copolymers: efﬁcient carriers for intelligent nanoparticulate drug targeting and...

Review

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Copolymers: Efficient Carriers for IntelligentNanoparticulate Drug Targeting and GeneTherapy

Mehrdad Hamidi,* Mohammad-Ali Shahbazi,* Kobra Rostamizadeh

Copolymers are among the most promising substances used in the preparation of drug/genedelivery systems. Different categories of copolymers, including block copolymers, graftcopolymers, star copolymers and crosslinked copolymers, are of interest in drug delivery. Avariety of nanostructures, including polymericmicelles, polymersomes and hydrogels, have beenprepared from copolymers and tested successfullyfor their drug delivery potential. The most recentarea of interest in this field is smart nanostruc-tures, which benefit from the stimuli-responsiveproperties of copolymeric moieties to achievenovel targeted drug delivery systems. Differentcopolymer applications in drug/gene deliveryusing nanotechnology-based approaches withparticular emphasis on smart nanoparticles arereviewed.

Introduction

Thedevelopment of novel drugdelivery systems (NDDSs) is a

rapidly evolving field that is underpinned by other classic as

well as modern fields, such as chemistry and biotechnology.

Dr. M. HamidiDepartment of Pharmaceutics, School of Pharmacy, ZanjanUniversity of Medical Sciences, Postal Code 45139-56184, Zanjan,IranE-mail: [email protected]. M.-A. ShahbaziDepartment of Pharmaceutics, Faculty of Pharmacy, ShirazUniversity of Medical Sciences, P.O. Box 71345-1583, Shiraz, IranE-mail: [email protected]. K. RostamizadehDepartment of Medicinal Chemistry, School of Pharmacy, ZanjanUniversity of Medical Sciences, Postal Code 45139-56184, Zanjan,Iran

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Scientists have quoted novel delivery systems as a key factor

contributing to therapeutic achievements, improved drug

efficacy and safety, and enhancements in drug physico-

chemical characteristics as well as pharmacokinetic profiles.

In recent years, copolymers have opened up new and highly

progressing horizons ahead of the limitations of the conven-

tional drug dosage forms to become one of the main

compounds used as drug delivery carriers/vehicles.[1–3]

Copolymers are heteropolymers prepared not only via the

polymerization of a variety of monomer combinations with

different properties, but also by attachment of varying block

lengths of repeating structural units, resulting in structures

with different unique properties.[4] The combination of

different chemical units in copolymer structures results in

novel materials with several novel properties. Accordingly,

the copolymers can be used in the preparation of different

nanostructures with a broad variety of characteristics.

Their synthetic segment makes it possible to change their

library.com DOI: 10.1002/mabi.201100193

Mehrdad Hamidi is Associate Professor of Phar-maceutics and Dean of the School of Pharmacy,Zanjan University of Medical Sciences, Iran. Hereceived his Pharm. D. and Ph. D. from TehranUniversity of Medical Sciences, Iran, and after hisPh. D completed two postdoctoral fellowships atthe University of Alberta and the University ofToronto. Dr. Hamidi’s main current researchinterest is the field of nanoparticulate drugdelivery, with several active research projectson nanogels, solid lipid nanoparticles, nanosus-pensions and erythrocyte-based drug delivery.Dr. Hamidi has published several articles inthe drug delivery field in international journals,with some of them being selected as the hottestarticles of the year.

Mohammad Ali Shahbazi finished his Pharm.D.at the Shiraz University of Medical Sciences, Iran,in 2010. At present, he is working as a researcherin Prof. Hamidi’s laboratory at the Zanjan Uni-versity of Medical Sciences, developing someprojects focused on the development of nano-particulate drug delivery systems using poly-meric biomaterials including micelles, vesiclesand hydrogels.

Kobra Rostamizadeh is Assistant Professor ofMedicinal Chemistry and Associate Dean ofResearch at the School of Pharmacy, Zanjan

Copolymers: Efficient Carriers for Intelligent Nanoparticulate Drug . . .

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properties by controlling themolecular weight and addition

or elimination of chemical functional groups, therefore

forming structures with specific desired functions. Copoly-

mers, basedontheir chemicalarchitecture,have theability to

dissolve, entrap, adsorb, attach or encapsulate therapeutic

agents with the ultimate goal of controlled drug release,

intended for both temporary and/or spatial drug delivery

purposes. In this context, copolymers canberegardedas ideal

solutions for delivery of challenging agents, especially

drugs with poor aqueous solubility and new therapeutic

agents suffering from limited bioavailability or stability in

biological compartments.

The present work is aimed at providing an extensive

systematic review of the drug delivery possibilities of the

copolymers exploited and used by the pharmaceutical

world. Since there are different types of copolymers with a

variety of structures and properties, which have been

completely described in many original and review articles,

the first part will summarize very briefly the most

important copolymers evaluated so far in drug and gene

delivery formulations. Afterwards, a thorough and com-

prehensive summary of the latest developments in several

kinds of copolymer-based nanostructures, such asmicelles,

polymersomes and hydrogels, will be presented with a

particular focus on recent data and findings published in

the literature. Finally, the perspectives and the future

trends in the field will be discussed.
University of Medical Sciences, Zanjan, Iran.She received her B. Sc. (2000), M. Sc. (2003)and Ph.D. (2007) degree in Applied Chemistryfrom Tabriz University, Tabriz, Iran. Her currentresearch interests are the design, synthesis andcharacterization of polymeric drug deliverysystems, including micelles, nanogels andmolecularly imprinted carriers, bioconjugatesand magnetic targeted drug delivery systems.
General Overview of Copolymers

A copolymer, by definition, is a polymer made of different

monomers attached to each other by various types of

chemical bonds. Different kinds of copolymers, both

naturally occurring and synthetic, have been identified,

evaluated and used in order to reach new and novel

physicochemical properties, for desired applications in

Figure 1. The schematic structures of the most common types of copolymers used inbiomedical applications ( A monomer, B monomer, C monomer). a) Blockcopolymer (AB); b) Triblock copolymer (BAB); c) Triblock copolymer (ABA); d) Triblockcopolymer (ABC); e) Graft copolymer; f) Star copolymer; g) Crosslinked copolymer.

different fields. The most common copo-

lymers used in pharmaceutical applica-

tions are block copolymers, graft copoly-

mers, star copolymers and crosslinked

copolymers. For example, block copoly-

mers are comprised of two or more

repeating chemical monomers that are

covalently bonded to each other in

different ways. Figure 1 shows a sche-

matic representation of copolymer archi-

tectures and Table 1 summarizes the

general characteristicsof themostapplic-

able polymers that are used to prepare

efficient copolymers. Following this,

some classes of nanoparticles based on

these chemical structures are discussed.

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Table 1. Overview of the most usable copolymers in drug delivery.

Copolymer type Characteristics Ref.

PEG-based copolymers PEGs play an important role in controlling the swelling process by adjusting the

hydrophilicity of drug delivery carriers, and can provide stealth and minimize

aggregation of particles; another potential advantage provided by hydrophilic

PEG is improvement of the biocompatibility of the delivery vehicle

[5,6]

acrylic-based

copolymers

because of pendant carboxyl groups in their structures, acrylic base copolymers

can aggregate and form supramolecular assemblies in a specific pH range

to increase the oral bioavailability of highly lipophilic compounds; this

aggregation behavior occurs at pH< 4.7, while at higher pH they dissociate

partially or completely as a result of ionization in carboxylic groups

[7,8]

poly(amino acid)-based

copolymers

although poly(amino acid)-based copolymers originate from natural proteins

and are thus biocompatible in the body, their immunogenicity and weak

mechanical properties are reasons to not consider them as potential

biomaterials; promisingly, scientists have solved this deficiency by grafting

amino acids as side chains onto a backbone of a synthetic polymer

[9,10]

polyester-based

copolymers

biodegradable, biocompatible hydrophobic copolymers that hydrolyze

non-enzymatically in the body; the degradation of these polymers mainly leads

to the production of carbon dioxide and water, which can be excreted via

the kidney; high mechanical strength decelerates their degradation rate,

subsequently leading to very slow drug release

[11,12]

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M. Hamidi, M.-A. Shahbazi, K. Rostamizadeh

Copolymer-Based Drug Delivery Systems

Micellar Drug Carriers

Micelles made up of amphiphilic block copolymers are a

group of drug delivery complexes of nanoscale size

(5–100nm) with structures characterized by a core/shell

architecture. The innercore is comprisedof thehydrophobic

blocks, creating a cargo space for the solubilization

(a central requirement for the administration of poorly

water soluble drugs), storage, controlled release and

protection of unstable lipophilic drugs from chemical

degradation (metabolism) via biological agents. The outer

Figure 2. Micelle formation. Drugs are incorporated into the hydrophobic inner core.

shell is a palisade or corona composed of

thehydrophilicblocksof theamphiphiles

that provides a protective interface

between the core and the external

environment through steric stabilization

effects, as well as minimizing unfavor-

able interactions between the surround-

ing water molecules and the drug or

hydrophobic groups of the amphi-

philes[13,14] (Figure 2).

In fact, micelles are made by the self-

assembly of amphiphiles above a thresh-

old level of concentration in solution,

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named the critical micelle concentration (CMC). The excess

amphiphile concentration above the CMC causes thermo-

dynamic stabilization ofmicelles, while dilution below the

CMC leads to dissociation and the production of single

copolymer chains referred to as unimers. Therefore, a

significant challenge associated with in vivo delivery

systemsbased onmicelles is their thermodynamic stability

following intravenous (IV) administration. The rate of

dissociation is closely dependent on the structure of the

amphiphiles, interactions between the chains and the glass

transition temperature (Tg) and melting temperature

of the copolymers (Tm).[4,15] An optimized set of the

aforementioned properties may cause kinetic stabilization

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of micelles even below the CMC for extended periods of

time.[16]

The solubilization of lipophilic drugs by copolymers is

madepossiblebyhydrophobicand/or covalent interactions

between (a part of) the polymer structure and the drug

molecule. Beyond solubilizing hydrophobic drugs, block

copolymer micelles are able to direct their encapsulant to

specific tissues through either passive or active targeting.

