothelial cells that form the exterior of the blood–brain barrier, and could potentially interfere with the transport of a number of modulators and homeostatic mediators in the brain [77] . Nanomicelles can induce different gene expressions depending upon the polymeric monomers used in the preparation of nano micelles. A change in gene expression has been www.futuremedicine.com

663

Review


reported in certain cells after cisplatin delivery with nanomicelles when compared with that of free cisplatin treatment. Cisplatin nanomicelles downregulated the gene expression of integrin and matrix metalloprotease families, whereas free cisplatin upregulated them. The results suggest that downregulation of integrin and matrix metalloprotease genes by cisplatin nanomicelles may lead to potential toxic effects [78] . A major risk of using stimulus-responsive targeted nanomicelles for cancer therapy is that they have not been extensively characterized in clinical settings (i.e., pharmacokinetics, biodistribution and toxicity) [29] . In one study, safety of PEG‑b‑poly(aspartate) block copolymer-based nanomicelles was determined by histological and immunohistochemical analysis. The only marked change observed was activation of the mononuclear phagocyte system. The results suggest that no, or a very low level of in vivo toxicity is observed after multiple intravenous injections of nanomicelles [79] . Indeed, nanomicelles appear to be safer as they have fewer adverse effects in comparison to other nanomedicines [18] . These adverse effects included hand-foot syndrome for doxorubicinincorporated liposomes and infusion-related reactions for an antibody–anticancer drug conjugate. These adverse effects are a major concern for other nanomedicines used for similar purposes. Therefore, the safety aspect of nanomicelles is of great importance [16–18] . In most cases, nanomicelles are hindered mainly by premature drug release before they reach the specific cancer target [12] . Premature

anticancer drug release into the blood circulation leads to systemic adverse effects and a failure to deliver the drug to the tumor site. Therefore, the design of stimulus-responsive active targeted nanomicelles may be a safer alternative for intracellular targeting of cancer cells to achieve maximal therapeutic efficacy with fewer adverse effects [29] . Some of the stimulus-responsive targeted micelles currently under progress for the treatment of cancer are listed in Table 1. Conclusion Studies on nanomicellar nanomedicine have already developed some novel or next-generation nanoplatforms for cancer therapy. This article summarized the recent advancements in nanomicellar nanomedicines incorporated with a targeting ligand (i.e., folic acid) and stimulusresponsive function to achieve maximum anticancer efficacy. The multifunctional capabilities of the nanomicelles are needed to overcome the therapeutic challenges of cancer (e.g., MDR). Finding important cancer biomarkers, and knowledge of molecular pharmacology, physio logy, biotechnology, synthetic polymer chemistry and nanotechnology will further improve the design of stimulus-responsive targeted nanomicelles. Furthermore, the toxicity and safety of nanomicelles must be investigated to prove their safe and efficacious use. Future perspective In the coming years, research on nanomicelles for cancer therapy will lead to breakthroughs that enable their effective application. In addition,

Table 1. Selected targeted micelles currently under investigation for the effective treatment of cancer. Type of targeted micelles Stimulus function

Size of the Level of development Practical success nanomicelles (nm)

Active targeted nanomicelles

63

Ref.

Preclinical/in vitro anticancer effect

Better cytotoxic effect

[54]

Passive targeted nanomicelles Temperature responsive Not mentioned


Temperature-dependent cytotoxic effect

[59]

Active targeted nanomicelles

Temperature responsive 46

In vitro development

Promising carrier for anticancer drug

[60]

Passive targeted nanomicelles Redox potential/ 90 multistimulus responsive


Effective carrier for site-specific drug delivery

[56]

Passive targeted nanomicelles Magnetically responsive 200


Suitable for magnetically guided drug delivery

[67]

Passive targeted nanomicelles Ultrasound responsive

12.9

Preclinical/in vivo anticancer effect

Targeted anticancer effect

[71]

Multifunctional active targeted nanomicelles

46


Enhanced uptake to cancer cells

[74]

