ABSTRACT
3. Smart Polymeric Materials for CDR 1 Polymeric Capsules
3.1.1 Preparation and Properties
Polymeric capsules (PCs) are materials with core-shell structures. The fi rst report about capsules appeared in 1835, which became an important material with applications in pharmaceutical, cosmetic, food, adhesive and agriculture industries. The fi rst investigation of PCs for CDR was not realized until 1970s.
Due to its easy preparation, controllable particle size from nanometer to micrometer, and facile functionalization, PCs have received considerable attention in the field of CDR. PCs are prepared by several methods, including pan coating, spray drying, centrifugal extrusion, and emulsion- based methods (Esser-Kahn et al. 2011). Each method has advantages on the control of shell wall thickness and permeability, chemical composition and mechanical integrity of the shell wall, and capsule size. Specifi cally, emulsion methods have drawn lots of attention, which could be divided into emulsification polymerization, layer-by-layer (LbL) assembly of polyelectrolytes, coacervation, and internal phase separation (Fig. 8) (Esser-Kahn et al. 2011). The major benefi t of LbL method is their versatility, which can be utilized to fabricate PCs with various templates, giving sizes varying from a few nanometers to hundreds of micrometers, and their
Figure 8. Schemes of emulsion strategies for preparation of polymeric capsules (Esser-Kahn et al. 2011). Copyright 2011 American Chemical Society.
chemical and mechanical properties can be precisely tailored by modulating the thickness and constitution of the shell.
The controlled release of cargo from PCs is of considerable interest in applications such as self-healing materials, nutrient preservation, fragrance release, and drug delivery. By modifying with smart molecules on the surface of capsules, or preparation of capsules with responsive molecule- conjugated polymers, the capsule vectors could encapsulate and deliver drugs and genes to target sites. Smart PCs for CDR have been developed increasingly in the past decades, including triggers as chemicals, pH, temperature, light, and magnetic fi eld.
3.1.2 Recent Advances of Stimuli-Responsive PCs
(1) pH-Responsive PCs
As mentioned above, there exists the difference of pH between normal cells, blood circumstance cancer cells/infl ammatory tissues and lysosome/
endosome, which promotes pH-responsive materials one of the most interesting issues. PCs with pH-responsive property can be fabricated by assembly of polymers modifi ed with pH-sensitive molecules or linkers, which would induce the change of permeability of capsule shells or rapture of the shells, resulting in the release of cargo in target sites.
In 2007, Na et al. reported a pH-responsive smart PCs vehicle, which could release the loaded cargo in acidic organelle quickly. They fabricated the capsules using poly(D, L-lactic-co-glycolic acid) (PLGA) which has been widely used as a carrier material for CDR owing to its excellent biocompatibility and biodegradability (Na et al. 2007). Anticancer drugs and sodium bicarbonante (NaHCO3) were loaded into the capsules. NaHCO3 played an important role in the controlled release process. When the capsule vehicles were transported into an endocytic organelle of a live cell, the protons infi ltrating from the compartment reacted with NaHCO3 to generate CO2 gas, which caused the shell to rupture due to the increased internal pressure. In addition, they trapped fl uorescent DiO in capsule shells, which could be used to trace the release of drug inside cells (Fig. 9) (Ke et al. 2011).
(2) Chemical-Responsive PCs
Chemical signals include Ca2+, Ba2+, sugar, and dithiothreitol (DTT) molecules. In cancer cells, there are rich antioxidants (e.g., dihydrolipoic acid or GSH), which is usually utilized to design redox-responsive systems.
Disulfi de bond is the unique linkage most investigated in this fi eld. Yan et al.
demonstrated a smart PCs vector for delivery of DOX to colon cancer cells.
The submicrometer PCs with size from 600 nm (pH 4.0) to 850 nm (pH 8.0) were fabricated by alternate assembly of poly(methacrylic acid) (PMAA)
and poly(vinyylpyrrolidone) (PVPON) on the SiO2. After the cross-linking of the third group and removal of SiO2, the capsules were obtained. Studies showed the PCs could deliver anticancer to the colon cells and cause great cytotoxicity to the cancer cells (Yan et al. 2010).