In addition, they exert somemerits, such as relatively high

stability owing to their low CMC and prolonged circulation

due to their high water solubility.[17,18] These structures

also have some critical advantages compared to the

solubilizing agents currently in use [such as polyethoxyl-

ated castor oil (Cremophors1 EL)], including lower toxicity,

less opsonization and decreased clearance by the reticu-

loendothelial system (RES), which result in prolonged

circulation times.[19,20] These advantages are attributed

to the hydrophilic shell. Despite all the advantages related

to the use of micelles for drug solubilization, it is worth

mentioning that the aggregates (micelles) disassociate at

low concentrations (upon dilution in a biological environ-

ment) and are no longer able to maintain hydrophobic

drugs within the core. To overcome this shortcoming,

physically and chemically stabilized micelles have been

developed. Actually, by increasing the hydrophobic block

length, the dissociation rate of block copolymers is

controllable because the critical micelle temperature

(CMT)will be decreased. In addition, crosslinking (chemical

or physical) and/or changing the terminal hydroxyl

groups of the micellar shell are other strategies for the

improvement of micellar stability, reaching a tunable shell

permeability.[2,21]

There are a wide variety of methods for the formation of

block copolymermicelles depending on the solubility of the

copolymer being used. These methods include direct

dissolution[22] and the film casting method[4] for relatively

water soluble copolymers and dialysis[23] or oil-in-water

emulsion[24] for copolymers that are not readily soluble in

water.Theshapeofmicelles isaffectedbytheconcentration

of the copolymer, the length of the hydrophilic or

hydrophobic segments and temperature.[25] When the

hydrophilic block is longer than the hydrophobic block,

the shape of micelles is spherical, while increasing

the length of the hydrophobic block beyond that of the

hydrophilic chains may generate various non-spherical

structures, including rod-like structures and lamellae.[26]

The pharmacokinetic and biodistribution of block

copolymer micelles are influenced by a variety of factors

including the chemical nature of the hydrophobic and

hydrophilic blocks, CMC, size, polydispersity, morphology,

surface charge and stability.[27,28] These physicochemical

properties of micelles are critical to the design of new

formulations with enhanced therapeutic efficacy. In

addition, various properties of the corona-forming block,




suchas the charge, thickness, presenceof functional groups,

density and surface bound targeting moieties may affect

the extent of opsonization and clearance. For example,

the presence of poly(ethylene glycol) (PEG) as a non-ionic

hydrophilic polymer prolongs the circulation time of

micelles via avoiding opsonization and subsequent clear-

ance by the mononuclear phagocytic system (MPS).[29]

Micelles for Delivery of Therapeutic Agents

In recent years, core/shell structures self-assembled from

amphiphilic copolymers have attracted growing attention

asdrug carriers for controlled release.Unlike liposomes that

are successfully used for the transport of water soluble or

hydrophilic drugs encapsulated in the inner water volume,

micellesaregenerallydesigned for carryingwater insoluble

or hydrophobic drugs.[30] In micelles, the miscibility,

compatibility and degree of interaction between the drug

and the core-forming polymer block are important factors

for achieving efficient drug loading, stability and drug

release. In addition, factors such as carrier size, polymer

composition and surface characteristics are the main

factors affecting long circulation properties.[31] There are

different kinds of block copolymers capable of beingused in

drugdelivery. For example, Pluronics [poly(ethyleneoxide)-

block-poly(propylene oxide)-block-poly(ethylene oxide),

PEO-b-PPO-b-PEO), also known as poloxamers or synpero-

nics, are ABA block copolymers which have been widely

studied for micellar drug delivery.[32] Ethylene oxide (EO)

andpropyleneoxide (PO)are responsible for thehydrophilic

and hydrophobic properties of Pluronics, respectively. The

hydrophilicity and hydrophobicity of Pluronics are adjus-

table depending on their composition (the number of EO

and PO units) and molecular weight, thus making them

suitable for the solubilization of different drugs. Pluronics

are able to dissolve in water as unimers or self-assemble

as micelles by increasing the hydrophobicity of the PO

block.[16,33] TheCMCofpluronics ishighlydependenton the

temperature and the ionic strength of themediumused.[34]

Pluronics, due to their solubilization properties, the

presence of a very broad range of compositions with

different EO/PO ratios, commercial availability, high

biocompatibility, low side effects in vivo and their ability

to form micellar nanocarriers, have gained popularity and

are widely used for drug delivery purposes.[21] Besides

Pluronics, there are many different kinds of block

copolymers that have been studied and used for drug

delivery purposes. For example, Cheng Chen and cow-

orkers[35] designed, synthesized and characterized amphi-

philic triblock copolymers of poly(3-hydroxybutyrate)-

block-poly(ethylene glycol)-block-poly(3-hydroxybutyrate)

(PHB-PEG-PHB) using the ring-opening copolymerization of

b-butyrolactone for the delivery of hydrophobic drugs.

These triblock copolymers result in biodegradable nano-

particles with core/shell structures having much smaller

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CMCsandbetter drug loadingpropertieswhencompared to

PEO-PHB-PEO copolymers.[36] In their work, nanoparticles

with different PHB block lengths exhibited sizes in the

range 30–116nm. The drug release profile, monitored by

fluorescence, clarified that the release of pyrene as an

imitative drug showed second order exponential decay

behavior.

In recent years, due to forward movement and growing

progress with formulations based on block copolymer

micelles, someformulationsareindifferentstagesofclinical

evaluation,withmany of them inpre-clinical development.

Currently, Pluronic micelles containing propofol, a poorly

water-soluble anesthetic drug, have been developed as an

injectable emulsion. This formulation releases the drug

rapidly after intravenous administration, due to the rapid

break-up of Pluronics in blood. This novel injectable

formulation of propofol/Pluronic has successfully com-

pleted clinical trials.[37] In addition, Paclitaxel (TAX)-loaded

PEO-b-PLAmicelles are an example that has progressed into

clinical trials for the treatment of ovarian cancer and breast

cancer.[38] This formulation ismuch less toxicwith a further

tolerated dose than its standard formulation called Taxols,

which contain Cremophor EL.[39] SP1049C, the first anti-

cancer micellar formulation to reach clinical evaluation

contains doxorubicin (DOX) in mixed micelles of Pluronic

L61 and Pluoronic F127.[40] Also, at present, a TAX-loaded

micelle referred to as NK105 with a loading capacity of

23wt% is in clinical trials. These micelles are composed of

PEG as the hydrophilic block andmodified PAsp as the core-

forming segment representing more anti-tumor activity,

higher plasma concentration and a significant reduction in

side effects compared to free TAX.

Despite much progress with viral vectors due to their

ability to deliver high transfection efficiency and sustained

expression of a foreign gene, they show some problems

Table 2. A summary of attempts made to use different copolymers

Polymer Encapsulant

PEG-L2-PCL DOX

mPEG45-b-PCL100-b-PPEEA12 siRNA

MePEG-b-PBTMC ellipticine

PCL67-PEEP36-CDI TAX

(PLL-PEG-PLL) prednisone

[MPEG-b-P(OPD-co-CL)] DOX

(PNIPAAm-b-PSMA-b-PSt) folic acid

DMAEMA- PAA- BMA siRNA

(CSO)-g-Pluronic DOX

PEOz-PDLLA docetaxel



associated with scale-up and safety, leading to increased

efforts by scientists to synthesize effective non-viral gene

delivery vectors. Today, the capacity of polymers to

enhance gene-transfer efficiency in vivo has drawn

growing attention.[41] Hu and coworkers[42] characterized

a micelle-like structure of stearic acid grafted to chitosan

oligosaccharide (CSO-SA). These micelles were synthesized

via a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide-

mediated coupling reaction by self-aggregation in aqueous

solution. The CMC of CSO-SA was about 0.035mg �mL�1

with a size below 100nm. The cationic property of the

micelleswas themajor reason to compact the plasmidDNA

to form micelle/DNA complex nanoparticles, which could

efficiently protect the condensed DNA from enzymatic

degradation by DNase I. These micelles, due to low

cytotoxicity and biodegradability, have been of interest

as an effective DNA condensation carrier for gene delivery

systems. At present, despite many developments in

micellar gene delivery, scientists are confronted with

pivotal challenges, such as low transfection efficiency

and transient expression of the foreign gene. Some

examples of micelles for the delivery of active therapeutic

agents are summarized in Table 2.

Active Drug Targeting Using Copolymeric Micelles

Systemic toxicity and a variety of undesirable side effects

ofmicelles intended forpassive targetingmade itnecessary

todevelopactively targetedmicelles to impart anaffinity to

deliver drugs to specific tissue, especially cancer cells, by

adding surface grafted recognition moieties. Due to the

rapid proliferation of tumor cells, several receptors, such as

folic acid, vitamins and sugars, are over-expressed in

cancerous cells to enhance the uptake of nutrients.

There are different substances and strategies for

the surface modification of nanoparticulate micellar

as micelle-forming agents in drug/gene delivery.

Size CMC Ref.

[nm] [mg � L�1]

20–150 16 [43]

98 2.7 [44]

95 4.09 [45]

70 0.89 [46]

50 95 [47]

70–95 51.2 [48]

100–120 at 40 8C 18.6 at pH¼ 2.1 [3]

120–160 at 25 8C 21.1 at pH¼ 6.9

40 2 [49]

43.6 35 [50]

28.7 1 [51]

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systems to efficiently deliver loaded agents and

accumulate them in target tissue(s). The most important

active targeting moieties used for micellar drug delivery

include antibodies,[52] galactose,[53] epidermal growth

factor,[54] peptides[55] and folate.[56] Some important aims

of functionalization of the hydrophilic shell of micelles

are to adjust the biodistribution, reduce the clearance

and increase cellular internalization, especially to

fight against cancerous cells via receptor-mediated

endocytosis that facilitates the entrance of drugs to the

cancer cells or endothelial cells of the tumor blood

vessels.[57,58] One promising example of active targeting

was based on poly(ethylene glycol)-block-poly[(propyl

methacrylate)-co-(methacrylic acid)] [PEG-b-P(PrMA-co-

MAA)] complexedwithpoly(amidoamine) (PAMAM)/small

interfering RNA (siRNA) to form core/shell type polyion

complexmicelles (PICMs).[59] Themicelleswere conjugated

with an antibody fragment against the transferrin receptor

(anti-CD71) via a maleimide/activated ester bifunctional

linker, resulting in a higher circulation half-life and more

stability compared to micelles with the cleavable disulfide

conjugation of the targeting moiety that are inefficient

due to cleavage in the blood. These micelles exhibited

theability todown-regulate theexpressionof theBcl-2anti-

apoptotic oncoprotein by losing their shell and releasing

the PAMAM/nucleic acid core under the acidic condition of

cancer cells.