664

pH responsive

pH responsive


future science group


anticancer drug-loaded nanomicelles may avoid the need for risky adminstration due to uncontrolled biodistribution of anticancer drugs, thereby promoting patient compliance and therapeutic effects. Stimulus-responsive targeted nanomicelles for cancer therapy may avoid the adverse effects (e.g., immunosuppression, cardiomyopathy and neurotoxicity) of traditional cancer therapies. The architectural control characteristic of cancer-targeted stimulus-responsive nanomicelles will lead to positive outcomes from in vitro and in vivo studies that can overcome the therapeutic challenges of cancer (e.g., MDR). Nanomicelles developed using multifunctional approaches (incorporating imaging agents, targeting ligands and stimulus-responsive functions) will hopefully enable the development of various novel products for effective cancer therapy. Although these products may offer various advantages over conventional anticancer drug-delivery systems, the issue of their safety should not be ignored.

Review

Not all cancer cells will respond to a single anticancer drug owing to their molecular and phenotypic heterogeneity [80] . Theranostic nanomicelles that incorporate both therapeutic and diagnostic agents may be promising carriers for personalized cancer therapy (specific for an individual cancer patient) to increase the specificity and efficacy of anticancer drugs [81–83] . The design of nanomicelles incorporated with diagnostic agents can identify the phenotypic expressions and treatment efficacy within cancer cells for an effective cancer therapy [81] . Financial & competing interests disclosure The authors acknowledge financial support from the University Grant Commission, New Delhi, India, in terms of a Research Fellowship. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or finan‑ cial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.

Executive summary Various stimulus-responsive targeted nanomicelles and stimulus-responsive multifunctional targeted nanomicelles are currently under intense development for effective therapeutic applications in cancer treatment. Stimulus-responsive targeted nanomicelles have been developed with various stimulus-responsive functions such as pH, temperature, redox potential, magnetism and ultrasound. Targeted drug delivery of stimulus-responsive nanomicelles can be achieved by either passive or active targeting. Toxicity studies on different types of nanomicelles and other composition involved in their design can provide the safety and toxic limits on the usage of the nanomicelles. The stimulus-responsive multifunctional targeted nanomicelles are capable of simultaneously diagnosing and controlling delivery of anticancer drugs based on stimulus responses to cancer cells by incorporating anticancer drugs, targeting ligands, stimulus functions and imaging agents. Enhancement of drug solubility, self-hydrophilic surface, active targeting, stimulus functions, multidrug resistance-overcoming mechanisms and imaging functions of upcoming nanomicelles will provide additional benefits during multidrug resistance cancer therapy.

Bibliography

5

Papers of special note have been highlighted as: n of interest 1

2

3

4

Moghimi SM, Hunter AC, Murray JC: Long-circulating and target-specific nanoparticles: theory to practice. Pharmacol. Rev. 53, 283–318 (2001). Roco MC: Nanotechnology: convergence with modern biology and medicine. Curr. Opin. Biotechnol. 14, 337–346 (2003).

n

6

7

Wilkinson JM: Nanotechnology applications in medicine. Med. Device Technol. 14, 29–31 (2003). Green JJ, Shi J, Chiu E et al.: Biodegradable polymeric vectors for gene delivery to human endothelial cells. Bioconjug. Chem. 17, 1162–1169 (2006).


8

Torchilin VP: Targeted pharmaceutical nanocarriers for cancer therapy and imaging. AAPS J. 9, E128–E147 (2007).

9

Jones M, Leroux J: Polymeric micelles – a new generation of colloidal drug carriers. Eur. J. Pharm. Biopharm. 48, 101–111 (1999).

Provides a comprehensive review of the development of targeted nanomedicines.

10

Adams ML, Lavasanifar A, Kwon GS: Amphiphilic block copolymers for drug delivery. J. Pharm. Sci. 92, 1343–1355 (2003).

11

Gaucher G, Dufresne MH, Sant VP et al.: Block copolymer micelles: preparation, characterization and application in drug delivery. J. Control. Release 109, 169–188 (2005).

