biosignal-sensitive polyion complex micelles for the delivery of biopharmaceuticals
TRANSCRIPT
REVIEW www.rsc.org/softmatter | Soft Matter
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Biosignal-sensitive polyion complex micelles for the delivery ofbiopharmaceuticals
Yan Leeab and Kazunori Kataoka*abc
Received 22nd May 2009, Accepted 2nd July 2009
First published as an Advance Article on the web 10th August 2009
DOI: 10.1039/b909934d
The application of polyion complex (PIC) micelles into therapeutic fields is rapidly increasing due to
simple and efficient encapsulation of biopharmaceuticals and outstanding biocompatibility among
various polymer-based drug delivery carriers. Ionic biopharmaceuticals, such as DNA, RNA, and
proteins can interact with ionic block copolymers to form PIC micelles with a core-shell structure.
In this review, the development of smart PIC micelles that can respond to biosignals and the application
of the biosignal-sensitive PIC micelles to the drug delivery are discussed. The change of ionic strength or
pH-dependent protonation–deprotonation can be useful for the selective dissociation of PIC
micelles because the ionic interaction between the block copolymer and counter-charged compounds is
a main driving force for the formation of PIC micelles. The release of encapsulated biopharmaceuticals
of PIC micelles can be effectively controlled by degradation of the chemical bonds in the block
copolymer responding to the change of pH or reduction potential. Temperature-dependent
hydrophilic–hydrophobic phase transition of block copolymers can also induce the destabilization of
PIC micelles. Progress in smart PIC micelle as efficient, specific, and safe drug delivery system is indeed
supported by the development of biosignal-sensitive block copolymers.
1. Introduction
Drug delivery technology is being used in over half of the drugs
in the world market. The US drug sales related to the drug
delivery technology were over $64.1 billion in 2005, and are
aCenter for Nanobio Integration, The University of Tokyo, 7-3-1 Hongo,Bunkyo-ku, Tokyo, 113-8656, Japan. E-mail: [email protected] for Disease Biology and Integrative Medicine, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, JapancDepartment of Materials Engineering, Graduate School of Engineering,The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656,Japan
Yan Lee, PhD is a Project
Assistant Professor of Graduate
School of Medicine, the
University of Tokyo, Japan. He
received his BS (1999), MS
(2001) and PhD (2005) from
School of Chemistry and
Molecular Engineering, Seoul
National University, Korea. His
current research is focused on
the development of biolpolymers
with pH and reduction potential
sensitivity for drug and gene
delivery.
3810 | Soft Matter, 2009, 5, 3810–3817
projected to reach $153.5 billion by 2011.1 Along with the rapid
growth, much of the research on drug delivery systems is focused
on the development of an ideal drug delivery technology that is
safe, efficient, and has specificity. The final goal of drug delivery
research is to deliver drugs into a target site with high efficiency
and without any side effects, a goal which has been partially
accomplished by applying nanotechnology, material science, and
the development of new biopharmaceuticals.
One of the frontier technologies in the drug delivery fields is
the polyion complex (PIC) micelle, which was developed by our
laboratory and other groups.2 The PIC micelles are formed when
a block copolymer with a neutral hydrophilic block and an ionic
Kazunori Kataoka, PhD, is
a Professor of Graduate School
of Engineering and Graduate
School of Medicine, the
University of Tokyo, Japan. He
received his BS (1974) and PhD
(1979) from the University of
Tokyo. He has been a Fellow
of the American Institute of
Medical and Biological Engi-
neering since 1999, and a vice
president of the Society of
Polymer Science, Japan since
2008. His current major
research interest includes the
development of new polymeric carrier systems, especially block
copolymer micelles, for drug and gene targeting.
