Polymers with Nano-Encapsulated Functional Polymers

CHAPTER

POLYMERS WITH NANO-ENCAPSULATED FUNCTIONAL POLYMERS

9

ENCAPSULATED NANOPARTICLES FOR TREATMENT OF CANCER CELLS Marli Luiza Tebaldi*, Rose Marie Belardi*, Sérgio Roberto Montoro†,‡ Universidade Federal de Itajubá, Itajubá, Minas Gerais, Brazil* Laboratory of Polymers, Chemical Engineering Department, Engineering School of Lorena, University of São Paulo, Lorena, São Paulo, Brazil† Centro Estadual de Educação Tecnológica “Paula Souza” (CEETEPS), Pindamonhangaba, São Paulo, Brazil‡

­C HAPTER OUTLINE HEAD 9.1 Introduction ...................................................................................................................................171 9.2 NPs for Treatment of Cancer ...........................................................................................................172 9.2.1 General Considerations .............................................................................................172 9.2.2 Nanocarriers Based on Polymeric Materials ................................................................174 9.2.2.1 Targeted Delivery ...................................................................................................175 9.3 Nanostructures for Anticancer Therapeutics: Future Tendencies .......................................................177 9.3.1 Anticancer Polymer Prodrug Nanocarriers ...................................................................181 9.4 Conclusions and Future Directions ..................................................................................................183 References............................................................................................................................................183

9.1 ­INTRODUCTION Researchers worldwide are continuously searching for advanced materials that fulfill high demands in a range of applications involving a multidisciplinary field with elements from engineering, physics, chemistry, biology, and medicine. New and modern methodologies have been developed and optimized for this purpose. The search for materials with unique properties to satisfy further demands has promoting the development of materials with dimensions at the nanoscale domain. The rationale for nanotechnology is the nanoscience. The interface among nanoscience, nanotechnology and other

Design and Applications of Nanostructured Polymer Blends and Nanocomposite Systems. http://dx.doi.org/10.1016/B978-0-323-39408-6.00008-X © 2016 Elsevier Inc. All rights reserved.

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interdisciplinary sciences has opened new scenarios for many industrial and consumer sectors. In this way, this knowledge frontier has been regarded as the focus of a new industrial revolution [1,2]. While nanoscience concerns an understanding of physical, chemical, and biological properties on atomic scales, nanotechnology employs controlled manipulation of these properties to create materials and functional systems with unique capabilities. The properties of nanomaterials depend on their composition and structure, and are related to new phenomena associated with quantized effects and the very substantial influence of surfaces and interfaces [3,4]. Nanotechnology developed by nature without the efforts of researchers uses enzymes and catalysts to organize atoms and molecules into complex structures making life possible. The explosion of the technology in the last decades in all industrial sectors led new techniques for working at the nanoscale to be developed fast and new nanotechnological products began to appear on the market. Nanostructured materials have the potential to improve health treatments, reduce drug toxicity and increase therapeutic efficacy. In this nanotechnological context the science of polymer materials holds a significant stake. In fact, it is not possible to imagine life without polymers at the present time [5]. These soft materials play a significant role in most parts of the industrial sector. No materials other than polymers present structural differences and process variables so closely linked to performance. Due to its versatility polymer science has been crucial in solving most of humanity's problems. The advancements in polymer science have helped to promote materials composed of natural polymers [5] for a new range of applications. Natural polymers are usually insoluble in solvents and this characteristic hinders the processing methods. The chemical modification of natural polymers improves solubility, controls degradation rates, and improves mechanical properties. Understanding the correlation between structure and properties can lead to controlled functionality in the macromolecules, which is the key to achieve potential advances for a wide range of innovative applications. The discovery of new polymerization processes with unprecedented control over synthesis has allowed tailored topologies, compositions and microstructures resulting in precise and complex macromolecular architectures [6]. Among these new synthetic procedures, the controlled radical polymerization (CRP) processes, such as atom transfer radical polymerization (ATRP) [7–9], reversible addition-fragmentation chain transfer (RAFT) [10–12], and nitroxide-mediated radical polymerization (NMP) [13] can be cited as valuable tools to prepare vinyl polymers with complex and functional macromolecular architectures and distinctive properties. These processes open up wide prospects in molecular design including the possibility of obtaining polymeric nanostructures. Most impressive and promising is, certainly, the possibility of conjugation of drugs or prodrugs in the functionalized polymer chains obtained by CRPs [14]. The understanding of polymer science combined with CRP techniques makes it possible to prepare a series of nanomaterials for drug delivery, particullarly in cancer therapy. This chapter focuses specifically on polymeric nanoparticles (NPs) for the treatment of cancer, exploring examples from the literature and discussing some future trends related to this topic.