In addition to the aforementioned receptors, cancerous

cells express a group of specific integrin receptors,

especially avb5 or avb3, that have enough potential to bind

to the arginine-glycine-aspartic acid (RGD) tripeptide

sequence. Thus, RGD modification can be used as a

method to direct micelles to tumor cells.[60] Also, the use

of aptamers that recognize antigens on cancer cells,

monoclonal antibodies that recognize specific epitopes

on tumor cells, and transferrin as an iron-binding protein

are other methods for the targeting of micellar delivery

systems to tumor sites.[61,62]

A new method recently introduced for targeted drug

delivery is the biotin–avidin system that can be used as a

reagent for the pre-targeting method in chemotherapy.[63]

In this method, for overcoming the probable denaturation

of ligands during synthesis and direct coupling in organic

solvent, the recognizing moieties, such as biotinylated

monoclonal antibodies conjugated with avidin, are first

administered to localize and concentrate in the tumor.[64]

After this, the administered biotinylated therapeutic

carriers will arrive at unhealthy tissue through the strong

affinity of biotin–avidin system (BAS).

Although the number of in vivo studies on the impact of

active targeting based on block copolymer micelles on the

cellularuptakeofdeliverysystems is few,pre-clinicalanimal

modelshaveshownconsequential improvements inefficacy

in comparison to non-targeted drug-loaded micelles.[65]




Decorating the corona of micelles with targeting

ligands through post-modification of nanoparticulate

copolymers with multifunctional spacer molecules or

direct synthesis of targeted block copolymers contributes

to site specific drug delivery, and thereby higher drug

efficacyand fewer sideeffects.Nevertheless, despiteall the

developments made in active targeting micelles, their

effects on the pharmacokinetics, biodistribution and

tissue accumulation of nanosystems is a controversial

issue because some studies have shown an increase in

tumor accumulation, while, in other cases, the degree of

tissue accumulation remained constant before and after

targeted micellar delivery, suggesting the enhanced

permeability and retention (EPR) phenomenon is the

primary factor in tumor localization.[66] However, scien-

tists believe that some factors, such as tumor physiology,

the nature of the targeting part and the density of the

ligand at the surface of the micelles may cause greater

cellular uptake and improved efficacy than their unmo-

dified counterparts.[66] Table 3 shows different polymeric

micelles targeted for the delivery of drugs and other

therapeutic compounds.

Passive Drug Targeting by Micellar Copolymers

Although the use of appropriate surfactants as excipients

has improvedthesolubilityofmanyhydrophobicdrugs, the

therapeutic efficacy of many conventional therapeutic

agents is still limited due to poor aqueous solubility,

intrinsic toxicity and the lack of preferential distribution to

the desired site within the body.[75,76]

In cancer therapy, achievement of an acceptable

therapeutic response requires maximizing the drug expo-

sure to the tumor site rather than healthy tissues. This aim

isachievable throughdesigningdeliveryvehiclescapableof

carrying large quantities of chemotherapeutics with drug

release only occurring once the vehicle has reached the

target tumor site. Passive drug targeting is a usual method

used for tumor therapy via the EPR effect that diminishes

drug extravasation into normal tissues, thereby decreasing

side effects and promoting drug efficacy. The EPR effects

related to prolonged circulation of the micelles, including

the high permeability of tumor blood vessels (leaky

vasculature), poor lymphatic drainage and lack of renal

clearance, enable theaccumulationofnanosizedmicelles at

tumor sites.[77,78] Recently, theeffect ofpassive targetingon

drug accumulation to tumors using micelles has been

evaluated by many scientists. For example, Wu and

coworkers[79] showed that the cell viability of hyper-

branchedpoly(amineester)/poly(e-caprolactone) (HPAE-co-

PCL) doxorubicin-conjugatedmicelles, free doxorubicin and

HPAE-co-PCL nanoparticles is 19.5� 3.8%, 77.1� 1.3% and

19.5� 3.8%, respectively, 4 h after co-incubation with HeLa

cells, revealing that the EPR effect and the unique structure

and properties of the DOX encapsulated HPAE-co-PCL

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Table 3. Active targeting examples based on copolymer-structured micelles.

Polymer Targeting moiety Encapsulant Ref.

PEG-b-PDLLA glucose N/A [67]

Pluronic P105 lactobionic acid silybin [68]

PAA-b-PMA mannose N/A [69]

P(NIPAAm-co-DMAAm-co-AMA)-b-PUA folate DOX [70]

PEG-b-PCL folate TAX [71]

PLGA-b-PEG HAb18 F(ab0)(2) DOX [5]

PEG-b-PEI transferrin Plasmid DNA [72]

Pluronic anti-HIF-1alpha TAX [52]

PAA-PMA HIV-1 Tat protein N/A [73]

PLGA-g-HA hyaluronic acid DOX [74]

PEEP-b-PCL galactose TAX [46]

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copolymer can facilitate the entry of drug loaded nano-

particles.

Generally, drug encapsulation within nanosized bio-

compatible and/or biodegradable amphiphilic block copo-

lymer micelles has shown promising results for reducing

systemic toxicity, increasingdrugsolubility andprolonging

circulation time, thereby enhancing tumor suppressing

effects compared to free drug.[80] Recently,Hennink et al.[81]

have developed biodegradable DOX-loaded micelles com-

posedofpoly(ethyleneglycol)-block-poly[N-(2-hydroxypro-

pyl)methacrylamide lactate] [mPEG-b-p(HPMAm-Lacn)]

diblock copolymers with small size (80nm), acceptable

polydispersity index (0.15), good colloidal stability, pro-

longed circulation times, adequate drug release kinetics,

enough drug accumulation via the EPR effect and better

anti-tumor activity in comparison to the free drug in mice

bearingB16F10melanomacarcinoma.Atpresent,passively

targetedmicelles comprised of poly(ethylene glycol)-block-

poly(acrylic acid) (PEG-b-PAA) loaded with cis-platin are

undergoing clinical trials.[82]

Amongthefactors thatplaysignificant roles inEPReffects,

prolonged circulation of the micelles as the most important

one is related to the size andphysicochemical characteristics

(e.g., surface properties) of the micelles. Also, the extent of

extravasationofmicelles into tumors is affected bya variety

of factors, including the density and heterogeneity of the

vasculature at the tumor site, transport of macromolecules

in the tumor interstitium and interstitial fluid pressure.

Therefore, passive targeting is not only influenced by

the properties of the micelles themselves, but also by the

physiopathological features of the tumor.

Block Copolymer Vesicles (Polymersomes)

In recent years, scientists have studied andutilized the self-

assembly properties of a diverse array of high molecular



weight amphiphilic block copolymers (containing hydro-

philic and hydrophobic blocks) to develop new drug

delivery systems, namely block copolymer vesicles (poly-

mersomes). Theyhavemore desirable anddistinctmechan-

ical and physical properties, including greater stability

and storability due to a thicker and tougher membrane,

versatility, tunablemembraneproperties such as elasticity,

permeability, and mechanical stability (by controlling

the molecular weight of the hydrophobic block of the

copolymer as membrane fluidity generally decreases with

increasing molecular weight) and low permeability in

comparison to liposomes and simple lipid-based vesicles

formed by small molecule surfactants.[83–85] In addition to

the above-mentioned advantages, the hydrophilic shell of

polymersomes reduces the interaction of vesicles with

macrophages, thus conveyingsurface-protectiveproperties

and long circulation to the drug carrier.[86]

The first report on polymeric vesicleswas approximately

fifteen years ago and involved the self-assembly of

polystyrene/poly(acrylic acid) (PS-PAA) block copolymers.[26]

At that time, a low understanding by scientists of the

thermodynamic stability of vesicles led to a hypothesis

about the non-equilibrium structure of prepared nanopar-

ticles; however, this was rejected after a few years.[87]

Polymer vesicle formation is more complex than

originally anticipated. Nowadays, there are different

reliable methods for nanometer scale vesicle preparation,

including the use of triggering factors such as tempera-

ture,[88] pH[89] or theuseof aplasticizing co-solventwhich is

then removed by dialysis or evaporation (solvent switch

method),[90] and also film rehydration followed by sub-

sequent extrusion.[91] Other methods include water/oil/

water (w/o/w) double emulsions,[92] microfluidic

devices[93] and electroformation.[94] Microfluidics allow

the directed self-assembly of monodisperse vesicles from

water/oil/water (w/o/w) double emulsions[95] and electro-

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formation makes the production of giant unilamellar

vesicles by maintaining a driving force and interfacial

concentration gradient possible.[96] Different preparation

methods lead to different encapsulation efficiencies.

Among all the aforementioned methods, scientists have

shown that the rehydration method is the most efficient

one.[97] However, microfluidic methods seem to produce

polymersomes with a narrower size distribution in

comparison to the other techniques.[98]

Micelle or vesicle formation is dependent on the ratio of

hydrophilic tohydrophobic blockvolume fractions,[84]with

micelle formation being favored when the ratio of

hydrophilic blocks to total polymer mass is greater than

45� 5%, while the occurrence of membrane structures is

preferred in the corresponding ratio of 35� 10%, andfinally

invertedstructuresaredominantwhenthe ratio is less than

25� 5%. This phenomenon is completely explained by Qiu

and coworkers[99] who showed micelle and polymersome

formation by using different ratios of PEG in the synthesis

of methoxypoly(ethylene glycol)/ethyl-p-aminobenzoate-

polyphosphazene (PEG/EAB-PPP) graft copolymers. In this

study, graft copolymers with higher fractions of PEG

assembled into micelles with a mean particle size of 93,

while copolymerswith a lower fraction of PEG formed large

vesicles. Nevertheless, there are some exceptions. Although

Li and coworkers[100] expected to develop poly(dimethylsi-

loxane)-block-poly(ethylene glycol) (PDMS-b-PEG)micelles,

vesicles were formed via introducing water into a tetra-

hydrofuran (THF) solution of the copolymers. Actually,

PDMShydrophobicity causesavery lowcritical aggregation

concentration of PDMS-b-PEG in water, therefore, the

aggregation and self assembly of PDMS segments when

water is introduced into a THF solution of PDMS-b-PEG is

attributed to microphase separation between hydrophilic

chains. At first, micelles are formed by adding THF solvent.