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block is mixed with counter-charged compounds. The PIC
micelles have a core-shell structure with a core consisting of the
polyion complexes and a shell consisting of the neutral block
(Fig. 1). The main driving force for the formation of the
PIC micelles is the electrostatic attraction between the ionic
block and counter-charged compounds. Various types of bio-
pharmaceuticals, such as plasmid DNA,3 oligo DNA,4 siRNA,5
and proteins,6 etc. have been reported as the counter-charged
compounds which can form PIC micelles.
Raw biopharmaceuticals can be easily deactivated by an
enzymatic attack or aggregation in the body. Moreover, low-
molecular-weight molecules are easily excreted in the kidney
through glomerular filtration,7 and significant portions of the
injected dose are removed by the reticuloendothelial system
(RES) in the liver, spleen, and lung.8 The core-shell structures of
the PIC micelle help to overcome the deactivation process due to
the high steric repulsion effect of the hydrophilic shell and
effective shielding of the internal cargo from external attack.9
The biopharmaceuticals in PIC micelles can also avoid glomer-
ular filtration due to the high molecular weight of the PIC
micelles, and the biocompatible surface could prevent the RES
recognition. Therefore, the elongated circulation of effective
biopharmaceuticals could be obtained by packaging them in PIC
micelles.10 Long-circulating macromolecular carriers have
a tendency to accumulate in solid tumor without any targeting
due to the ‘‘enhanced permeability and retention (EPR) effect’’.11
The highly permeable characteristics of solid tumor probably
result from the overexpression of the vascular permeability factor
(VPF) and vascular endothelial growth factor (VEGF), and the
secretion of the basic fibroblast growth factor (bFGF) along with
bradykinin, nitric oxide, and prostaglandin.12 Considering that
the vascular cut-off value of the tumor tissue is around 500 nm,
supramolecular carriers of that size such as liposomes or poly-
meric micelles can be attractive candidates for the treatment of
cancer.13
Despite the advantages of supramolecular carriers, the
controlled release of biopharmaceuticals from the carriers is one
of the major obstacles in the development of an ideal drug
delivery system. The capabilities of stable encapsulation of
biopharmaceuticals and prompt release in the target site can
only be obtained by ‘‘smart’’ delivery systems that can respond
Fig. 1 The schematic diagram o
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to the change of bioenvironment—biosignals. Both internal
signals such as pH, temperature, or reduction potential, and
external signals such as light or ultrasound, can be used as the
biosignals.
In this review, after a short introduction to the PIC micelles
for the delivery of biopharmaceuticals, we will discuss the
development of smart PIC micelles that can respond to various
biosignals, the application of the biosignal-sensitive PIC micelles
to the drug delivery, and future perspectives on the role PIC
micelles play in enabling an efficient, specific, and safe drug
delivery system to be developed.
2. The PIC micelle structure and biosignal response
As mentioned earlier, the shell of PIC micelles consists of a neutral
hydrophilic segment of the block copolymer. Because the shell is in
direct contact with the outer biological environment, it must have
biocompatibility. This is probably one reason why poly (ethylene
glycol) (PEG) is the most frequently selected as a neutral hydro-
philic block among various polymers including PEG, poly (acryl
amide) (PAAm),14 poly (glyceryl methacrylate) (PGMA),15 poly
(hydroxyethylacrylate) (PHEA),16 poly (N-(2-hydroxypropyl)-
methacrylamide) (PHPMA),17 poly (isopropylacrylamide) (PNI-
PAAM),18 and poly (isopropyl oxazoline) (PiPrOx)19 (Fig. 2). PEG
has been approved by the US Food and Drug Administration
(FDA) due to its non-toxicity and weak immunogenicity based on
the high degree of hydration and flexibility of the backbone.20
Although some of the neutral polymers on the surface of the PIC
micelles showed a response to biosignals such as thermo-sensitive
PNIPAAM or PiPrOx, the main role of surface polymers is to
protect the internal cargos from external destabilization. More-
over, the surface of PIC micelles can be modified with various
ligands for targeted drug delivery. Several types of ligands were
developed previously,21 but there have been limited examples of
the introduction of these ligands into the PIC micelles. Owing to
the development of heterobifunctional polymers,22 lactose23 and
cyclic RGD peptide (c(RGDfK))24 ligands for specific and efficient
gene delivery were recently introduced.