9.2 ­NPs FOR TREATMENT OF CANCER 9.2.1 ­GENERAL CONSIDERATIONS The most recent data and projections show that the incidence of new cancer cases is expected to rise from 14 to 22 million per year within the next two decades (World Health Organization: WHO, 2014). The main factors that may be related to cancer are consumption of alcohol, tobacco and sugar-sweetened

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beverages, and exposure to occupational and environmental carcinogenic risks, including air pollution and stressful lifestyle. The global increase of obesity can also be linked to some types of tumors such as esphageal cancer, due to excessive consumption of processed foods leading to increased incidence of in tropical countries another problem is excessive exposure to the sun, which may cause skin cancer [15–18]. The most widely used therapy for cancer treatment is chemotherapy, in which several combinations of chemotherapeutic agents (CTAs) are applied to the patient. However, this strategy is very aggressive leading to several side effects, such as nausea, fatigue, neuropathy, and hair-loss and a common consequence is damage to healthy tissues [19,20]. In order to circumvent these limitations it is necessary to improve our knowledge about cancer physiopathology, to discover new anticancer drugs and to develop novel biomedical technologies that can release the drug specifically into the tumor. Many researchers have focused their efforts on this area, with the aim of obtaining a precise understanding of the differences between a cancer cell and a healthy cell. In fact, the development of new therapies has led to increased survival times and a better quality of life for cancer patients, but there is still a need for improvement [14,21]. Many scientists have focused their attention on the development of effective systems that selectively destroy disease cells without damaging the healthy cells, releasing drugs or bioactive agents at the desired site of action. These materials are also called controlled drug-delivery systems. In recent years the number of publications associated with “cancer drug delivery” has increased considerably, as shown in Figure 9.1. One of the most promising strategies for the diagnosis and treatment of diseases comprises the polymeric NPs. Despite the significant advances reported for several treatments using drug loaded-NP, some limitations still remain. The insufficient understanding of events at the nano-bio interface both

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FIGURE 9.1 Number of publications produced per year since 2000 as ascertained from a PubMed search using “cancer drug delivery” as keyword.

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in vitro and in vivo, and the low reproducibility of NPs synthesis at scales suitable for clinical development and commercialization are among the main limitations of treatments involving drug loaded-NP [22–24]. Furthermore, there is little understanding of the toxicological properties of NPs and their impact on human health [25].

9.2.2 ­NANOCARRIERS BASED ON POLYMERIC MATERIALS The polymeric materials used in nanomedicine must be nontoxic, biocompatible and eliminated easily from the body, for instance by biodegradation or bioerosion. In general, they are made from synthetic or natural polymers, such as poly(ethylene glycol) (PEG), poly(lactic acid) (PLA), poly(ε-caprolactone) (PCL), poly(lactic-co-glycolic acid) (PLGA), poly(2-hydroxyethyl methacrylate) (PHEMA), polysaccharides, proteins, etc. Nanocarriers based on polymers have attracted much attention due to their advantages compared to other conventional materials. Polymeric materials offer major flexibility in the methods of synthesis; a broad diversity in terms of nature, properties, and composition; easy functionalization; and the possibility to be stimuli responsive [14]. Nano-sized polymeric carriers have been extensively applied, because, in general, loaded nano-sized carriers can improve drug solubility and stability in vivo, and can target a drug to the tumor site protecting the healthy tissue. Polymeric NPs are colloidal particles composed of polymers and low weight substances as surfactants and drugs, which may range around 100 nm in size [26]. An important class of polymeric NP comprises nanospheres and nanocapsules. In the nanospheres the drugs may be adsorbed at the sphere surface and/or incorporated within the particle matrix (Figure 9.2). Nanocapsules are vesicular systems in which the drug is confined to a cavity consisting of an inner oily core surrounded by a polymeric wall. Thus, active substances are usually dissolved into the inner oily core, but may also be adsorbed at the capsule surface. Polymeric NPs from biodegradable and biocompatible polymers have been studied extensively as carriers in the pharmaceutical and medical fields [26]. NPs should be able to load, target and release the drug. Also, after releasing the drug, NPs should dissolve by erosion or degradation in the biological medium to be eliminated [1,2]. Blending different polymers is an extremely attractive way of obtaining NPs with synergistic properties and yet conserving some of the individual properties from each polymer. The advantages of this strategy include easy fabrication of devices, manipulation of device properties, higher drug Polymer shell Oily core