(3) Enzyme-Responsive PCs
The selectively responsive property of PCs is critical for biomedical application. Enzymes represent the typical biomolecules that are rich in living organisms and react selectively with substrate. De Geest et al. reported an enzyme-responsive polyelectrolyte PCs vehicle. The PCs were composed of poly-L-argine (PARG) and as the polycation and dextran sulfate (DEXS) as the polyanion, which were fabricated by alternate deposition of PARG and DEXS on CaCO3, followed by the removal of cores. The capsules would deliver fl uorescein isothiocyanate (FITC)-conjugated dextran to African green monkey kidney cells (VERO-1 cells) with the release driven by enzyme-catalyzed degradation (De Geest et al. 2006). Johnston et al.
designed a novel PCs vehicle assembled by DNA molecules. The DNA molecules were engineered to contain a specifi c site in the sequences, which could be responsive to the restriction enzyme, resulting in the release of encapsulated proteins. This work was interesting because it provided a new strategy to design smart PCs responsive to different stimulus by programming the DNA sequences (Johnston et al. 2009).
Figure 9. Scheme illustrating the capsules loaded anticancer drug and NaHCO3, which could release drugs in the acidic condition (Ke et al. 2011).
(4) Light-Responsive PCs
Triggered release of contents based on PCs using light is appealing for a broad application. For applications in cosmetics and agriculture, capsules with UV- and visible-sensitive properties are applied because of the abundance of UV and visible light. In the fi elds of biomedicine, NIR- absorbing capsules are of greater interests because of the deep penetration of NIR in tissues without causing harm.
In 2012, Li et al. reported a light-triggered PCs vehicle prepared by LbL assembly of polysaccharide chitosan (CHI) as the polycation and 5-(4-aminophenyl)-10,15,20-triphenyl-porphyrin (APP) conjugated PASP (PASP-g-APP) as the polyanion. Porphyrins are a typical class of tetrapyrroles, which can yield reactive oxygen species (ROS) upon light exposure and are wide used as photosensitive chemicals. Upon light irradiation (400 nm), APP containing porphyrin moiety produced ROS to break down the polymeric chains and destroy capsule structure, subsequently causing the release of the drug (Li et al. 2012). Tao and Li et al. also reported a light-responsive capsule system composed of azo dyes, and polymers. The resulting hollow capsule displayed a sensitive response to visible light. Investigation found the photochemical reaction of the assembled hollow PCs depends strongly on the matrix. The permeability of the hollow capsule shells can be photo-controlled easily at varying irradiation time (Tao et al. 2004).
3.1.3 Conclusion
As promising candidates for medicine, PCs have been developing for decades. The unique properties, such as degradability, biocompatibility, controlled size and facile functionalization have promoted the rapid development of PCs in this fi eld of CDR. Various kinds of smart release systems have been constructed, and tested in vitro, and the fi rst commercial capsules appeared in 2001. Further efforts have to be devoted to smart capsule vehicles in vivo studies.
3.2 Polymeric Hydrogels 3.2.1 Introduction
A gel is a solid, jelly-like material with the properties ranging from soft to hard. It behaves like solids due to a three-dimensional cross-linked network within the liquid. Hydrogel is a kind of materials defi ned as a three dimensional biopolymeric network formed by chemical crosslinking
by covalent bonds or physical crosslinking by non-covalent bonds, which have the tendency to retain a large amount of water. The fi rst report on
“hydrogel” appealed in 1894, which involved a colloidal gel of inorganic salts. The term hydrogel as it is known today, was fi rst introduced by Wichterle and Lim in 1960. The absorbed property of water is attributed to the specifi c hydrophilic groups in the three international networks, such as –OH, -CONH2, -SO3H, -CONH, and -COOR. The soft and rubbery surface, structure, and physic-chemical properties of hydrogels mimic that of human tissue, which makes them potential candidates in biomedicine.
The advance in polymer science and technology promotes the discovery of smart biodegradable polymeric hydrogels (PHs), whose behavior would change in response to the physical or chemical triggers. The unique property of such smart PHs makes them intelligent candidates for CDR (Vashist and Ahmad 2013). The discovery of micro/nano PHs provided perspicacious means of drug delivery systems, which could be triggered by internal and external stimulus, such as pH, light, temperature, and chemicals.