Both cationic and anionic polymers can be used as the ionic
block in the block copolymer depending on the counterionic
biopharmaceuticals they contain (Fig. 2). Cationic blocks
f the PIC micelle formation.
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consisting of poly (L-lysine) (PLL), poly (aminoalkyl asparta-
mide), and poly (aminoalkyl methacrylate) have been used for
anionic biopharmaceuticals such as DNA, siRNA, heparin,25
anionic dendrimers,26 and even virus,27 whereas anionic blocks
consisting of poly (methacrylate) (PMAA) and poly (aspartate)
(PAsp) have been used for cationic biopharmaceuticals such as
chitosan,28 cationic proteins,29 and cationic dendrimers.30 In
place of synthetic polymers, some ionic biopharmaceuticals such
as short DNA31 or siRNA32 can act as an ionic block themselves.
Because the enthalpy gains by electrostatic interaction and the
entropy gains from the release of small counterions due to their
exchange with counter-polyelectrolytes are the main factors in
the formation of PIC micelles, the change of charge density or
polarity of the ionic block by protonation–deprotonation, bond
degradation, and geometric transition responding to a specific
biosignal is used for the controlled release of biopharmaceuticals
in most of the biosignal-sensitive PIC micelles. In contrast to the
hydrophobic inner core of the simple polymeric micelles, the
ionic inner core of the PIC micelles is easily accessible by
hydrophilic stimulates in the exterior aqueous phase, and it is
therefore feasible to construct a system with high sensitivity.
In addition, some ionic blocks facilitate endosomal escape,
thus improving the intracellular delivery efficiency of the
biopharmaceuticals.33
The linker between the neutral block and the ionic block is also
important to control the stability of the PIC micelles. The bio-
signal-sensitive degradation of the linker removes the hydrophilic
neutral shell from the micellar structure. The exposed ionic core
of the PIC micelles can directly interact with the external envi-
ronment to produce a rapid response to the biosignal and
destabilization.
3. Ionic strength-sensitive PIC micelles
PIC micelles dissociate above a critical ionic strength (Ic) because
the coulomb force between the ionic block and counter-charged
compounds as well as the chemical potential of free ions, two
main factors that determine the stability of the PIC micelle, are
directly influenced by the external ionic strength. In general, the
Ic of the PIC micelles is highly dependent upon the external
salt type, micelle concentration, and mixing ratio between the
oppositely charged ions in the PIC micelles, pH, and tempera-
ture.34 The ionic strength-dependent association–dissociation of
the PIC micelles can be applied to an on-off control of
Fig. 2 Various polymers
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encapsulated enzyme activity. Because the encapsulated enzymes
in the PIC micelles (i.e. lysozyme) could not directly contact with
large-size external substrates (i.e. Micrococcus luteus cell walls),
no enzymatic activity was detected unless the PIC micelles
dissociated. The addition of salts above Ic could induce the
dissociation of the PIC micelles to rapidly increase the enzymatic
activity, while the enzymatic activity decreased immediately after
the dilution of the salts below Ic. Therefore, the enzymatic
activity could be on-off controlled by a simple salt addition–
dilution procedure.35
However, the ionic strength-sensitive PIC micelles have
significant limits for in vivo application. The physiological ionic
strength should be maintained at about 150 mM because varia-
tion or change in the ionic strength can be fatal to the biosystem.
Moreover, several types of the ionic strength-sensitive PIC
micelles dissociate at that physiological ionic strength to release
their internal cargos before arrival at the target site. Additional
stabilization forces, such as hydrophobic interaction36 or even
cross-linking,37 were occasionally introduced to prevent the
premature dissociation of the PIC micelles.