Polymer matrix Drug

Drug

Nanocapsule

FIGURE 9.2 Schematic representation of nanocapsules and nanospheres.

Nanosphere

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175

loading and utilization of the dispersed phase domains as microreservoirs for specific controlled release properties [27,28]. Dendrimers are a particular class of polymeric NPs composed of repeatedly branched polymeric macromolecules with numerous arms extending from a center, resulting in a three-dimensional geometric pattern. They have three main components: an initiator core, branches, and terminal functional groups. The key feature of dendrimers is to provide domains with high surface-area to which therapeutic agents and targeting molecules could be attached [29,30]. Polymeric micelles, another relevant class of polymeric NP, are obtained by self-assembly of amphiphilic block copolymers in aqueous media. They tend to dissociate and/or aggregate as a result of extensive dilution as well as interactions with the complex substrates present in the blood stream [31].

9.2.2.1 ­Targeted delivery Current chemotherapy normally causes many side effects related to indiscriminate drug distribution towards the diseased and healthy cells. These side effects can also be linked to poor water solubility of the anticancer drugs, which need organic ­solvents or surfactants to improve their solubility for medical applications. Drug loaded nanocarriers present several advantages over conventional chemotherapy such as drug delivery to specific targets in the body; rate control of the drug release from the carrier; drug protection until the target; enhanced drug absorption into tumors and the cancerous cells themselves; interaction of the drug with normal cells is avoided reducing side effects, etc. [32,33]. Size and polydispersity of polymeric NPs are relevant characteristics because they are related to kinetic stability, cellular uptake, biodistribution, and drug release kinetics. In general, the particle size of the polymeric NPs with an average diameter less than 100 nm could target different organs as they can pass the blood capillary easily. Jeevitha et al. [34] described the synthesis and characterization of a nanoblend consisting of anthraquinone (AQ) loaded to chitosan (CS) and poly(lactide) (PLA) NPs. The effectiveness of the NPs was evaluated in human carcinoma (HepG2) cells. According to this study, this nanoparticulate system seems to be a promising vehicle for the administration of chemopreventing agents and promotes cytotoxicity in HepG2. The accumulation of drug in an organ or specific tissue is called drug targeting. The targeting of drugs to the action site in the body can be achieved using two different approaches: “passive” or “active” targeting. • Passive targeting Selective accumulation of nanocarriers in tumor tissues occurs by the enhanced permeability and retention (EPR) effect. This discovery was made in the late 1970s by Maeda et al. [35]. The tumor-selective passive accumulation of macromolecules was attributed to the defective tumor vasculature with disorganized endothelium at the tumor site and a poor lymphatic drainage system. Since then, researchers have used this concept for the delivery of various drugs loaded to NPs or conjugated with polymers. Passive targeting of NPs via EPR effect increases the concentration of drug-loaded nanocarriers at the tumor region by about 10-50-fold the concentration observed in normal tissue within 1-2 days [36]. This phenomenon is due to the increased spacing among the endothelial cells due to an inflammatory process, as well as secretion of proteoglycans and collagen by cells, increasing the viscosity of the interstitial liquid (Figure 9.3). On the other hand, an accumulation in the liver, lymph nodes and spleen may also

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Healthy tissue

Normal blood vessel

Endothelium

Tumor

Leaky blood vessel

FIGURE 9.3 Passive targeting of nanoparticles via EPR effect.