3.2.2 Recent Advances of Stimuli-Responsive PHs
(1) pH-Responsive PHs
The pH-responsive PHs could encapsulate and release drugs or genes in different pH, which is of signifi cance in the potential application of cancer treatment. The release process is based on the pH-hydrolyzed linker or volume expanding. In 2012, Zeng et al. fabricated a pH-sensitive poly(ethylene glycol)-poly-L-histidine hydrogel, which was used to load adeno-associated virus serotype 2 containing the green fl uorescent protein gene (rAAV2-GFP). Polyhistidine (polyHis, pKa 6.10) was incorporated into the hydrogel system, and played a critical role in the controlled process. Under acidic condition the amine groups in polyHis became protonated, which caused the increase of water uptake by the hydrogel. Investigations in vitro indicated the rAAV2-GFP released from hydrogels in pH 6.0, which was dependent on the ratio of polyHis in Hydrogel (Zeng et al. 2012). Zan et al. recently demonstrated a dual pH- responsive PHs vehicle for CDR, the nano PHs were fabricated by host- guest interaction between adamantly (AD)-benzoic imine-conjugated poly[poly(ethylene glycol)monomethyl ether methacrylate]-co-poly(2- hydroxyethylmethacrylate) (PPEGMA-co-PHEMA) and polymer composed of DOX-hydrazone and β-CD-conjugated poly[N-(2-hydroxypropyl) methacrylamide]-co-poly(3-azidopropyl methacrylate) (PHPMA-co- PAzPMA) (PHPMA-co-PPMA-DOX-CD). The nanogels would reorganize into polymer-DOX nanoparticles with smaller size in acidic condition in tumor cells. Furthermore, the smaller polymer tides could release DOX
quickly in intracellular endolysosomes at pH 5.0 due to the existence of pH-cleavable hydrozone linkage, causing greater cytotoxicity to Hela cells (Fig. 10) (Zan et al. 2014).
(2) Light-Responsive PHs
Light is an attractive manner to control the drug delivery and release with controllable applied time, intensity and wavelength, which would achieve the remote activation of materials with relatively high spatial and temporal precision. Specifi cally, NIR-stimuli is more appealing because of its deep penetration into skins in the range of 700 nm to 1100 nm (Schwarz
Figure 10. Polymeric nanogels formed by host-guest assembly of polymers with dual-pH stage release property (Zan et al. 2014). Copyright 2014 Royal Society of Chemistry.
PPEGMA-co-PHEMA-AD Self-assembly
~ 220 nm
pH 6.5
~ 25 nm Reorganization
PHPMA-co-PPMA-DOX-CD
DOX release
~ pH 5
PPEGMA-co-PHEMA-AD PHPMA-co-PPMA-DOX-CD
O
O O
O O
44 O
O O
NH O
O O
x y n
m
m O
O
O
O HOO
HO N
N N
N
N N
N N
OH
OH OH
N OH H
O NH2 O
et al. 2002). Kang et al. described a NIR-controlled core-shell PHs carrier for targeted drug delivery. The PHs were obtained with Au-Ag nanorods as the cores and DNA cross-linked polymers as the shells, which was further functionalized with PEG to avoid aggregation and site-specifi c ligands to achieve the active tumor targeting. Upon NIR irradiation, the absorbance of Au-Ag cores would increase the temperature surroundings, leading to the melting of polymeric shells and subsequent release of guest molecules.
The in vitro investigation further confi rmed the feasibility of the core-shell PHs vehicle for targeted delivery of DOX to tumor cells with NIR light stimuli (Kang et al. 2011).
(3) Thermo-Responsive PHs
Thermo-responsive PHs can be categorized to negatively responsive PHs and positively responsive PHs depending on their temperature volume phase transition properties. PNIPAM is an outstanding thermo-sensitive polymer for fabrication of negatively temperature responsive hydrogel, which would undergo volume shrinkage above the LCST due to the phase transition from a hydrophilic coil to hydrophobic globule. PNIPAM was the polymer frequently utilized to obtain PHs for CDR. For example, Shin et al. fabricated a thermo-sensitive PHs based on PNIPAM and nanoporous silica. They incorporated PNIPAM into a porous silica host, which loaded indomethacininside. The hybrid hydrogel would exhibit a uniform release when the temperature was kept higher than 40ºC, at which the PNIPAM shrank to squeeze the entrapped molecules to run out (Shin et al. 2001).