4. pH-sensitive PIC micelles
Among various biosignals for the PIC micelles, pH has been the
most frequently used because significant variations in pH are
often observed in the biosystem even though an extreme case
such as a gastric pH value of 2 is very rare. For example, the
pH of late endosomes during the uptake process of extracellular
materials is lowered to the final value of 4 by the proton pump
in the endosomal membrane.38 It was also reported that the pH
value of tumor tissue is around 6.39 Therefore, PIC micelles
which can release biopharmaceuticals by a pH trigger have
great potential in the field of intracellular drug delivery and
cancer therapy.
The established pH-sensitive PIC micelles can be classified into
two categories according to their response mechanism. The first
category is based on the protonation–deprotonation of the
functional groups in block copolymers. The pKa values of the
functional groups determine the protonation–deprotonation
degree. The negatively-charged carboxylates are protonated into
uncharged ones at a pH below pKa, whereas uncharged amines
are protonated into positively-charged ones at a pH below the
pKa of their conjugate ammonium ions. Because the stability of
PIC micelles is highly dependent upon the charge balance
for the PIC micelles.
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between the counterions, the abrupt ionic change can induce the
destabilization of the PIC micelles, causing them to release
their internal cargos. For example, PIC micelles consisting of
PEG-b-poly(aspartic acid) (PEG-pAsp) and cationic zinc
porphyrin dendrimer were dissociated at pH 6.2 due to the
partial protonation of the aspartic acid groups.40
Endosomal destabilization at the early stage (pH 5–6) of
endosomal acidification is required for the successful delivery of
biophamaceuticals into the cytoplasm or nucleus because inter-
nalized biopharmaceuticals are finally degraded or deactivated in
lysosome. The endosomal destabilization can be induced by the
protonation–deprotonation of the functional groups in the block
copolymers. PIC micelles containing KALA, a fusogenic peptide
responding to pH drop could be a good example.31b The endo-
some buffering hypothesis suggests that the unprotonated
amines in a polymer could act as a proton buffer to destabilize
the endosome during the acidification process,41 and the same
concept was used for the PIC micelles to improve delivery
efficiency. An ABC-type triblock copolymer, PEG-b-poly[(3-
morpholinopropyl)aspartamide]-b-poly(L-lysine) (PEG-PMPA-
PLL), has two ionizable blocks with different pKa values
(Fig. 3A).42 Only the PLL blocks with a high pKa could condense
negatively charged DNA at pH 7.4 to form three-layered
PIC micelles with the intermediate layer of unprotonated
PMPA blocks. However, the PMPA groups are gradually
protonated, which results in endosomal disruption during the
acidification process. The intermediate silamine block in the
Fig. 3 (A) pH-sensitive endosomal disruption by PEG-PMPA-PLL PIC
micelles. (B) Two-step protonation of the PEG-pAsp(DET) block
copolymer.
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PEG-b-poly(silamine)-b-poly[2-(N,N-dimethylamino)ethyl meth-
acrylate] (PEG-PSAO-PAMA) triblock copolymer acted as the
same pH-sensitive endosomal disruption moiety (proton-sponge).23b
The enhancement of DNA delivery efficiency by the pH-
sensitive protonation of ionic blocks could also be observed in
diblock copolymers. The 1,2-diaminoethyl moiety in the
ionic block of the PEG-b-poly{N-[N0-(2-aminoethyl)-2-amino-
ethyl]aspartamide} (PEG-pAsp(DET)) diblock copolymer
showed two-step protonation behavior (Fig. 3B).43 Because the
pKa of the first protonation is 9.1 and that of the second is 6.3,
the protonation degree of the pAsp(DET) block increased about
37% during acidification from pH 7.4 to pH 5.0. The diproto-
nated pAsp(DET) at the early endosomal pH could induce
endosomal disruption efficiently, which was supported by the
observation that the pAsp(DET) had pH-sensitive membrane
destabilization capability. Resultantly, the PIC micelles consist-
ing of PEG-pAsp(DET) showed outstanding DNA delivery
efficiency on various cell lines.33b
The second category of the pH-sensitive PIC micelles uses
pH-sensitive bond degradation in block copolymers (Fig. 4).