occur due to the nanocarrier uptake by phagocytic cells of the monocytic system via opsonization. A strategy to avoid opsonization increasing the circulation time of the nanocarrier in the blood is the coating of the nanocarrier with hydrophilic macromolecules. The hydrophilic surface prevents binding of opsonins [37] repelling plasma proteins and reducing clearance by macrophages. As a consequence, the uptake by the reticuloendothelial system (RES) and non-target tissues is avoided [20,38]. PEGylated nanocapsules prepared using poly(lactide-co-glycolide-PEG) and containing a liquid core of perfluorooctyl bromide showed the ability to detect tumors 7 h after administration in mice. This appeared to be a promising system to prolonging the circulation time in the bloodstream through the EPR effect [39]. Chaudhari et al. [37] prepared PEGylated poly(nbutylcyano acrylate) (PBCA) NPs containing docetaxel for treating metastatic breast cancer. According to the authors, the PEGylated PBCA formulation presented high entrapment, the desired particle size and seemed to prevent RES uptake prolonging the circulation half-life. The advances in synthetic polymers as drug delivery platforms focusing on PEGylation are very well discussed by Joralemon et al. [40]. • Active targeting One of the greatest challenges of cancer treatment is the transport of drug-loaded NPs selectively to the cancerous tissue. The strategy of the active targeting, in contrast to passive targeting, is the ability of the NP to bind to the surface of a specific cell type via a ligand-receptor mediated mechanism. The receptor is expressed at the surface of the target cell. The ligands from the NP can be vitamins, lectins, carbohydrates, antibodies, antibody fragments, etc. After the specific interaction between the NP and the cell, the drug can be released or the cell can uptake the NP via receptor-mediated endocytosis [20]. Danhier et al. [41] compared the anticancer efficacy of a novel anticancer cyclin dependent kinase inhibitor called NJ-7706621, by passive or active targeting using polymeric micelles and NPs as drug carriers. According to the authors the passive targeting of micelles was more effective than passive targeting of NPs. However, the greatest delay in tumor growth was obtained from NJ-7706621-loaded tripeptide arginine-glycine-aspartic acid-NPs (RGD-NPs) in combination with Paclitaxel (PTX) by active targeting to the tumor endothelium. Yonenaga et al. [42] developed a new liposomal system of RGD-grafted distearoylphosphatidyl ethanolamine-polyethylene glycolpolycation liposomes (RGD-PEG-PCL) for systemic and actively targeted delivery of siRNA (small

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interfering RNA). The system was able to prolong the circulation without degradation by plasma nucleases. Both strategies, active and passive targeting, can be combined to reduce the interaction of the nanocarrier with the healthy cells achieving the tumor shrinkage with lower drug doses [43].