Muthu MS, Singh S: Studies on biodegradable polymeric nanoparticles of risperidone: in vitro and in vivo evaluation. Nanomedicine 3, 305–319 (2008). Muthu MS, Singh S: Targeted nanomedicines: effective treatment modalities for cancer, AIDS and brain disorders. Nanomedicine 4, 105–118 (2009). Sahoo SK, Labhasetwar V: Nanotech approaches to drug delivery and imaging. Drug Discov. Today 8, 1112–1120 (2003).

www.futuremedicine.com

12 Mahmud A, Xiong X, Montazeri H et al.:

Polymeric micelles for drug targeting. J. Drug Target. 15, 553–584 (2007). 13 La S, Okano T, Kataoka K: Preparation and

characterization of the micelles-forming polymeric drug: indomethacin-incorporated

665

Review


poly(ethylene oxide)–poly(b-benzyl l-aspartate) block copolymer micelles. J. Pharm. Sci. 85, 85–90 (1996). 14

15

16

17

18

19

Yamamoto Y, Yasugi K, Harada A et al.: Temperature related change in the properties relevant to drug delivery of poly(ethylene glycol)–poly(d,l-lactide) block copolymer micelles in aqueous milieu. J. Control. Release 82, 359–371 (2002). Nishiyama N, Kataoka K: Current state, achievements, and future prospects of polymeric micelles as nanocarrier for drug and gene delivery. Pharmacol. Ther. 112, 630–648 (2006). Uziely B, Jeffers S, Isacson R et al.: Liposomal doxorubicin: antitumor activity and unique toxicities during two complementary Phase I studies. J. Clin. Oncol. 13, 1777–1785 (1995). Muggia F, Hamsworth JL, Jeffers S et al.: Phase II study of liposomal doxorubicin in refractory ovarian cancer: antitumor activity and toxicity modification by liposomal encapsulation. J. Clin. Oncol. 15, 987–993 (1996). Gordon AN, Fleagle JT, Guthrie D et al.: Recurrent epithelial ovarian carcinoma: a randomized Phase III study of pegylated liposomal doxorubicin vs. topotecan. J. Clin. Oncol. 19, 3312–3322 (2001). Matsumura Y, Hamaguchi T, Ura T et al.: Phase I clinical trial and pharmacokinetic evaluation of NK911, micelle-encapsulated doxorubicin. Br. J. Cancer 91, 1775–1781 (2004).

26 Pridgen EM, Langer R, Farokhzad OC:

21

Ferrari M: Cancer nanotechnology: opportunities and challenges. Nat. Rev. Cancer 5, 161–171 (2005).

22 Nishiyama N, Kataoka K: Development of

polymeric micelles for the delivery of antitumor agents. Gan To Kagaku Ryoho 36, 357–361 (2009). 23 Uchino H, Matsumura Y, Negishi T et al.:

Cisplatin-incorporated polymeric micelle (NC-6004) can reduce nephrotoxicity and neurotoxicity of cisplatin in rats. Br. J. Cancer 19, 678–687 (2005). 24 Mizumura Y, Matsumura Y, Yokoyama M

et al.: Incorporation of the anticancer agent KRN5500 into polymeric micelles diminishes the pulmonary toxicity. Jpn J. Cancer Res. 93, 1237–1243 (2002). 25 Negishi T, Koizumi F, Uchino H et al.:

NK105, a paclitaxel-incorporating micellar nanoparticle, is a more potent radiosensitizing agent compared with free paclitaxel. Br. J. Cancer 95, 601–606 (2006).

666

Folate receptor expression in carcinomas and normal tissues determined by a quantitative radio ligand binding assay. Anal. Biochem. 338, 284–293 (2005).

27 Yoo HS, Park TG: Folate receptor targeted

biodegradable polymeric doxorubicin micelles. J. Control. Release 96, 273–283 (2004). 28

Sawant RM, Hurley JP, Salmaso S et al.: “SMART” drug delivery systems: doubletargeted pH-responsive pharmaceutical nanocarriers. Bioconjug. Chem. 17, 943–949 (2006).