Various types of acid-sensitive bonds such as acetals, hydra-
zones, and orthoesters were developed44 and applied into
the PIC micelles-based delivery of biopharmaceuticals.
Although the rate of the bond degradation is slower than that
of protonation–deprotonation, the bond degradation is often
accompanied by abrupt changes of the polymer characteristics.
The linker bond between the neutral and ionic blocks, and
other bonds in the ionic blocks can be the targets of bond
degradation.
The linker degradation strategy is useful when the ionic block
is a biopharmaceutical itself, such as an oligonucleotide or
siRNA (Fig. 4A). A phosphoramidate bond was used as the
linker between PEG and c-myc antisense DNA (ODN) in
a PEG-b-ODN.31b The phosphoramidate was degraded at the
late endosomal pH of 4.7 to release the therapeutic antisense
DNA. b-thioester bonds, which could be degraded at an early
endosomal pH of 5.5, were also used for antisense DNA
delivery.31a A similar block copolymer with a b-thioester linker
between PEG and a siRNA block was used for the siRNA
delivery into tumor cells and spheroids.32 The functional groups
in the ionic block could be cleaved as shown in the PIC micelles
with a charge-conversional block copolymer, PEG-b-poly[(N0-
citraconyl-2-aminoethyl)aspartamide] (PEG-pAsp(EDA-Cit)
(Fig. 4B).45 Because the citraconic amide in the side group of
the ionic block had a negative charge at pH 7.4, PEG-pAs-
p(EDA-Cit) could form a PIC micelle with a cationic protein,
lysozyme. However, the citraconic amide degraded rapidly to
expose a cationic amine at pH 5.5, and the resulting charge-
conversion of the block copolymer from negative to positive
generated a repulsive force to dissociate the PIC micelles and to
release the lysozyme cargos.
The pH can be an important factor for controlling the stability
of PIC micelles and the selective release of biopharmaceuticals.
The sensitivity to a smaller pH change, the faster response to
a pH change, and the more distinct characteristic change would
be the main objectives of the development of the next generation
of pH-sensitive block copolymers to facilitate the application
of their corresponding PIC micelles to a biopharmaceutical
delivery.
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Fig. 4 (A) Degradation of pH-sensitive linkers. (B) Degradation and charge-conversion of the ionic block in PEG-pAsp(DET-Cit).
Fig. 5 Dissociation of reduction potential-sensitive PIC micelles based
on the disulfide crosslinker.
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5. Reduction potential-sensitive PIC micelles
Glutathione, a sulfhydryl buffer in the biosystem, can distinguish
between the extracellular and intracellular environment. The
reduced form of the glutathione has a 500–1000 times higher
concentration than the oxidized form in cytoplasm, which is
clearly different from the ratio in the oxidizing environment of
extracellular matrix.46 It was also reported that the glutathione
concentration near cancer tissues was 7 times higher than normal
tissues.47 Therefore, the reduction potential generated by the
reduced form of the glutathione in the intracellular environment
and cancer tissue could be an effective biosignal for triggering
selective release in such target sites.