9.3 ­NANOSTRUCTURES FOR ANTICANCER THERAPEUTICS: FUTURE TENDENCIES Despite numerous efforts of researchers in the area of drug delivery for anticancer therapy, the translation of laboratory into the clinic has been slow and is still a challenge. Most of these difficulties are related to the inherent characteristics of the polymeric materials, such as the heterogeneity of chain length (broad molar mass distribution) resulting from conventional methods of polymerization. Advances in synthetic methodologies that have occurred over the last two decades are now playing a fundamental role in obtaining polymeric architectures with fine control of the molecular weight, low molecular mass distribution, and chain functionality. These synthetic strategies include living radical polymerizations (CRPs) such as RAFT (reversible addition fragmentation chain transfer) polymerization, ATRP (atom transfer radical polymerization), and NMP (nitroxide-mediated radical polymerization). The macromolecules obtained by living radical polymerization may provide a wide range of functional groups with different reactivity and polarity, because initiators or chain transfer agents can be chosen depending on the application required, making a diversity of conjugation reactions possible. In addition, narrow molecular weight distributions can facilitate the design of various polymeric nanostructures [11]. Through the use of CRP techniques, synthetic strategies for drug bioconjugation can be carried out before polymerization (using a biofunctionalized initiator or chain transfer agent) [44], conjugating the drug to a pre-synthesized polymer [45] or using methods to introduce chain ends with living polymerization initiators, for example, conversion of the terminal alcohol groups of PCL, PEG, etc., to a macroinitiator ATRP [46,47]. In addition, the CRP techniques make the building of supramolecular structures with segments of different polarities possible. As a consequence, amphiphilic copolymers with self-assembly properties can be obtained. They may present a series of different supramolecular structures, such as spherical, worn, and multicompartment micelles, vesicles, etc. Self-assembled nanostructures are formed because of the balance between hydrophilic and hydrophobic polymeric segments in the amphiphilic copolymers. The copolymers can carry a therapeutic agent through van der Waals interactions, hydrogen bonding, electrostatic attraction, etc. Such interactions occur during the self-assembly process in which the therapeutic molecules (proteins or genes) are incorporated in the hydrophobic core (Figure 9.4). Furthermore, due to the characteristic size range of these structures EPR effect is improved. In this case, polymer micelles passively release drugs targeted at the tumor tissue up 100 times more than the observed at normal tissues [48]. During the last 14-year period, a significant increase in the number of published papers about micellar systems has been observed (Figure 9.5). Nanostructures from amphiphilic functional copolymers such as di-, tri-, and multiblock copolymers may form micelles spontaneously in aqueous solution at concentrations above the critical micelle concentration. This phenomenon occurs due to hydrophobic interactions, steric repulsion, and solvation. The balance between hydrophobic and hydrophilic segments and a narrow size distribution of the polymers are important parameters to ensure higher efficiency of the nanocarrier [49].

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Hydrophilic blocks Water

Drug Hydrophobic block

FIGURE 9.4 Triblock copolymer with drug loaded to the hydrophobic core. 350

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FIGURE 9.5 Number of publications produced per year since 2000 about polymer micelles for drug delivery as ascertained from PubMed search using “polymeric micelles for drug delivery” as the keywords.

Nowadays, CRPs are one of the best ways to build block copolymers. Besides the advantages of controlled molecular weight of the polymer chains, CRPs offer the possibility of obtaining polymers with reactive functionalities. These functionalities can conjugate bioactive moieties aiming the preparation of active targeting nanocarriers. There are two basic synthetic routes for bioconjugation: (i) using a biofunctionalized initiator [11,49,50] together with amphiphilic block copolymer in aqueous solution; and (ii) bioconjugation at the interface after the formation of the micelles [51]. Micelles for biomedical applications should be nontoxic to various cell lines, and hence biocompatible. Polymeric systems that can respond to small changes in the environment conditions, such as pH, temperature, ionic strength, and light are attracting increasing attention from the academic community. These systems are called “smart” or “stimuli-sensitive” polymers [48,52] and are designed to release drugs at specific sites in response to environmental stimuli such as temperature and/or pH [53]. Smart polymers that are pH-sensitive appear among the most promising systems for drug carriers, especially for cancer treatment. This is related to the variations in the pH values in different tissues and

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179

H+

FIGURE 9.6 Schematic figure of micellar system with encapsulated drug at neutral pH (left) and dissociated micelle at acidic pH (right).