41 Sinha R, Kim GJ, Nie S, Shin DM:

Nanotechnology in cancer therapeutics: bioconjugated nanoparticles for drug delivery. Mol. Cancer. Ther. 5, 1909–1917 (2006). 42 Harris AL, Hochhauser D: Mechanisms of

multidrug resistance in cancer treatment. Acta Oncol. 31, 205–213 (1992).

29 Torchilin V: Multifunctional and stimuli-

sensitive pharmaceutical nanocarriers. Eur. J. Pharm. Biopharm. 71, 431–444 (2009). n

43 Tsuruo T, Naito M, Tomida A et al.:

Molecular targeting therapy of cancer: drug resistance, apoptosis and survival signal. Cancer Sci. 94, 15–21 (2003).

Presents the development of stimulusresponsive multifunctional nanoparticles.

30 Ganta S, Devalapally H, Shahiwala A,

44 Jabr-Milane LS, van Vlerken LE, Yadav S,

Amiji MM: Multi-functional nanocarriers to overcome tumor drug resistance. Cancer Treat. Rev. 34, 592–602 (2008).

Amiji M: A review of stimuli-responsive nanocarriers for drug and gene delivery. J. Control. Release 126, 187–204 (2008). n

31

Presents the development of stimulus-responsive particles. Singh S, Nalwa HS: Nanotechnology and health safety-toxicity and risk assessments of nanostructured materials on human health. J. Nanosci. Nanotechnol. 7, 3048–3070 (2007).

45

medicine to solid tumors. Adv. Drug Deliv. Rev. 46, 149–168 (2001).

multidrug resistance by transferrinconjugated liposomes co-encapsulating doxorubicin and verapamil. J. Pharm. Pharm. Sci. 10, 350–357 (2007). 47 van Vlerken LE, Duan Z, Seiden MV,

Amiji MM: Modulation of intracellular ceramide using polymeric nanoparticles to overcome multidrug resistance in cancer. Cancer Res. 67, 4843–4850 (2007).

33 Yamamoto Y, Nagasaki Y, Kato Y et al.:

Long-circulating poly(ethylene glycol)– poly(d,l-lactide) block copolymer micelles with modulated surface charge. J. Control. Release 77, 27–38 (2001).

48 Devalapally H, Shenoy D, Little S et al.:

Poly(ethylene oxide)-modified poly(b-amino ester) nanoparticles as a pH-sensitive system for tumor-targeted delivery of hydrophobic drugs: part 3. Therapeutic efficacy and safety studies in ovarian cancer xenograft model. Cancer Chemother. Pharmacol. 59, 477–484 (2007).

34 Torchilin VP: Micellar nanocarriers:

pharmaceutical perspectives. Pharm. Res. 24, 1–16 (2007). 35

Steinhauser I, Spankuch B, Strebhardt K et al.: Trastuzumab-modified nanoparticles: optimisation of preparation and uptake in cancer cells. Biomaterials 27, 4975–4983 (2006).

49 Venne A, Li S, Mandeville R et al.:

Hypersensitizing effect of pluronic L61 on cytotoxic activity, transport, and subcellular distribution of doxorubicin in multiple drug resistant cells. Cancer Res. 56, 3626–3629 (1996).

36 Tiefenauer LX, Kuhne G, Andres RY:

Antibody–magnetite nanoparticles: in vitro characterization of a potential tumor-specific contrast agent for magnetic resonance imaging. Bioconjug. Chem. 4, 347–352 (1993). 37 Hood JD, Bednarski M, Frausto R et al.:

50 Lee ES, Na K, Bae YH: Doxorubicin loaded

pH-sensitive polymeric micelles for reversal of resistant MCF-7 tumor. J. Control. Release 103, 405–418 (2005).

Tumor regression by targeted gene delivery to the neovasculature. Science 296, 2404–2407 (2002). 38 Pasqualini R, Ruoslahti E: Organ targeting

in vivo using phase display peptide libraries. Nature 380, 364–366 (1996). 39 Pasqualini R, Koivunen E, Ruoslahti E:

a v integrins as receptors for tumor targeting by circulating ligands. Nat. Biotechnol. 15, 542–546 (1997).