Because almost all types of disulfide bonds are stable outside
the plasma membrane but degrade rapidly by a disulfide
exchange reaction with glutathione molecules after internaliza-
tion into cytoplasm,48 a block copolymer with disulfide bonds
has great potential for the preparation of reduction potential-
sensitive PIC micelles. Similarly to the pH-sensitive linker, PEG
and an antisense DNA molecule were conjugated to each other
via a disulfide bond to form the PIC micelles, which released
antisense DNA in the reductive cytosolic condition.49 The
disulfide linker degradation could be applied to the enhancement
of the endosomal escape efficiency. Although the neutral block
on the surface of PIC micelles provides physiological stability for
in vivo application, it reduces the interaction of the PIC micelles
with plasma or endosomal membrane, which is important for
efficient delivery into cells. For example, the PIC micelles con-
sisting of PEG-pAsp(DET) showed less DNA delivery efficiency
than the polymer-DNA complexes of homo pAsp(DET) without
the PEG block. For both physiological stability and high delivery
efficiency, PEG-SS-pAsp(DET) with a disulfide linker between
the PEG and pAsp(DET) block was synthesized.50 The PEG
block in PEG-SS-pAsp(DET) could be detached from the PIC
micelles when the PIC micelles were internalized into cells or
placed near cancer. The exposed pAsp(DET) block can directly
contact the membranes to destabilize them, as described above.
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The DNA delivery efficiency of PEG-SS-pAsp(DET) was
comparable to that of homo pAsp(DET).
Disulfide bonds are especially useful for the improvement of
the physiological stability of the PIC micelles as a reversible
crosslinker of PIC micelles (Fig. 5). Even though some cross-
linkers such as glutaraldehyde37 provided enough stability in
a physiological salt concentration to the PIC micelles, the
crosslinking was irreversible and the crosslinked PIC micelles
could not release the internal cargos, even at the target site.
Therefore, disulfide crosslinking, which can be degradable only
at the cell interior or near cancer, is very attractive for the
selective destabilization of PIC micelles and the controlled
release of internal biopharmaceuticals responding to the reduc-
tion potential. The 3-amine groups in PEG-b-poly(L-lysine)
(PEG-PLL) can be easily modified into thiols by reaction with
2-iminothiolane or N-succinimidyl-3-(2-pyridyldithio) propio-
nate (SPDP). The resulting 4-thiobutanoic amidines or 3-thio-
propionic amides can crosslink each other by disulfide bonds in
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the oxidative condition. The core of the PIC micelles between
thiolated PEG-PLL and anionic polymer,51 DNA,52 or siRNA,53
was stabilized by disulfide bonds. The stability of the disulfide
crosslinking in the oxidative condition enabled even lyophiliza-
tion of the PIC micelles,54 and the reduction potential-sensitive
degradation could help selective release of the internal cargos.
The delivery efficiency of the crosslinked PIC micelles could be
further improved by a ligand conjugation.55
Fig. 6 Temperature-sensitive phase transition and aggregation of the
PIC micelles.
6. Temperature-sensitive PIC micelles
Temperature-sensitive polymer can be a valuable ingredient for
biosignal-sensitive PIC micelles. The temperature-sensitive drug
delivery carrier can be a useful tool for targeting inflamed tissue,
the temperature of which is well over the normal body temper-
ature of 37 �C. The temperature difference can also be generated
by an external stimulation. Considering that the tumor tissue is
more sensitive to high body temperatures ranging from 40 �C to
43 �C (hyperthermia) than normal tissue, a dual anti-cancer
therapy using both local heating of cancer tissue by far infrared
irradiation and the corresponding release of anti-cancer drugs
from temperature-sensitive drug delivery carriers can be very
effective.56
After the development of PNIPAAM which undergoes low
critical solution temperature (LCST) transition,57 many
researchers attempted to apply its thermosensitivity to the field of
drug delivery and controlled release.58 The LCST of the original
PNIPAAM is 32 �C but the LCST is easily controllable by simple
modification of PNIPAAM. Polyethylenimne (PEI), a well-
known polymer for gene delivery, was conjugated with PNIPAM
to produce PEI-g-PNIPAAM with a LCST of 35–40 �C.59 The
increased DNA delivery efficiency of PEI-g-PNIPAAM at
a temperature above the LCST is probably originated from
the phase transition of the PNIPAAM from hydrophilic to
hydrophobic at the LCST. The resulting hydrophobic surface of
the PEI-g-PNIPAAM/DNA complex can interact more strongly
with the plasma membrane and endosomal membrane to
enhance cellular uptake and endosomal escape. A PIC micelle
using a block copolymer based on PNIPAAM was also reported.