cellular domains. For example, the pH value of tumor sites is more acidic (pH ~5-5.5) than blood and normal tissues. A cleavable bond at specific pH value ranges may cause collapse of the micelle structure enabling the release of a higher amount of drug at tumor sites when compared to normal tissues (Figure 9.6). Gillies et al. [52] developed a pH-sensitive block copolymer incorporating acetal groups on the core of a micelle-forming block copolymer. Doxorubicin (DOX) was used as drug model and loaded to the micelles. At pH 5, this system experienced rapid hydrolysis (half-live = 60 min at 37 °C) becoming hydrophilic. The micelles were disrupted releasing the drug. Lee et al. [53] investigated the interactions and uptake of a pH-sensitive multifunctional polymeric micelle into adenocarcinoma breast cancer cell line (MCF-7). The multifunctional system interacted with biotin on the tumor cell surface under slightly acidic conditions and was internalized by biotin receptor-mediated endocytosis in a short time period. Below pH 6.8, the micelles were destabilized enhancing the DOX release. Poloxamers are a class of commercially available non-ionic amphiphilic copolymers based on Poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG), which can form micelles and are have been used for obtaining drug-delivery systems. Choo et al. [54] modified a micellar system of PEG-bPPG-b-PEG triblock copolymer (Pluronic, P123) with poly(acrylic acid) (PAA) to improve the drug loading efficiency. Firstly, P123 was modified by esterification reaction of hydroxyl end groups of PEG with α-bromoisobutyryl bromide. In the second step the resulting Br-P123-Br macroinitiator was used to start the polymerization of tert-butyl acrylate (tBA) by ATRP. P123-PAA was obtained by acidolysis of tBA groups. The pH-responsive acid function was introduced to improve drug loading efficiency. According to the authors, a burst release of DOX occurred in more acidic medium suggesting that these systems could be potentially applied in cancer therapy. Rzayev et al. [11] synthesized an end-chain carboxyl-trithiocarbonate functionalized polymer by RAFT polymerization of maleic anhydride in order to investigate the physical and chemical interactions of these materials with cancer cells and healthy cells. The presence of carboxyl groups, trithiocarbonate groups, and conjugated double bonds improved the physical and chemical interactions with biomolecules, suggesting that this copolymer may show an effective and selective anticancer activity and it is a strong candidate for use in clinical cancer therapy. Mendez et al. [55] synthesized degradable amphiphilic block copolymers presenting polyesters with disulfides labeled on the main chain at regular intervals (ssPES-OH) by bromination of ssPES-OH to ssPES-Br with subsequent chain extension by ATRP with water-soluble methacrylate. Nile Red (NR) was used as hydrophobic drug model and the results suggested that thiol-responsive degradation of ssABP-micelles enabled fast release of encapsulated NR. Licciardi et al. [56] synthesized a folic acid (FA)-functionalized ATRP initiator to obtain diblock copolymers for gene-delivery applications and ­encapsulation of hydrophobic drugs.

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pH-responsive micellar vehicles were based on 2-(methacryloyloxy)-ethyl phosphorylcholine and 2-(diisopropylamino) ethyl methacrylate (FA-MPC-DPA) copolymers. The results of release behavior and cell viability suggested their potential for cell targeting. Among the thermo-responsive polymers, Poly(N-isopropylacrylamide) (PNIPAAM) has been extensively investigated due to its “smart” nature and lower critical solution temperature (LCST) in water just below human body temperature (around 32 °C). NIPAAM can be copolymerized together with hydrophilic and hydrophobic monomers. An ideal balance between them can adjust the LSCT leading to phase change close to physiological temperature ranges [57]. Nabid et al. [58] reported the synthesis of folate decorated and thermo responsive amphiphilic star block copolymers based on 4s[poly(εcaprolactone)-b-2s(poly(N-isopropylacrylamide-co-acrylamide)-b′-methoxy PEG/PEG-folate)] aiming to produce systems for tumor targeting delivery. The systems were produced by sequential ring opening polymerization of ε-caprolactone (CL) followed by bromation of PCL-OH to produce the PCL-Br macroinitiator and subsequent ATRP reaction with N-isopropylacrylamide (NIPAAM) and acrylamide (AAM). After azidation, click chemistry [59] of copolymer N3 with PEG-folate and methylPEG was carried out to produce 4s[PCL-b-2s(P(NIPAAM-co-AAM)-b-MPEG/PEGFA)] (PTXPCIAE-FA). Then, the composition of the blocks was adjusted to tune the LCST according to the temperature of the tumor tissue. The anticancer drug paclitaxel (PTX) was used as a drug model to be incorporated into the micelles. Higher uptake of the PTX-PCIAE-FA was obtained and PTX releasing was enhanced at the simulated tumor tissue condition (T = 40 °C). It is well known that the presence of unreacted monomer residues with acrylamide-based polymers may be responsible for the toxicity of this material hindering the approval for in vivo applications. However, there are other N-substituted poly(acrylamides) that are alternatives to acrylamide-based polymers, which exhibit similar behavior in aqueous solution, such as poly(N-vinylcaprolactam) (PVCL). This polymer has a cyclic amide as repeat unit in which nitrogen is directly attached to the hydrophobic polymer backbone. Due to this, PVCL is not capable of producing small amide derivatives upon hydrolysis. This feature together with its low toxicity and biocompatibility make PVCL an interesting material, particularly for the biomedical field. In a previous study, our group synthesized a new temperature and pH-sensitive poly(2-(dimethylamino) ethylmethacrylate-b-vinylcaprolactam-b-(2-(dimethylamino) ethyl methacrylate)), PDMAEMA-bPVCL-b-PDMAEMA, triblock copolymer by RAFT polymerization. PDMAEMA is dual sensitive (pH and temperature) and its LCST was around 50 °C, while PVCL is a thermo-­responsive polymer and its LCST was around 32 °C. By modifying the structure and adjusting the composition of these polymers it is possible reduce the LSCT and pKa values in order to ensure ­compatibility with physiological ­temperature and pH (see Table 9.1, Triblock 1). The results showed that these polymers can self-­assemble into micelles in different environments [60]. Table 9.1  LCST of Homopolymers and Different Copolymers (1 wt% in Water) (from ref. [60]) Sample