Then, by introducing water, small micelles rapidly transfer

into large micelles, such as worm-like and subsequently

disc-like bilayer micelles, which finally wrap up and form

small vesicles. Conversely, Maskos et al reported cross-

linked micelles generated on the basis of heterotelechelic

PDMS-b-PEG copolymers.[101]

Oneof theadvantagesofpolymersomes incomparisonto

micelles is the ability to encapsulate both hydrophilic and

hydrophobic molecules in their aqueous internal part and

thick lamellar membranes, respectively, while micelles are

only capable of encapsulating hydrophobic therapeutics

unless strong linkages like covalent bindings enclose

aqueous soluble drugs.[102] There are several strategies

for drug encapsulation within polymersomes that are

different based on the physicochemical properties, parti-

cularly the hydrophilicity/hydrophobicity of the drug to be

loaded. For hydrophilic drugs, the most common loading

methods include the formation of w/o/w emulsions,[103]

direct encapsulation during polymersome formation,[83]




the use of a pH gradient[104] and the use of ammonia salt

gradients.[105] Encapsulation methods for hydrophobic

drugs within polymersomes, on the other hand, are more

limited than hydrophilic species and mostly involve the

use of w/o/w double emulsions[103] and diffusion.[105]

Polymersomes for Delivery of Active TherapeuticAgents

Polymersomes, due to slow rates of dissociation, low

critical aggregation concentrations and slow chain

exchange dynamics, may be promising for developing

new drug delivery carriers that release their payload for a

long period of time, depending on the characteristics of the

hydrophobic block of the copolymers.[105] The first pivotal

step for successful drug delivery using polymersomes was

efficient encapsulation of therapeutic agents. Today,

efficient loading of different drugs with various degrees

of hydrophilicity in the aqueous lumen of the vesicles or

the hydrophobic cores of the membranes have been

reported. For example, Eisenberg and co-workers[106] have

synthesized poly(ethylene oxide)-block-polycaprolactone-

block-poly(acrylic acid) (PEO-PCL-PAA) triblock copolymers

loaded with fluorescence-labeled bovine serum

albumin (FITC-BSA), a hydrophilic model protein, as the

encapsulant.

Unfortunately, the limited bioavailability of hydropho-

bic drugs, as well as their toxic side effects and non-specific

in vivo biodistribution, may render their therapeutic value

ineffective, but drug targeting by novel drug delivery

systems, such as polymersomes, holds extraordinary

promise for the local delivery of highly toxic therapeutics

with tuned pharmacokinetics to greatly increase thera-

peutic efficacy. Polymersomes can be directed to specific

sites in vivo, when the end group of their hydrophilic

polymer block (hydroxyl end-group) is conjugated with

molecular targeting agents (active targeting).[107] A chal-

lenging problem associated with active targeting of

polymersomes is that ligand conjugation may cause the

alteration of the hydrophilic block to total mass ratio,

thereby changing the structural morphology (e.g., from

vesicles to micelles).

At present, many studies have been completed or are

ongoing for improving targeted drug delivery using

polymersomes into specific areas within the human body.

Lin et al[108] functionalized polymersomes of poly(ethylene

glycol)-block-poly(butadiene) (PEG-PBD) with anti-ICAM-1

antibody using modular biotin/avidin chemistry for

targeting to vascular endothelial cells during inflamma-

tion. Conjugation of anti-human IgG (a-HIgG) or anti-

human serum albumin to carboxyl groups of PEG-PLA or

PEG-PCL polymersomes through covalent attachment

showed promising results for specific targeting to human

IgG or human serum albumin.[83] Recently, as a new

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approach, Opsteen and colleagues[109] have developed

polystyrene-block-poly(acrylic acid) (PS-PAA) polymer-

somes peripherally covered with azide groups that can

be used for further surface modification to increase the

efficiency of drug targeting. Another work involved the

modificationofpoly(2-methyl-2-oxazoline)-poly(dimethyl-

siloxane)-poly(2-methyl-2-oxazoline (PMOXA-PDMS-

PMOXA) polymersomeswith polyguancylic acid in order to

target a macrophage scavenger receptor SRA1.[110] This

scavenger receptor is known as a pattern-recognition

antigen with the ability to up-regulate specifically in

activated tissue macrophages, not in monocytes or

even monocyte precursor cells. These aforementioned in

vitro cell targetingworks have not yet been translated to in

vivo, where intricacies and entanglements may occur due

to mysterious occurrences, such as opsonization by serum

components, as well as competitive or non-competitive

interactions with other cells in the body.

Although polymersomes can be used for targeted drug

delivery by conjugation of biologically active ligands,

particularly antibodies, to exterior brush surfaces (active

targeting), the anatomical, pathological and physiological

abnormalities of tumor tissue, such as the irregular dilated

shape of the vascular system, leaky blood vessels,

fenestrations in disorganized endothelial cells and poor

lymphatic drainage, can improve localized delivery and

maintenance of therapeutics at the tumor site via passive

effects.[111] These phenomena can cause high local con-

centrations of macromolecules at the tumor site without

specific targeting.

At present, there is a controversy about the therapeutic

efficacy of anti-cancer drugs because of poor water

solubility and a wide variety of side effects. Therefore,

researchers have examined new systems to increase water

solubilityanddecrease their toxic sideeffects. Basedonthis,

Mishra and colleagues[112] developed docetaxel-loaded

polymersomes of poly(g-benzyl L-glutamate)-block-poly-

hyaluronan with high in vitro toxicity in MCF-7 and

U87 cells compared to free DOC. These polymersomeswere

successfully accumulated at the tumor site due to both

passive accumulation (EPR effect) and active targeting

(CD44-mediated endocytosis) in Ehrlich Ascites Tumor-

bearing mice.

Another application of polymersomes is its potential for

use in cancer combination therapy due to the synergistic

effect of drugs administered in the polymersomes that

increases therapeutic outcome. For example, Ahmed and

coworkers[105] designed, optimized and characterized

efficient PEG-b-PLA/PEG-b-PBD vesicles encapsulated with

DOX in their hydrophilic segment and TAX in the

hydrophobic segment to passively treat tumor-bearing

mice. The polymer vesicles and drug combination were

spontaneously self-assembled upon mixing together. The

results of thisworkwere as followswhenanti-cancer drugs



were administered in vesicles rather than as free drugs or

empty polymersomes:

1) L

12, 1

H &

ong circulation due to PEGylation;

2) H
igh EPR effect and strongly positive cell death;
3) T
riggered apoptosis in the tumor (a 2-fold and 245-fold
increase in comparison to free drug and empty

polymersomes, respectively);

4) E
nhanced synergistic effect due to combination
therapy;

5) M
ore tolerability.
Previously, there was no efficient carrier system for

carryingtwodrugs toaspecific tumor.Hence, thepromising

results of thisapproachneed furtherwork togeneratea safe

andfullybiodegradable formulation. Figure3 isa schematic

illustration of the delivery of both hydrophilic and

hydrophobic drugs via active targeting of polymersomes.

In addition to drug delivery, gene therapy via character-

ized polymersomes has been an area of interest. One of the

attractive studies for vesicular gene delivery was based on

cetyltrimethylammonium bromide (CTAB) and sodium

octyl sulfate (SOS) as cationic and anionic surfactants,

respectively.[113] Although these vesicles were able to

encapsulate DNA by complexation, they were not taken

into account as a viable carrier owing to the cytotoxic

nature of CTAB. To overcome this shortcoming, Lindman

and colleagues[114] replaced CTAB with a cationic surfac-

tant, arginine-N-lauroylamide dihydrochloride. These

cationic vesicles showed similar DNA intercalation to

CTAB/SOS systems; however, controlled release and in vivo

circulation were difficult to achieve due to complexation

andnegative charges in the surface, creating an impetus for

more work to overcome these difficulties and generate

efficient carriers. Some examples of gene carrier polymer-

somes include poly(butadiene)-block-poly(N-methyl-4-

vinylpyridinium) (PBD-P4VPQI),[115] an acid-labile block

copolymer of poly[(2-dimethylamino)ethyl methacrylate]

(PDMAEMA) and PEG connected through a cyclic ortho

ester[6] and the diblock copolymer of poly(2-methyacry-

loyloxyethylphosphorylcholine)-block-poly(2-diisopropyl-

aminoethyl methacrylate) (PMPC-PDPA).[116]

A significant limitation of the initial polymersomes was

that they were not biodegradable andwere likely to not be

fully biocompatible. Therefore, designing biocompatible

block copolymer vesicles is most important for scientists,

due to their promising potential for the in vivo delivery of

many drugs, especially anti-cancer drugs and even genes.

Actually, for drug delivery applications, biocompatibility of

the hydrophilic block and biodegradability of hydrophobic

blocksaredesired toachieveprolongedcirculation timeand

low toxicity.[117]

Initially, only aliphatic polyesters were known as

biodegradable blocks, but polycarbonate block copolymers

2, 144–164

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Figure 3. Polymersomes for targeted delivery of both hydrophilic and hydrophobic drug.


www.mbs-journal.de

have recently been suggested due to their enzymatic

degradability, biocompatibility and low toxicity.[118] Based

on their promising characteristics, Lecommandoux and

coworkers[119] developed a viable drug delivery system of

DOX (apoorlywater-soluble drug) loaded poly(trimethylene

carbonate)-b-poly(L-glutamic acid) (PTMC-b-PGA) polymer-

somes by a nanoprecipitationmethodwith benefits such as

biodegradability, biocompatibility, high loading efficiency,

high stability and sustained drug release that could be

increased in acidic environments. Compared to polyester

block copolymers such as PEO-b-PCL and poly[a,b-N-(2-

hydroxyethyl)-L-aspartamide]-graft-poly(e-caprolactone)(PHEA-g-PCL), PTMC shows much slower non-enzymatic

hydrolysis and thus less influence on the release profile.

Besides applications of biocompatible polymersomes to

deliver poorly water soluble drugs, Qio et al. [99] prepared

the first system based on amphiphilic graft copolymers

of poly(ethylene glycol)/4-aminobenzoate-polyphospha-

zenes (PEG/EAB-PPPs) to fabricate polymersomes for water

soluble anti-cancer drug delivery. A unique advantage of

polyphosphazene is its active chloride groups, which can

easily be substituted by different functional groups,

representing multi-functionalized polyphosphazene with

tunable physicochemical/biological properties.