Gottesman MM, Fojo T, Bates SE: Multidrug resistance in cancer: role of ATP-dependent transporters. Nat. Rev. 2, 48–58 (2002).

46 Wu J, Lu Y, Lee A et al.: Reversal of

32 Jain RK: Delivery of molecular and cellular

20 Peer D, Karp JM, Hong S et al.: Nanocarriers

as an emerging platform for cancer therapy. Nat. Nanotechnol. 2, 751–760 (2007).

40 Parker N, Turk MJ, Westrick E et al.:

Biodegradable, polymeric nanoparticle delivery systems for cancer therapy. Nanomedicine 2, 669–680 (2007).

51

Bae Y, Nishiyama N, Fukushima S et al.: Preparation and biological characterization of polymeric micelle drug carriers with intracellular pH-triggered drug release property: tumor permeability, controlled subcellular drug distribution, and enhanced in vivo antitumor efficacy. Bioconjug. Chem. 16, 122–130 (2005).



52

Bae Y, Fukushima S, Harada A, Kataoka K: Design of environment-sensitive supramolecular assemblies for intracellular drug delivery: polymeric micelles that are responsive to intracellular pH change. Angew. Chem. Int. Ed. 42, 4640–4643 (2003).

61

54 Bae Y, Jang WD, Nishiyama N et al.:

Multifunctional polymeric micelles with folate-mediated cancer cell targeting and pH-triggered drug releasing properties for active intracellular drug delivery. Mol. Biosyst. 1, 242–250 (2005). 55

Lee ES, Na K, Bae YH: Super pH-sensitive multifunctional polymeric micelle. Nano Lett. 5, 325–329 (2005).

56 Klaikherd A, Nagamani C,

Thayumanavan S: Multi-stimuli sensitive amphiphilic block copolymer assemblies. J. Am. Chem. Soc. 131(13), 4830–4838 (2009). 57 Lee ES, Gao Z, Kim D et al.: Super

pH-sensitive multifunctional polymeric micelle for tumor pHe specific TAT exposure and multidrug resistance. J. Control. Release 129, 228–236 (2008). 58 Liu SQ, Tong YW, Yang YY: Thermally

sensitive micelles self assembled from poly(N-isopropylacrylamide-co-N,Ndimethylacrylamide)-b-poly(d,l-lactide-coglycolide) for controlled delivery of paclitaxel. Mol. Biosyst. 1, 158–165 (2005). 59 Chung JE, Yokoyama M, Yamato M et al.:

Thermo-responsive drug delivery from polymeric micelles constructed using block copolymers of poly(N-isopropylacrylamide) and poly(butylmethacrylate). J. Control. Release 62, 115–127 (1999). 60 De P, Gondi SR, Sumerlin BS: Folate

conjugated thermoresponsive block copolymers: high efficient conjugation and solution self-assembly. Biomacromolecules 9, 1064–1070 (2008).


72 Pruitt JD, Pitt WG: Sequestration and

ultrasound-induced release of doxorubicin from stabilized Pluronic P105 micelles. Drug Deliv. 9, 253–258, (2002). 73 Yang X, Chen Y, Yuan R et al.:

62 Cavallaro G, Campisi M, Licciardi M

et al.: Reversibly stable thiopolyplexes for intracellular delivery of genes. J. Control. Release 115, 322–334 (2006).

53 Bae Y, Nishiyama N, Fukushima S et al.:

Preparation and biological characterization of polymeric micelle drug carriers with intracellular pH-triggered drug release property: tumor permeability, controlled subcellular drug distribution, and enhanced in vivo antitumor efficacy. Bioconjug. Chem. 16, 122–130 (2005).

Saito G, Swanson JA, Lee KD: Drug delivery strategy utilizing conjugation via reversible disulfide linkages: role and site of cellular reducing activities. Adv. Drug Deliv. Rev. 55, 199–215 (2003).