Mn,SEC (g/mol)

PDMAEMA (%) (mol)

LCST (°C)

PDMAEMA PVCL Triblock 1 Triblock 2 Triblock 3

13,700 9000 19,600 19,200 11,200

100 0 70 50 18

52 32 38 42 34

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Table 9.2  Examples of Block Copolymer Micelles and Self-Assembled Polymer Conjugate Nanoparticles for Therapeutics Under Preclinical Evaluation Sub Class

Examples

Composition

Status

Block copolymer micelles Block copolymer micelles Block copolymer micelles Self-assembled polymer conjugate nanoparticles Self-assembled polymer conjugate nanoparticles

SP1049C NK 105 NK-6004 IT-101

Doxorubicin block copolymer micelle Paclitaxel block copolymer micelle Cisplatin block copolymer micelle Polymer conjugated-cyclodextrin nanoparticle-camptothecin Polymer-conjugated cyclodextrin-nanoparticle-siRNA

Phase I/II Phase II Phase II Phase II

CALAA 01

Phase I

Adapted from Ref. [32].

Tsvetanov et al. [61] described the trends in modern polymer science in which one of the most important parameters is the synthesis of the multi-stimuli sensitive polymers, which may have different types of response to pH and temperature changes. In addition, the authors showed the importance of the CRPs in the precise control of the macromolecule structure. Incorporating blocks of different chemical natures in controlled positions of these macromolecules lead to self-assembly in aqueous medium resulting in a diversity of structures with multiple types of interactions. The review by Kang and colleagues summarizes very well the developments in CRPs, in particular in bioactive surfaces and biomaterials prepared by ATRP [8]. Although there is a large number of studies involving polymer-drug conjugates and advanced block copolymer micelles, these systems are not on the market yet. However, there are some nanomedicine products based on self-assembled and block copolymer micelles under preclinical tests (phase I/II) as showed in Table 9.2. Other information about polymer therapeutics in the market and clinical evaluation can be found in the review by Duncan et al. [33].

9.3.1 ­ANTICANCER POLYMER PRODRUG NANOCARRIERS The number of publications based on new therapeutics for cancer treatment has increased significantly over the last few years, particularlly those focused on targeted therapy. Although very promising results and advancements have been reported, only a few formulations were clinically approved, because many drug-delivery systems have strong limitations that may prevent their clinical tests. The recent trends in the design of anticancer polymer prodrug nanocarriers were recently well described by Nicolas [14]. Among these limitations are: the fast release just after administration, which can lead to high toxicity in vivo; the fact that poorly soluble drugs require organic co-solvents to prepare the formulations, which increases the toxicity; and high concentration of nanocarriers to the patient due to poor drug encapsulation. In order to overcome these drawbacks, many efforts have been made to design new drug-carrier systems with greater therapeutic efficacy. Prodrugs are bioreversible derivatives of original compounds that undergo an enzymatic and/or chemical transformation in vivo to release the active form of the drug with pharmacological effect. The prodrug concept is historical and one of the most common examples is aspirin (acetylsalicylic acid), synthesized in 1897 by Felix Hoffman (Bayer, Germany), and introduced into medicine in 1899 [62]. However, polymeric prodrugs for delivery of therapeutic agents for cancer therapy have been studied with more attention over the last three decades. The criteria for a water-soluble polymer system includes: a water-soluble polymer