Feijen and colleagues[120] prepared a group of polymer-

somes with biocompatible and biodegradable properties

using PEG and polyesters or polycarbonates. They selected

PEG due to its biocompatibility and resistance to both

protein adsorption and cellular adhesion and selected

polyesters or polycarbonates due to their biodegradability.

Lee and coworkers designed biodegradable polymersomes

of poly(2-hydroxyethylaspartamide) grafted with lactic

acid oligomers.[117]

In addition to drug delivery, scientists have developed

non-toxic and biocompatible quasi-tubular synthetic




vesicles based on poly[2-(methacryloyloxy)ethylphosphoryl-

choline] poly[2-(diisopropylamino)ethyl methacrylate]

(PMPC-PDPA) to deliver green fluorescent protein (GFP)-

encoding DNA plasmid to living cells.[116] In these vesicles,

after rapid endocytosis, protonation of the hydrophobic

PDPAchains takesplacedue to the lower localpHwithinthe

endocytic organelle (pH¼ 5–6), leading to vesicle dissocia-

tion and thereby long time release of the encapsulant. One

of theprofitable resultsof thisworkwasassociatedwith the

high transfection efficiencies of the delivery vehicles.

Lately, some degradable polymersomes were functiona-

lized for siRNA and antisense oligonucleotide delivery.[121]

Also, biodegradable polymersomes of PEG-PCL conjugated

with mouse-anti-rat monoclonal antibody, OX26, were

investigated for peptide brain delivery.[122]

Ultimately, it is worthmentioning here that, if the rapid

developments of physical, chemical and biological func-

tions of desired polymersome formulations can be asso-

ciated with industrial and academic collaborations, the

possible applications appear endless and will help scien-

tists to move them from the bench to the bedside.

Hydrogels

In the past decade, hydrogels have gained increasing

attention frommany investigators for applications in drug

delivery owing to their inherent unique characteristics,

suchas theability toabsorba considerable amountofwater

while maintaining the gel structure integrity as well as

their flexible, while stable physical structure, along with a

high degree of hydrophilicity. Hydrogels are three-dimen-

sional structures that can be produced from both natural

and synthetic polymers. Hydrogels may be generated via

the self-assembly of block or graft copolymers. Unlike

micelles, hydrogels are able to encapsulate and deliver

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hydrophilic drugs, proteins, DNA and RNA. Besides the

advantage of encapsulating a wide range of hydrophilic

drugs, proteins and genes, scientists are attempting to use

hydrogels to improve some deficiencies, such as weak

mechanical strength, rapid erosion and fast release from

drugdeliverycarriers.[123–125] Somecopolymersused for the

preparation of hydrogels are those based on poly(vinyl

alcohol) (PVA), poly(2-hydroxyethyl methacrylate)

(pHEMA), polyacrylamides, chemically modified chitosan

and methacrylic acid (MA).

Smart Drug Carriers as Targeted DeliverySystems

One of the attractive advantages of amphiphilic copoly-

mers is related to their potential for decoration with

responsive or reactive groups through appropriate chemi-

cal procedures. Therefore, the most fascinating topic and

effort is the development of intelligent vehicles based

on copolymers that can be exploited for targeted drug or

gene delivery via responding to internal or external

stimuli, such as temperature, pH, oxidation, reduction,

light, ultrasound and magnetic field, either reversibly or

non-reversibly.[56,83,126]

Stimuli-responsive block copolymers are comprised of a

hydrophilic block conjugated to a stimuli-responsive block

capable of undergoing conformational changes upon

stimulation. They have mostly been synthesized by

reversible addition-fragmentation chain transfer (RAFT)

polymerization or atom-transfer radical polymerization

(ATRP) owing to the versatility in monomer selection and

mild reaction conditions offered by these techniques.

The main advantage of a stimuli-responsive system for a

drug targetingstrategy is responding toa stimulusunique to

adiseasepathology, allowing thedrug/genedelivery system

to respond specifically to the pathological ‘‘triggers’’. Smart

vehicles based on amphiphilic copolymers can be regarded

as active targeting systems since incorporated drugs release

from the vehicles in response to external or environmental

changes. The responses of amphiphilic block copolymers are

varied among sol-gel transition, micellization, aggregation,

hydrodynamic volume change, solubilization, cleavage of

block units, etc. At present, some of the formulations have

already passed several phases of clinical tests. In the

following sections, the recent developments in micelles,

polymersomes and hydrogels composed of stimuli-respon-

sive block copolymers are reviewed.

Smart Micelles

In recent years, smart micelles have been developed to

provide responsive drug release behavior. A brief and

comprehensive overview of the different types of pH-



responsive and temperature-responsivemicelles, aswell as

their applications in drug delivery, is discussed in the

following sections.

pH-Responsive Micelles

Generally, polymers containing alkaline or acidic func-

tional groups that respond to pH alterations are called pH-

responsive polymers. Due to the presence of acidic

carboxylic groups or basic amino groups, these structures

are capable of releasing or receiving protons in response

to pH variations in different parts of the host body. Unlike

acidic smart polymers that become ionized at a high pH,

basic polymers become ionized at a low pH. For example,

poly(acrylic acid) (PAA)becomes ionizedanddissolvesmore

at high pH, while poly(N,N-diethylaminoethyl methacry-

late) (PDEAEM) becomes ionized and dissolves or swells

more at low pH. The pH-sensitive copolymersmay respond

to changes in pH by a soluble-insoluble phase transition

or swelling-shrinking changes. The most important

pH-responsive polymers include PAA, poly(methacrylic

acid) (PMAA), poly(ethyleneimine), poly(propyleneimine),

chitosan, poly(L-lysine) and poly(L-histidine) as typical

examples.

Insufficient oxygen supply in tumor tissues causes

hypoxia, the production of lactic acid and also hydrolysis

of ATP, resulting in a decrease in pH (<6.5) compared to

the surrounding normal tissues (pH¼ 7.5). Therefore, pH-

responsive block copolymer micelles have been recom-

mendedasacarrier to release their incorporateddrugsupon

exposure to the tumor site. These systems should have a

narrow pH range for modulating their physical properties

in order to distinguish between the normal and tumor

tissues. In one studybyHsiue et al.,[126] DOXwasphysically

loaded into the pH-sensitivemicelles prepared frompoly(2-

ethyl-2-oxazoline) (PEOz) as the hydrophilic segment and

poly(L-lactide) (PLLA) as thehydrophobic segment. PEOzand

PLLA exhibited pH-sensitive and biodegradable properties,

respectively. The DOX-loaded micelles were prepared

successfully by a dialysis method and exhibited a narrow

size distribution with a mean diameter of around 170nm.

These carriers showed a cumulative release of 65% at

pH¼ 5.0 and 25% at pH¼ 7.4, a good criterion for drug

release in tumor tissue attributed to the pKa value of PEOz

that is near neutral pH. They have also developed a new

multifunctional mixed micelles system constructed from

poly(HEMA-co-histidine)-g-PLA and diblock copolymer

PEG-PLA with diagnosis, targeting and therapeutic possi-

bilities together.[127] PEG-b-PLA as the outer shell was

functionalized with Cy5.5 as a biodistribution diagnosis

moiety and folate for cancer-specific targeting, while

histidine was the pH-responsive moiety of the inner core

enabling biocompatibility, biodegradability, bioabsorb-

ability and intracellular drug delivery. DOX was loaded

in the core of micelles constructed from the hydrophobic

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blocks of copolymer and graft copolymer. The average size

of theDOX loadedmicelleswas around 200nm.At pH¼ 7.4

and 5.5, mixedmicelles exhibited�50 and 12% cumulative

release, respectively. A significant result of this research

was the IC50 of free DOX, micelles-associated doxorubicin

and folate-micelle-associated DOX being 0.92mg �mL�1,

9.49mg �mL�1 and 1.69mg �mL�1, respectively, after 48h

incubation. This finding indicates that the use of folate

micellesdecreases theeffectivedoseof thedrug remarkably

compared to micelles without the targeting moiety,

thereby reducing the overall risk of organism exposure to

unwanted hazards.

Thermo-Responsive Micelles

Thermo-responsive copolymers are capable of exhibiting a

phase transition at a certain temperature, leading to

changes in conformation, solubility and hydrophilic/

hydrophobic balance.[56] Polymers that become collapsed

and insoluble in an aqueous environment upon heating

have a so-called lower critical solution temperature (LCST),

while polymers that turn soluble upon heating are

characterized by an upper critical solution temperature

(UCST). To describe LCST or inverse temperature depen-

dence, at lower temperatures hydrophilic segments of the

copolymerandwatermolecules showdominatedhydrogen

bonding, leading to more dissolution, but, as the tempera-

ture increases, hydrophobic incorporations among hydro-

phobic segments become dominant and stronger in

comparison to hydrogen bonding which becomes weaker,

leading to shrinking of the polymers due to interchain

interactions. The thermosensitivity and LCST of block

copolymer micelles can be adjusted by changing the

concentration of the aqueous solution, the composition

of the copolymersandsoon.[5] For example, LCSTgoesdown

by using more hydrophobic monomers or by increasing

polymer molecular weight, while increasing the hydro-

philic monomers that form hydrogen bonds with thermo-

sensitive monomers raises the LCST point. The most

important thermo-responsiveblockcopolymersaresynthe-

sized from N-alkylacrylamides, especially poly(N-isopro-

pylacrylamide) (PNIPAM) since it possesses a LCST of 32 8C,close to physiological temperature (37 8C). Other polymers

capable of being used as thermo-responsive structures

include poly(N-acryloylpiperidine) (PNAPi), poly(N-

propylacrylamide) (PnPA) and poly(N-acryloylpyrrolidine)

(PAPy). In addition, block copolymers of PEG/poly(L-glycolic

acid) (PLGA)[128] and poly(ethylene oxide)/poly(propylene

oxide) (PEO/PPO)[129] have shown thermo-sensitivity and

capability for protein/peptide drug delivery because no

organic solvent is used during their preparation.[130] Other

examples of studied thermo-responsive polymericmicelles

include PNIPAM, PEG-PCL, poly(ethylene oxide)-block-

poly(propylene oxide)-block-poly(ethylene oxide) triblocks

(PEO-PPO-PEO), chitosan glycerolphosphate, ethyl(hydroxy-




ethyl)cellulose (EHEC) formulated with ionic surfactants

and PEG-PLA-PEG.