(4) Multi-Responsive PHs
Multi-responsive release PHs are capable of controlling the release of drugs under multi-stimulus, which is helpful in delivering drugs and genes in complex environment. The purpose of multi-responsive PHs is to achieve high chemotherapeutic effi cacy and lowest toxicity, which is attractive for recent attention. Zhao et al. fabricated a thermo- and pH-responsive hydrogel by photo-cross-linkers. The hydrogel was composed of poly(L- glutamic acid) and PNIPAM. The investigation of hydrogel in different temperatures, pHs, and ionic strengths showed the hydrogel underwent volume shrinkage under acidic condition or at temperature above their collapse temperature, and would swell in neutral or basic media or at lower temperature. The reversible swelling/deswelling process could achieve the entrapment and release of loaded cargo. The in vitro study on the release of bovine serum albumin (BSA) from hydrogel at pH 6.8 and diverse temperatures indicated that hydrogels presented a slower release rate at temperature above their LCST. Additionally, the release rates of cargo from hydrogels increased sharply when pH was changed from 1.2 to 6.8 (Zhao et al. 2012). Xing et al. described a hollow nano PHs carrier
consisting of a PAA and a PNIPAM network, which was fabricated by colloidal template method and loaded with isoniazid (an antibubercular drug) by controlling the equilibrium temperature. The nano PHs carrier was a dual-responsive system, and could be regulated by temperature and pH. The in vitro evaluation indicated the carrier was effi ciently triggered to release by acidic pH (Xing et al. 2011).
3.2.3 Conclusion
Besides the smart PHs mentioned above, there are many other reports about the intelligent PHs as the candidates for CDR, including magnetic, chemicals, enzymes, and so forth. PHs have been one of the most promising materials for biomedical application, especially the emergency and development of nanoscale smart hydrogels. The further development of hydrogels depends on the advances of novel biodegradable polymers and nanotechnology. In the future, further efforts are needed in the investigation on the nanoscale PHs vectors with biodegradable/biocompatibility involving a long circulation time in the bloodstream, and with recognition of the target site.
3.3 Polymersomes
Polymersomes are a class of artifi cial vehicles composed of bilayered amphiphilic copolymers enclosing aqueous cores. Polymersomes are formed by the assembly of the amphiphilic copolymers with hydrophobic part contacting with each other and hydrophilic part facing water to minimize the energy (Fig. 11). They are in the size range from 50 nm to tens of micrometers. After its fi rst reports in 1995 (Zhang and Eisenberg 1995), polymersomes have received much attention in medicine, pharmacy, and biotechnology due to their unique characteristics. Polymersomes can be prepared by dissolution of the block copolymers in an organic solvent suitable for all blockers, followed by the addition of water, which will cause the assembly of copolymers to hollow polymersome structure. Alternatively, they can also be prepared by polymer rehydration techniques. Polymers are fi rst dissolved in organic solvent. Then, the evaporation of solvent gives the polymer fi lm. By addition of water to the fi lm, polymersomes will be obtained.
Polymersomes exhibit improved mechanical and chemical stability due to the higher molecular weight of copolymers. In addition, polymersomes are attractive from technological points of view. Polymersomes possess an aqueous core and hydrophobic shell, which could incorporate hydrophilic, such as drugs, enzymes, proteins, and DNA in the core and hydrophobic anticancer drugs in the shells, and are wide exploited in
CDR. Furthermore, as the polymersome is formed by self-assembly of amphiphilic copolymers in water, chemical methods are used to modify the copolymers to improve the hydrophilic property, which improve the interaction between polymersome and the circumstance. For instance, PEG is a famous polymer used to modify nanoparticles to improve the time of recycle. PEG containing block copolymers assemble into polymersomes with a highly hydrated and yet neutral polymer brush which has very limited interaction with proteins, which allows the PEG-polymersomes to withstand biological fl uids without interacting with the immune system and reach tumor site via enhanced permeability and retention (EPR) effect.