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Polymeric backbone

Biodegradable spacer

Drug Solubilizing group Targeting moiety

FIGURE 9.7 Model of polymeric prodrug [62].

and biocompatible backbone, a therapeutic agent, a spacer to separate the therapeutic agent from the backbone and a targeting moiety (Figure 9.7) [63,64]. As well as other carries, polymeric prodrugs can be passively or actively targeted. Today it is estimated that about 10% of worldwide marketed drugs can be classified as prodrugs. This strategy is opening new doors in the challenging field of drug discovery and revolutionizing the art of drug development, because drug candidates are often rejected due to poor pharmacokinetic properties and/or high toxicity. The global incidence of cancer has increasing enormously and new strategies are urgently required. Due to factors previously cited, bioreductive prodrugs can be an efficient alternative in order to improve the pharmacokinetic properties and to reduce the toxicity of the active drug. Nanocarriers, such as micelles and liposomes in which the drugs are physically incorporated, tend to dissociate and release the drug just after intravenous administration, while polymeric prodrugs covalently linked to the polymer backbone generally prolong in vivo circulation and reduce adverse effects [14,65]. Cisplatin is one of the most effective CTAs against many forms of cancer, generally used to treat around 50% of all cancers. However, this drug is associated to significant dose-limiting toxicities thus reducing the clinical benefit. New strategies to increase the safety and decrease the side effects of cisplatin therapy have been encouraged. Studies carried out by Dhar et al. [66] demonstrated that targeted delivery of cisplatin as prodrug using NP for delivery is more effective for prostate cancer therapy when compared to that of cisplatin administered in its conventional dosage form, improving its tolerability and efficacy in vivo. Yuan et al. [47] synthesized well-defined triblock copolymers based on PEG-b-poly(tertbutyl acrylate)-b-poly(2-hydroxyethyl methacrylate) (PEG-b-PtBA-b-PHEMA), by ATRP process. The self-assembly behavior of the copolymers was investigated. In addition, the authors studied the drugcopolymer conjugation and the potential application of this material for drug delivery. DOX was linked to cis-aconityl to obtain the prodrug cisaconityl-doxorubicin (CAD), which was conjugated to the hydroxyl groups of PHEMA. This strategy was used to ensure the effective release of the polymer-linked drug to the desired target and thus reducing side effects. The results showed that all copolymers could selfassemble into spherical micelles and PEG-b-PtBA-b-P(HEMA-CAD) prodrugs reached loading content up to 38%. In this way, these systems present great potential as drug-delivery carriers. Polymeric prodrugs improve in vivo applications and reduce significantly drug-associated side effects. Recently, numerous polymeric prodrugs have been approved for different phases of clinical trials [14,67].

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9.4 ­CONCLUSIONS AND FUTURE DIRECTIONS Despite intense effort and investment, the vast majority of them of drug-based treatments fails at the clinical stage. However, the expressive advancement in polymer science and modern polymerization processes, it is estimated that a breakthrough cannot be far away. For the polymer therapeutics, characteristics such as molecular weight and molecular weight distribution, particle size, chemical functionalities, and structural architecture are very important for the efficiency of the system itself. The new polymerization techniques, in particular RAFT and ATRP, offer the possibility of reducing heterogeneity and enhancing the above-mentioned characteristics of polymers for therapeutics. In addition, due to the intrinsic functionalities in the macromolecules a wide variety of post-polymerization modifications become possible. Consequently, these strategies allow for the preparation of well-defined polymers with exceptional versatility and tolerance for diverse functional groups. Over the last decade, many new anticancer therapeutic systems have been developed and only a few have entered in the market. However, many are still undergoing the clinical development phase. Thus, it is expected that the next decade will reveal how these massive efforts will help to improve the routine of patients with cancer. Nevertheless, the toxicity issues of the developed nanomedicines must be investigated to prove their safe and efficacious use.

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