Some limitations of temperature-sensitive diblock copo-

lymers that have prevented significant progress with their

application in intracellular drug delivery include lack of

biocompatibility, difficulties in controlling release because

of evident polydispersity that broadens and obscures the

phase transition temperature and poor micellar stability.

To overcome the aforementioned problems, Ging-Ho

Hsiue and coworkers[131] developed a new class of mixed

micelles composed of methoxypoly(ethylene glycol)-block-

poly(N-propylacrylamide-co-vinylimidazole) [mPEG-b-

P(NnPAAm-co-VIm)] and methoxypoly(ethylene glycol)-

block-poly(D,L-lactide) (mPEG-b-PLA) characterized as tem-

perature-sensitive diblock copolymers and biocompatible

CMC diblock copolymers, respectively. The methods of

preparation for [mPEG-b-P(NnPAAm-co-VIm)] copolymers,

mPEG-b-PLA copolymers andmixedmicelles frommPEG-b-

P(NnPAAm-co-VIm) and mPEG-b-PLA were free-radical

polymerization, ring-opening polymerization and hot

shock protocol, respectively. These polymers were in

soluble form during storage (at a temperature below the

LCST) and formed micelles to protect drug molecules upon

intravenous injection. Physicochemical studies of these

mixed micelles revealed that CMC diblock copolymer

improved the micellar stability of temperature-sensitive

diblock copolymers by reducing its mobility and tempera-

ture-sensitive diblock mitigated the disintegration of

mPEG-b-PLA after dilution in the bloodstream, exhibiting

significantly improved stability under various physiologi-

cal conditions. This approach will be practically applied for

the intravenous drug delivery of some potent but toxic

drugs, such as cancer-fighting drugs.

Double-hydrophilic block copolymers (DHBC) consisting

of two different hydrophilic blocks are likely to have

promising applications in drug delivery. If one of the

hydrophilic blocks is thermo-responsive, the combination

will be able to undergo a hydrophilic-to-hydrophobic

transition as temperature increases and, consequently,

the formationofmicelleswillbeevident.[132]Basedonthese

systems, a multifunctional micellar drug carrier using

thermo-sensitive and biotinylated DHBC, biotin-PEG-b-

P(NIPAAm-co-HMAAm)) (LCST 41.5 8C), was designed and

prepared by Zhang et al.[133] to deliver methotrexate, a

poorly water soluble anti-cancer drug, to tumor tissue. The

drug release behavior in distilledwater showed an increase

from66 to 90% in 96hwith a temperature decrease from43

to 37 8C due to the changing hydrophobic property of the

core to hydrophilic, resulting in deformation of themicellar

structure (Figure 4). These nanoparticleswith a narrow size

distribution and diameter of around 273nm [peak 303nm,

polydispersity index (PDI)¼ 0.099] were able to specifically

and efficiently bond to cancer cells using the biotin/

transferrin pre-treatment, suggesting multifunctional

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Figure 4. Schedule illustration of the thermally-induced structure change of micellesself-assembled from biotin-PEG-b-P(NIPAAm-co-HMAAm) in aqueous solution.

156

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micellar drug carriers with great potential in tumor

targeting chemotherapy.

Today, scientists are trying todevelopmicelles that show

biodegradable properties so theyare cleared easily fromthe

body. Ren and colleagues[134] synthesized a series of

thermosensitive and biodegradable graft copolymers

(PNDH-g-PLLA) by the radical copolymerization of N-

isopropylacrylamide, N,N-dimethylacrylamide and N-

(hydroxymethyl)acrylamide followed by the ring-opening

polymerization of L-lactide. The average hydrodynamic

diameters of themicelleswere in the range 70–130nmand

showed a relatively narrow size distribution. The drug

loading and controlled release behavior of these PNDH-g-

PLLAmicelleswere studiedusing rifampicin, a hydrophobic

drug, that was loaded in the hydrophobic inner core of the

micelle by hydrophobic interactions. The encapsulation

efficiency of PNDH-g-PLLAmicelles loaded with rifampicin

was 12.7%. The release rate of the drug from micelles was

accelerated by increasing the temperature from 30 to 44 8C,due to temperature-induced structural changes in the

polymeric micelles (Figure 5). Actually, the PNDH shell

changed from hydrophilic to hydrophobic, leading to

deformation of the micellar structure. These micelles

Figure 5. The cumulative drug release from thermosensitivemicelles of PNDH-g-PLLA. Reproduced from Ren[134] with per-mission. Copyright 2009, Elsevier.


� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhe

showed potential as a drug carrier

for temperature-triggered controlled

release.

Smart Polymersomes

The ability of smart polymersomes as a

novel programmable delivery system for

modulating the release of encapsulated

drugs in a controlled program upon

arrivalat the target site leads toenhanced

bioavailability, improved therapeutic

efficacy and minimized side effects.[135]

Although the modes of responsiveness of smart polymer-

somes are different, including swelling/shrinking, dissolu-

tion/precipitation, hydrophilic/hydrophobic transition,

degradation, collapse or bond cleavage, the majority of

studies are based on changing the hydrophobicity/hydro-

philicity balance and fewer are directed to the collapse of

polymersomes due to bond cleavage triggered by a

stimulus. Nonetheless, developing new methods and

technical challenges for their preparation provide an

impetus for more research in this field. At present, there

are several types of block copolymer vesicles sensitive to

different stimuli factors that are summarized here.

pH-Responsive Polymersomes

Today, the acidic pH of tumors and inflammatory tissues

(pH � 6.8), as well as the endosomal (pH � 5.0–6.5) and

lysosomal (pH�4.5–5.0) compartmentsof the livingcells, is

attractive as apromise for localizeddeliveryof systemically

administered drugs by a pH-sensitive carrier.[136] Polyacids

and polybases are the most common polymers for the

preparation of pH-responsive polymersomes due to their

‘‘titratable’’ groups (pKa from 3–11), such as weak acidic

groups (carboxylic acids and sulfonic acids) andweak basic

groups (primary, secondary or tertiary amine groups).

Usually, pH-responsive polymersomes are formed at a pH

above the pKa of copolymers, where the hydrophobic block

contains essentially uncharged groups. By decreasing

the pH below the pKa, the ionization (protonation) of

both hydrophilic and hydrophobic pieces leads to more

hydrophilicity, electrostatic repulsions and, thereby, desta-

bilization of the formulations.

The common route for the delivery of pH-responsive

nanoscale polymersomes is intravenous injection. There-

fore, carriers must be stable at physiological pH (7.4) and

release incorporated drug when they are exposed to an

acidic pathological area (around pH¼ 5.0–5.5).

One of the problems associated with pH-dependent

hydrolysis for releasing sensitive biological macromole-

cules from pH-responsive polymersomes is the slow rate of

hydrolysis, which occurs over time scales of hours to days,

thus resulting in the destruction of loaded bioactive

im www.MaterialsViews.com


www.mbs-journal.de

molecules exposed to harsh conditions in polymersomes.

Therefore, designing sensitive polymersomes with

sharp transitions seems essential. Since amine-based

copolymers exhibit pH-dependent aqueous solubility (i.e.,

show hydrophobic properties in their neutral state but

behave as weakly cationic polyelectrolytes in protonated

situations), scientists generated poly(2-vinylpyridine)-

block-poly(ethylene oxide) (P2VP-b-PEO) polymersomes

showing disassembled characteristics in acidic solutions

and fast and complete release of encapsulates by convert-

ing the hydrophobic properties of P2VP into awater soluble

polymer after its protonation at pH< 5.0.[137] In a similar

work, Hubbell and coworkers[89] prepared polymersomes

made of PEG-PVP with rapid dissolving when pH dropped

from 7.4 to 5.5.

Polymersome preparation methods can affect their

efficacy. The most common method involves the use of

different organic solvents, such as tetrahydrofuran, N,N-

dimethylformamide or 1,4-dioxane, to prepare a polymer

solution, followed by the addition of a block selective

solvent, especially water. The most significant problem

associated with this method is the difficulty with

removing the whole organic solvent because its

presence may damage incorporated drug or bioactive

molecules such as proteins, oligonucleotides and DNA.

To overcome this defect, scientists have developed stimuli-

responsive block copolymers with the ability to form

polymersomes directly from aqueous solvent, i.e., without

the need for any organic solvent.[138]

As mentioned before, one of the advantages of polymer-

somes compared with micelles is their ability for synchro-

nous encapsulation of both hydrophilic and hydrophobic

drugs. Recently, Zhong and coworkers[89] designed a

pH-sensitive nanosized degradable polymersome based

on a diblock copolymer of PEG and an acid-labile

polycarbonate, poly(2,4,6-trimethoxybenzylidenepenta-

erythritol carbonate) (PTMBPEC), for triggered release of

both hydrophilic and hydrophobic anti-cancer drugs (DOX

and TAX). These polymersomes showed average sizes of

Figure 6. pH-based transition of PMPC-b-PDPA.

100–200nm and were prone to fast

hydrolysis under mildly acidic condi-

tions, resulting in a significant size

increase of the polymersomes to over

1 000nm, though they were stable at

pH¼ 7.4. Polymersomes that were devel-

oped in this study showed the ability to

release TAX and DOX in a controlled and

pH-dependent manner.

An interesting pH-responsive poly-

mersome was prepared by Armes and

coworkers[139] using poly[2-(methacry-

loyloxy)ethylphosphorylcholine] (PMPC)

as the biocompatible block and PDPA

as thepH-responsiveblock (pKa�5.8–6.6,




depending on the ionic strength) forming the vesicle walls.

Upon changing the solution pH from 2 to above 6, vesicles

were formed spontaneously due to neutralization of the

tertiary amine groups on the PDPA chains. They developed

stable vesicles at physiological pHwhile releasing the drug

at pH below 5.5 due to dissociation resulting from

converting hydrophobic segments to hydrophilic via

protonation of the tertiary amine groups on the PDPA

chains (Figure 6). Today, besides the aforementioned

developments, pH-responsive vesicles for rapid and non-

cytotoxic cellular delivery of DNA sequences have been

developed. These vesicles open up possibilities for genetic-

level disease treatment by efficient gene delivery.[135,140]

Thermo-Responsive Polymersomes

Polymersomes with temperature-sensitive properties are

prepared from thermo-sensitive polymers, which show a

phase transition at a certain temperature.[135] The most

important polymers exhibiting this property include

PNIPAAm as the mostly used block due to its relatively

sharp phase transition, poly(methyl vinyl ether) (PMVE),

N,N-diethylacrylamide, poly(N-vinylcaprolactam) (PNVCa)

and poly(N-ethyloxazoline) (PEtOx).[83,98,135] Most of the

thermo-sensitive polymersomes studied so far show an

LCST, with the conformation of polymers being changed

from a coil (hydrophilic) to a globule (hydrophobic) when

raising the temperature above the LCST.