An anionic block copolymer, PNIPAAM-b-polyacrylate (PNI-
PAAM-b-PAA) formed PIC micelles with a cationic detergent,
dodecyltrimethylammonium bromide (DTAB) below a critical
temperature (Tc¼ 35 �C), but formed significant aggregation due
to mutual hydrophobic interaction above the Tc.18
Temperature-sensitive polymers other than PNIPAAM were
also used for the development of temperature-sensitive PIC
micelles. PIC micelles consisting of poly(2-isopropyl-2-oxazo-
line)-b-poly(amino acid) (PiPrOx-b-PAA) showed unimodal size
distributions with diameters of 30–40 nm below the cloud-point
temperature (Tcp), but formed aggregation rapidly above the Tcp
due to the LCST transition of the PiPrOx block (Fig. 6).19 The
Tcp of the PIC micelles were around 35 �C, and showed depen-
dency upon ionic strength and concentration. Because uniform
physicochemical characteristics of block copolymers over graft
polymers help in the accurate control of temperature-sensitivity,
we will rapidly accelerate the development of PIC micelles using
various temperature-sensitive block copolymers for the delivery
of biopharmaceuticals.
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7. Concluding remarks
Many researchers in the field of drug delivery are interested in
polymeric micelles or polymersome due to their attractive
points over other delivery carriers such as liposome or nano-
particles.60 The PIC micelle, a special type of polymeric
micelles, is formed between block copolymers and important
water-soluble ionic biopharmaceuticals, i.e. DNA, RNA, and
proteins. Although the research on PIC micelles is still new
compared to that of other polymeric micelles, the application of
PIC micelles into therapeutic fields is rapidly increasing due to
their simple and efficient encapsulation of biopharmaceuticals
and high biocompatibility. In addition, biosignal-sensitive
characteristics have been introduced for the more variable and
selective applicability of PIC micelles. Although we focused on
PIC micelles responding to only ionic strength, pH, reduction
potential and temperature in this review, other biosignals such
as light or ultrasound,61 which have been applied successfully to
other polymeric micelles, could also be good triggers of the
biosignal-sensitive PIC micelles.
As we mentioned above, one of the biggest obstacles to the
application of PIC micelles in in vivo drug delivery is their
physiological instability. Because the formation of PIC micelles
is mainly driven by the electrostatic interaction, their thermo-
dynamic stability decreases significantly in the presence of a high
concentration of salt and charged serum proteins in the body.
The required physiological stability is expected to be accom-
plished by hydrophobic modification and crosslinking of the PIC
micelles. The introduction of thermosensitive polymers into the
block copolymer can be a viable method for providing suitable
stability at the body temperature. Biosignal-sensitive cross-
linking, such as disulfide crosslinking, also has great potential.
A reversible modification method to increase the charge density
of proteins was recently developed for the physiological stability
of the PIC micelles.62 In this charge-conversional method, the
modified protein can be reversed to the original one responding
to endosomal pH.
Efficiency, specificity, and safety are three of the most essential
factors for drug delivery, and are interconnected. Researchers are
developing highly efficient delivery carriers for bio-
pharmaceuticals, and adding specificity into the carriers by
introducing biosignal-sensitivity and target-specific ligands.
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Biocompatible or biodegradable materials are selected for the
safety of the delivery strategies. Improving biosignal-sensitive
PIC micelle so that it can become an ideal carrier for the delivery
of biopharmaceuticals will be supported by the development of
multi-functional block copolymers with efficiency, specificity,
and safety.
Acknowledgements
This work was supported by a Core Research for Evolutional
Science and Technology (CREST) grant from the Japan Science
and Technology Agency (JST).
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