Most importantly, the biodegradable copolymers conjugated with active moieties and stimuli-sensitive groups will yield smart polymersomes, which could accomplish release of payloads in target sites upon triggers, such as pH, light, temperature and so on (Onaca et al. 2009, Liu et al. 2012).
For example, polymersomes formed by triblock copolymers PEG- b-(2,4,6-trimethoxybenzylidene-1,1,1-tris(hydroxymethyl)ethane methacrylate)-b-PAA (PEG-PTTMA-PAA) could be responsive to acidity stimuli. The polymersomes were reported to load anticancer drug DOX
• HCl, which would be dissociated and release the encapsulated drugs when the acetals in polymersomes were hydrolyzed in acidic condition (Du et al. 2012). The amphiphilic copolymers can also be designed with multi-responsive moieties to achieve polymersomes which are responsive to multi-triggers. For instance, copolymers PEG-SS-poly(2-(diethylamino) ethyl methacrylate) (PEG-SS-PDEA) containing disulfi de linkers and acetal linkers assembled to generate polymersomes, which were capable of loading and release drugs in response to acidic circumstance and reductive condition (Zhang et al. 2012).
Polymersomes for CDR has been developing for over two decades. As a promising candidate material for biomedical application, polymersomes
Figure 11. Structure of polymersomes (LoPresti et al. 2009). Copyright 2009 Royal Society of Chemistry.
Hydrophilic block
Hydrophobic block
Self assembly in water
will be attractive in the future. But at the present stage, the efforts on designing smart polymersomes are not enough. New strategies are believed to promote the development of polymersomes towards clinical tests.
3.4 Polymeric Micelles
According to IUPAC, micelle is the particle of colloidal dimensions that exists in equilibrium with the molecules or ions in solution from which it is formed. When dispersed in aqueous media with the concentration above a critical concentrate (CMC), micelles are formed by the self-assembly of amphiphilic polymers with the hydrophilic head regions in contact with the surrounding water, and the hydrophobic tails touching each other. The amphiphilic molecules could also form inverse micelles when dispersed in oil. The formation of micelles was observed as early as 1913 by McBain at the University of Bristol (McBain 1913). Small molecular amphiphililes, or surfactants, are typically composed of a hydrocarbon chain and a hydrophilic head group, which are widely used as detergents, bioactive denaturing agents, microbiocides, and repellents, but are limited due to their relatively high CMC values and cytotoxicity. Polymeric micelles (PMs) have a much lower CMC, and possess several distinct advantages, such as nanoscale size (20–100 nm), and reduced cytotoxicity of anticancer drug, improved solubility, prolonged time in vivo circulation time and preferential accumulation at tumor site via EPR, which generate great interests in PMs for biomedical applications. Some micellar anticancer drugs have been approved for clinical trials in Japan, UK, and USA (Deng et al. 2012).
Over the past decades, numerous PMs have been developed for hydrophobic drug delivery, such as Pluronics consisting of hydrophobic poly(propylene oxide) and hydrophilic poly(ethylene oxide), poly(esters), and poly(amino acids). Recently, a new sugar-based amphiphilic polymer (SBAPs) comprised of hydrophobic sugar segments and hydrophilic PEG chains was developed by Tian et al. (Tian et al. 2004), which is stable against dilution because of its extremely low CMC. With proper sugar and PEG ratio, SBAPs-based micelles could be internalized by cells, and effi ciently transported to nucleus. Such micelles with particle size from 16–25 nm could evade renal clearance, penetrate tumor tissues, and avoid internalization by the reticuloendothelial system (RES), holding promise as a good vehicle for CDR (Djordjevic et al. 2008). Anticancer drug could be chemically conjugated to the hydrophobic segment of SBAPs via hydrazine linkage to yield drug-loaded SBAPs-micelle. The pH-cleavable property of the hydrazine linkage would lead to pH-triggered release of drugs (del Rosario et al. 2010).
However, there are some shortcomings existing in conventional PMs, which limit the advances of PMs towards in vivo applications. One of them