Higher local temperature in solid tumors compared to

normal body temperature besides hyperthermia therapy

have created a good situation for instantaneous tempera-

ture-induced drug release in tumor sites by adjusting the

LCST of the thermo-sensitive polymersomes between body

temperature and the higher temperature of the tumor.

The first elegant example of temperature-induced

polymersomes was reported by McCormick and

colleagues,[141] who prepared poly[N-(3-aminopropyl)-

methacrylamide hydrochloride]-block-(N-isopropylacryla-

mide) (PAMPA-b-PNIPAM) through RAFT polymerization,

where polymer chains exist as unimers in aqueous solution

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and self-assemble into vesicles upon temperature enhance-

ment above the LCST of the PNIPAM chain. Chen et al.[98]

designed and prepared thermo-responsive crosslinked

polymer vesicles from the self-assembly of poly(2-cinna-

monylethyl methacrylate)-block-poly(N-isopropylacryla-

mide) (PCEMA-b-PNIPAAm) through the reaction

between NH2 and COOH in PNIPAAm and PHEMA blocks,

respectively. They showed that this system could tune the

loading and release behavior of the polymer vesicles for 4-

aminopyridine, as a model compound, with varying

temperature under the 278nm UV light on-off conditions.

At present, some PNIPAAm-based thermo-sensitive

polymersomes are designed to deliver different therapeu-

tics.[83] In all these studies, PNIPAAm converts from soluble

(hydrophilic) to insoluble (hydrophobic segments) above

the LCST, thus forming a barrier membrane against

the release of encapsulated drug. Upon cooling to room

temperature, the lamellar hydrophobic bilayer of

PNIPAAm becomes hydrophilic, causing rupture, thus

initiating the release of the drug. These results are

promising for designing new drug delivery systems that

can release drugs via local cooling by, for example, external

ice packs.

Unfortunately, there are limitations against tempera-

ture-sensitive polymersomes as follows:

i) L
ow number of temperature sensitive copolymers
capable of polymersome formation;

ii) N
on-biodegradable and toxic properties of PNIPAAm
(as the most prevalent temperature sensitive polymer)

and its derivatives in the human body.

Owing to the above-mentioned problems, scientists

hope that they will generate polymersomes with upper

critical solution temperature (UCST) properties rather

than LCST properties for designing site-specific

intracellular drug delivery systems based on temperature

sensitivity.

Oxidation-Responsive Polymersomes

Naturally occurring oxidative conditions, which exist

physiologically and pathophysiologically in extracellular

fluids, e.g., inflamed tissues and tumor tissues, can be used

as a stimulus to make active the release of incorporated

molecules of nanocarrier systems at pathogenic sites.

Therefore, oxidation-responsive vesicles have been devel-

oped to undergo phase transitions in response to oxidative

potential, thereby releasing drugs. This form of vesicles

destabilizes on exposure to H2O2, as evidenced by cryo-

TEM.[142] Napoli et al.[143] demonstrated that vesicles

comprised of ABA triblock copolymer of poly(ethylene

glycol)-block-poly(propylene sulfur)-block-poly(ethylene

glycol) (PEG-b-PPS-b-PEG) could be destabilized by

the oxidation of the hydrophobic PPS block. By oxidation



of PPS, firstly it is converted to poly(propylene sulfoxide)

and then to poly(propylene sulfone), leading to more

hydrophilicity, thereby converting the morphology from

vesicles to worm-like micelles, then to spherical micelles

and ultimately to unimolecularmicelles.[142] This phenom-

enon causes biodegradability as a result of oxidation of the

PPS chain into small aqueous solublemolecules that can be

readily cleared.[143]

Reduction-Responsive Polymersomes

Reduction-sensitive polymersomes are designed for pro-

tecting biomolecules, such as peptides, proteins, oligonu-

cleotides and DNA, in the extracellular environment and

releasing the content within the cysteine-rich reducing

environment of the early endosome (intracellular) prior to

exposure to the harsh conditions encountered after

lysosomal fusion.[144] Actually, the presence of a reduc-

tion-sensitive part is essential for the generation of

reduction-responsive vesicles. For example, Cerritelli

et al.[144] have reported polymersomes comprised of PEG

and PPS with a reduction-sensitive disulfide link between

the two blocks (PEG-SS-PPS). This form of polymersome is

able to suddenly burst during endocytosis due to the

reductive environment in the endosome. Despite reductive

environments within the cell that can be exploited to

destabilize carrier systems, the reduction of disulfide bonds

may also occur during systemic circulation due to the

presence of low concentrations of cysteine and glutathione

(the most important reducing agents) which may lead to

premature release of drugs.

Smart Hydrogels

In past years, the stimuli-sensitive copolymer hydrogels,

networks formed by physical interactions, have received

increasing attention in drug delivery due to their sol-gel

phase transition in response to external stimuli (intelli-

gence property), biocompatibility and biodegradability. pH

and temperature are two of the most widely used stimuli

for stimuli-sensitive hydrogels because they have shown

practical advantages both in vitro and in vivo. These

systems will be discussed briefly.

pH- Responsive Hydrogels

In recent years, pH-sensitive hydrogels have attracted

increasing attention and have presented a promising

prospect. At present, chitosan-based hydrogels have been

focused on by many scientists as safe, biodegradable,

biocompatible and non-expensive systems for gene deliv-

ery. Despite all the benefits mentioned, it is worth

mentioning that partial protonation of the amino groups

on chitosan at physiological pH is a limiting factor against

tough interactions between chitosan and the gene of

interest. To overcome this shortcoming, investigators have

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developed some methods, including the use of covalent

cross-linkers like glutaraldehyde, ionic crosslinker like

Na2SO4 or tripolyphosphate,[145] or structural modification

of chitosan like quaternization of NH2 groups to form

trimethylchitosan (TMC).[146] Recently Dehousse and cow-

orkers[7] designed and characterized hydrogels prepared by

adding Eudragit1 S100 to TMC-siRNA to escape from the

endosomal pathway, enhance the cytoplasmic delivery

of siRNA, reinforce interactions between chitosan and

siRNA, and exhibit pH-sensitivity. In this system, upon

decreasing pH, dissociation and swelling occurred due to

charge imbalance and modification of the degree of

interaction between the polymers.

Unlike usualmedicines that can be taken orally and reach

the bloodstream intact, proteins, due to their particular

structural and physiological properties, must often be

injected directly into the blood stream. One of the major

preventiveobstacles fororal administrationofproteins is the

unpleasant conditions of the stomach associated with the

acidic environment and the presence of proteolytic enzymes

that often destroy most of the protein before it passes from

the membrane and reaches the bloodstream.[8] Unfortu-

nately, the parenteral route also leads to low compliance of

patients because of the injection pain; therefore, many

scientists are focused on finding efficacious alternate

methods of delivering proteins.[147] In recent years, utiliza-

tion of different copolymer hydrogels with remarkable

characteristics has indicated promisingly that this aim is

achievable. For example, Peppas et al.[8] have developed

nanospheres of crosslinked networks of methacrylic acid

grafted with PEG, and acrylic acid grafted with PEG, capable

of trapping insulin inside the copolymer carriers at a

surrounding pH near 3.0, while releasing them at pH values

of near-neutral. They used methacrylic acid and acrylic acid

for their pH-sensitive nature andpoly(ethyleneglycol) for its

ability to stabilize and protect proteins. They showed the

capability of these copolymers to improve insulin transport

across the cellular barrier in the upper small intestine.

Thermo-Responsive Hydrogels

Thermo-responsive hydrogels are free-flowing sols at

ambient temperature while, via in vivo injection, the

hydrogels formanon-flowinggel atbodytemperature, thus

sustaining drug release in vivo via drug entrapment in the

gel structure.[148] This behavior can be used for local drug

delivery via the injection of thermo-sensitive hydrogels.

As degradability is an important characteristic for

designing novel drug delivery systems, Corrigan et al.[149]

developed copolymer hydrogels with the dual functional-

ities of thermo-responsiveness and degradability,

composed of PNIPAAm and poly(lactic acid) diacrylate

macromer (PLAM). In this work, different molar ratios of

PLAM could modify the LCST, swelling properties and the

mechanical strength of the hydrogels, leading to versatile




release profiles and the encapsulation of a wider range of

drugs.

Unlike many works that have delivered hydrophilic

drugs, Qian et al.[150] characterized nanoparticles of poly(e-caprolactone)-poly(ethylene glycol)-poly(e-caprolactone)(PCEC) for the delivery of Honokiol, a model hydrophobic

drug. These nanoparticles were prepared by an emulsion

solvent evaporation method, and then incorporated into a

thermo-sensitive F127 hydrousmatrix. In this research, the

average particle size of the prepared blank and Honokiol-

loaded PCEC nanoparticles was 148 and 157nm, respec-

tively. Upon injection, this systemcould formagel andplay

a role as a depot for prolonged and sustained release of

drugs in situ. Thisworkhas seenmadepromising results for

being utilized in cancer therapy systems.

Copolymers and Gene Therapy

Today, many researchers have focused on development of

safe cationic polymeric nanoparticles for the delivery of

nucleic acids to treat both acquired and inherited genetic

disorders like immunodeficiency diseases, Parkinson’s

disease and cystic fibrosis. These efforts are due to more

benefitsof thismethod, including reduced immunogenicity

and toxicity, largeDNAloading capacity, increasingnuclear

deliveryby targeting the cell nucleus, adjustable structures,

the ability to add endosomolytic agents to disrupt the

endosome and ease of scale up, compared to viral vectors.