Folate-encoded and Fe3O4-loaded polymeric micelles for dual targeting of cancer cells. Polymer 49, 3477–3485 (2008). 74

63 Tartaj P, del Puerto Morales M,

Veintemillas-Verdaguer S et al.: The preparation of magnetic nanoparticle for applications in biomedicine. J. Phys. D Appl. Phys. 36, R182–R197 (2003).

65

Cinteza LO, Ohulchanskyy TY, Sahoo Y et al.: Diacyllipid micelle-based nanocarrier for magnetically guided delivery of drugs in photodynamic therapy. Mol. Pharm. 3, 415–423 (2006).

Nanomedicine: current status and future prospects. FASEB J. 19, 311–330 (2005). 76 Lam KH, Schakenraad JM, Esselbrugge H

et al.: The effect of phagocytosis of poly(l-lactic acid) fragments on cellular morphology and viability. J. Biomed. Mat. Res. 27, 1569–1577 (1993). 77 Batrakova EV, Li S, Alakhov VY et al.:

Optimal structure requirements for pluronic block copolymers in modifying P‑glycoprotein drug efflux transporter activity in bovine brain microvessel endothelial cells. J. Pharmacol. Exp. Ther. 304, 845–854 (2003).

66 Kim GC, Li YY, Chu YF et al.: A nanosized

thermo-sensitive drug carrier: self-assembled Fe3O4-OA-g-P(OA-co-NIPAAm) magnetomicelles. J. Biomater. Sci. Polym. Ed. 19, 1249–1259 (2008). 67 Talelli M, Rijcken CJF, Lammers T et al.:

Superparamagnetic iron oxide nanoparticles encapsulated in biodegradable thermosensitive polymeric micelles: towards a targeted nanomedicine suitable for image guided drug delivery. Langmuir 25, 2060–2067 (2009).

78 Nishiyama N, Koizumi F, Okazaki S et al.:

Differential gene expression profile between PC-14 cells treated with free cisplatin and cisplatin incorporated polymeric micelles. Bioconj. Chem. 14, 449–457 (2003). 79 Kawaguchi T, Honda T, Nishihara M et al.:

68 Hong G, Yuan R, Liang B et al.:

Folate-functionalized polymeric micelle as hepatic carcinoma-targeted, MRI-ultrasensitive delivery system of antitumor drugs. Biomed. Microdevices 10, 693–700 (2008).

Histological study on side effects and tumor targeting of a block copolymer micelle on rats. J. Control. Release 136, 240–246 (2009). 80 Tran TD, Caruthers SD, Hughes M et al.:

Clinical application of perfluorocarbon nanoparticles for molecular imaging and targeted therapeutics. Int. J. Nanomedicine 2, 515–526 (2007).

69 Marin A, Muniruzzaman M, Rapoport N:

Mechanism of the ultrasonic activation of micellar drug delivery. J. Control. Release 75, 69–81 (2001). 70 Husseini GA, Pitt WG: Ultrasonic-activated

micellar drug delivery for cancer treatment. J. Pharm. Sci. 98, 795–811 (2009). 71 Gao Z, Fain HD, Rapoport N:

Ultrasound-enhanced tumor targeting of polymeric micellar drug carriers. Mol. Pharm. 1, 317–330 (2004).

www.futuremedicine.com

Nasongkla N, Bey E, Ren J et al.: Multifunctional polymeric micelles as cancer-targeted, MRI-ultrasensitive drug delivery systems. Nano Lett. 6, 2427–2430 (2006).

75 Moghimi SM, Hunter AC, Murray JC:

64 Duguet E, Vasseur S, Mornet S et al.:

Magnetic nanoparticles and their applications in medicine. Nanomedicine 1, 157–168 (2006).

Review

81

Warner S: Diagnostic plus therapy = theranostics. Scientist 18, 38–39 (2004).

82 Sumer B, Gao J: Theranostic nanomedicine

for cancer. Nanomedicine 3(2), 137–140 (2008). 83 Jain KK: Role of nanobiotechnology in the

development of personalized medicine. Nanomedicine 4(3), 249–252 (2009).

667