The most important polymers that are able to entrap DNA

and form stable polymer/DNA nanoparticles via electro-

static interactions are poly(L-lysine) (PLL) and poly(ethyle-

nimine) (PEI); however, there are other efficient polymers

withwell-defined physicochemical properties to overcome

the extracellular and intracellular obstacles of gene

delivery.[151–153] In recent years, designing biodegradable

PEI derivates based on oligo(ethylenimine) (OEI) for

obtaining high efficient gene delivery and minimal

cytotoxicity have attracted increased attention.[154,155]

There are two main methods for designing degradable

polyethylenimine analogueswhich include: 1) crosslinking

of OEI using biodegradable crosslinkers[154] thatmay result

in poor solubility and unusual molecular structures due to

excessive intra- or intermolecular crosslinking; 2) grafting

of OEI to a biodegradable polymer backbone.[156] Xuesi

Chen[155] and coworkers designed and synthesized a

controlled and well-defined molecular structure from a

series of novel biodegradable poly(ethylene glycol)-block-

poly(carbonate-graft-oligoethylenimine) [mPEG-b-P(MCC-

g-OEI)] copolymers as non-viral gene carriers. In this study,

poly(ethylene glycol)-block-polycarbonate (mPEG-b-PMCC)

was considered as the biodegradable backbone. Actually,

aliphatic polycarbonates are biodegradable and biocompa-

tible polymers able to improve cytocompatibility and

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decrease the cytotoxicity of nanoparticles in gene delivery.

Also, PEG improved solubility and reduced cytotoxicity,

aggregation and opsonization by serum proteins in the

blood circulation.[155] The results showed that PPO600 and

PPO1800 would condense DNA into nanosized particles

(100–140nm)withpositive charges in the surfacewhenthe

PPO/DNA mass ratio was above 10:1. The most important

benefits of this research were the lower cytotoxicity and

higher gene transfectionefficiencyof PPO1800 compared to

many other developed gene carriers, resulting in an ideal

vehicle for gene therapy.

Today, polyionic complexes (polyplexes) formed from

plasmid DNA (pDNA) and cationic polymers are known as a

valid carrier because of their low immunogenicity and high

versatility in terms of chemical structure.[157] One of the

methods for enhancing the efficiencyof theseparticles is the

introductionofPEGmolecules tothesurfaceofnanoparticles

to increase particle stability and thus circulation time. The

advantage of PEGylated cationic polymers is producing

water soluble polyionic complexes (polyplexes) with plas-

mid DNA and consequently enhanced stability compared to

non-PEGylatedpolyplexes. Thisbenefit isoutweighedwitha

drawback, namely the considerable decrease in the transfec-

tion efficiency of polyplexes, probably as a result of poor

cellular internalization, difficulty in releasing theDNAcargo

fromthenanoparticulatecomplexesanddiminishedcellular

uptake of the polyplex particles by the target cells due to the

steric hindrance of PEG.[158] To overcome these deficiencies,

Pluronic block copolymers have been suggested by some

scientists for improving the transfection efficiency of

polyplexes, due to their ability to interact with the plasma

membrane and increase cellular uptake of various mole-

cules. For example, Kwon and colleagues[159] evaluated the

transfection efficiency of polyplexes using Pluronic P85-

based and PEG-based cationomers comprised of poly{N-[N-

(2-aminoethyl)-2-aminoethyl]aspartamide {P[Asp(DET)]}

cationic blocks. The results showed that, although the

stabilityofthePEG-basedpolyplexeswasbettercomparedto

P85-basedpolyplexes, theP85-basedpolyplexdemonstrated

significantly higher gene delivering ability due to improved

cellular internalization of the P85-based polyplexes.

In addition to the aforementioned gene carriers, block

copolymer micelle systems have also received interest in

recent years as promising gene delivery vehicles as a result

of prolonged blood circulation. For gene delivery, micelle

copolymerswitha core-shell structurehave thepotential to

load genes into their shell compartments. Some interesting

examples of such copolymers include poly(dimethylami-

noethyl methacrylate)-poly(e-caprolactone)-poly(dimethy-

laminoethyl methacrylate)[160] and poly{(N-methyldiethe-

neamine sebacate)-co-[(cholesteryloxocarbonylamido-

ethyl) methylbis(ethylene)ammonium bromide] seba-

cate}.[161]



Conclusion and Perspective

Over the last few decades, nanoparticulate copolymers

have resulted in great progress in drug and gene delivery.

Although copolymer-based drug delivery systems are a

relatively new area of interest and research, they have

received a vast amount of attention owing to their

characteristics, such as size, structure, stability and surface

chemistry, which suit the intended purposes in the drug/

gene delivery field. Despite all the developments, these

systems are still in the initial stages and are still far from

ideal for clinical use. To reach that goal, biocompatibility,

biodegradability, release mechanisms and formulation

issues, as well as other challenges, need to be evaluated

and optimized. Improvement of drug stability during

storage and delivery, longer circulation times, reducing

the frequency of administration, minimizing potential

side effects, decreasing dosing levels and subsequently

improving therapeutic effectiveness and patient compli-

ance are important aims of scientists using copolymers as a

vehicle.

It is possible to install target-specific moieties on

copolymer-based nanostructures to improve drug delivery

by directing therapeutic agents to the intended site(s)

within the host body. It is also clear that smart copolymers

will possibly provide an alternative method to traditional

polymers to be utilized as the most facile, multi-purpose,

versatile and efficient method in challenging therapeutic

areas like cancer therapy.

There are many interesting challenges for the future of

copolymers and their use in medical applications. Today,

many researchers formulate copolymers and therapeutics

to achieve controlled drug delivery systems, both tempo-

rally and spatially, with tunable release rates and/or

strategies ongoing. Thus, the next step in copolymer

formulations is likely to be their application as

a biomimetic and functionalized materials to control

macromolecular and cellular responses. Finally, it is

worth mentioning that advancing our understanding of

copolymer systems will not only increase our knowledge,

but will lead to incredible new applications not yet

visualized.

Abbreviations

ADR A

12, 12, 144–164

H & Co. KGaA, W

driamycin

AMA 2
-Aminoethyl methacrylate
ATRP A
tom-transfer radical polymerization
BMA B
utyl methacylate
BSA B
ovine serum albumin
CDI N
,N0-Carbonyldiimidazole
CMC C
ritical micelle concentration
einheim www.MaterialsViews.com


www.mbs-journal.de

CMT C

www.MaterialsVie

ritical micelle temperature

CSO C
hitosan oligosaccharide
CTAB C
etyltrimethylammonium bromide
DLLA D
,L-lactide
DMAAm N
,N-dimethylacrylamide
DMAEMA D
imethylaminoethyl methacrylate
DOX D
oxorubicin
EHEC E
thyl(hydroxyethyl)cellulose
EPR E
nhanced permeability and retention
FDA F
ood and Drug Administration
FITC-BSA F
luorescence-labeled bovine serum albumin
HA H
yaluronic acid
HMAAm N
-hydroxymethylacrylamide
HPAE H
yperbranched poly(amine ester)
HPMA N
-(2-Hydroxypropyl)methacrylamide
IC50 H
alf-inhibitory concentrations
LA L
actide
LCST L
ower critical solution temperature
MA M
ethacrylic acid
MPEG M
ethoxypoly(ethylene glycol)
MPS M
ononuclear phagocytic system
OPD 2
-Oxepane-1,5-dione
P2VP P
oly(2-vinylpyridine)
P4VPQI N
-methyl-4-vinylpyridinium
PAA P
oly(acrylic acid)
PAA P
oly(amino acid)
PAA P
ropylacrylic acid
PAMAM P
oly(amidoamine)
PAMPA P
oly[N-(3-aminopropyl)-methacrylamide
hydrochloride]

PAPy P
oly(N-acryloylpyrrolidine)
Pasp P
oly(aspartic acid)
PBD P
oly(butadiene)
PBTMC P
oly(5-benzyloxy-trimethylene carbonate)
PCEC P
oly(e-caprolactone)-block-poly(ethyleneglycol)-block-poly(e-caprolactone)
PCEMA P
oly(2-cinnamonylethyl methacrylate)
PCL P
oly(e -caprolactone) PDEAEM P oly(N,N-diethylaminoethyl methacrylate)
PDLLA P
oly(D,L-lactide)
PDMAEMA P
oly[(2-dimethylamino)ethyl methacrylate]
PDMS P
oly(dimethylsiloxane)
PDPA P
oly[2-(diisopropylamino)ethyl methacry-
late]

PEEP P
oly(ethylethylene phosphate)
PEG P
oly(ethylene glycol)
PEO P
oly(ethylene oxide)
PEOz P
oly(2-ethyl-2-oxazoline)
PEtOx P
oly(N-ethyl oxazoline)
PGA P
oly(glycolic acid)
PHB P
oly(3-hydroxybutyrate)
PHEA P
oly[a,b-N-(2-hydroxyethyl)-L-aspartamide]
PHEMA P
oly(hydroxyethyl methacrylate)
PICMs P
olyion complex micelles
ws.com



PLA P

12, 12, 144–164

H & Co. KGaA, We

oly(lactide)

PLAA P
oly(L-amino acid)
PLAM P
oly-lactic acid diacrylate macromer
PLGA P
oly(lactide-co-glycolic acid)
PLL P
oly(L-leucine)
PLLA P
oly(L-lactide)
PMA P
oly(methyl acrylate)
PMAA P
oly(methacrylic acid)
PMMA P
oly(methyl methacrylate)
PMOXA P
oly(2-methyl oxazoline)
PMPC P
oly[2-(methacryloyloxy)ethylphosphoryl-
choline]

PMVE P
oly(methyl vinyl ether)
PNAPi P
oly(N-acryloylpiperidine)
PNIPAAm P
oly(N-isopropylacrylamide)
PnPA P
oly(N-propylacrylamide)
PNVCa P
oly(N-vinylcaprolactam)
PPEEA P
oly(2-aminoethylethylene phosphate)
PPO P
oly(propylene oxide)
PPS P
oly(propylene sulfur)
PrMA P
oly(propyl methacrylate)
PS P
olystyrene
PSMA P
oly[styrene-alt-(maleic anhydride)]
PTMBPEC P
oly(2,4,6-trimethoxybenzylidenepentaery-
thritol carbonate)

PUA P
oly(10-undecenoic acid)
PVA P
oly(vinyl alcohol)
PVP P
olyvinylpyrolidine
RAFT R
eversible addition-fragmentation chain
transfer

RES R
eticuloendothelial system
RGD A
rginine-glycine-aspartic acid
SCL S
hell-crosslinked
SOS S
odium octyl sulfate
TAX P
aclitaxel
TMC T
rimethylchitosan
UCST U
pper critical solution temperature
online: October 17, 2011; DOI: 10.1002/mabi.201100193
Received: May 25, 2011; Revised: August 1, 2011; Published
Keywords: copolymerization; drug delivery systems; nanoparti-cles; nanotechnology

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