Self-assembled micelles of N -phthaloylchitosan-g-polyvinylpyrrolidone for drug delivery
Fengling Bian *,Lixia Jia,Wei Yu,Mingzhu Liu
College of Chemistry and Chemical Engineering,Lanzhou University,Lanzhou 730000,PR China
a r t i c l e i n f o Article history:
Received 24April 2008
Accepted 10November 2008
Available online 20November 2008Keywords:
N -phthaloylchitosan Polyvinylpyrrolidone Self-assembly
Polymeric micelles Prednisone acetate
Controlled drug release
a b s t r a c t
A novel amphiphilic graft copolymer N -phthaloylchitosan-g-polyvinylpyrrolidone (PHCS-g-PVP)was synthesized by grafting polyvinylpyrrolidone (PVP)onto a chitosan derivative whose amino groups were protected by phthaloyl groups.Polymeric micelles were prepared by the dialysis method,and showed a low critical micelle concentration (CMC)of 0.83mg/L detected by fluorescence spectroscopy.Prednisone acetate was incorporated in the polymeric micelles.The loading capacity was found to be around 44.6wt%.Morphological investigation by transmission electron micrograph (TEM)showed that the micelles were round in shape.The mean particle diameter of the drug-loaded micelles were about 143.3nm,much bigger than the unloaded micelles which had a unimodal size distribution with an aver-age diameter of 89.8nm as measured by dynamic light scattering (DLS).In vitro tests showed release of prednisone acetate from the micelles was continuous with no initial burst.All the results suggest that the nano-size core–shell micelles might be used in controlled drug delivery system.
Ó2008Elsevier Ltd.All rights reserved.
1.Introduction
Delivering water-insoluble drugs,reducing severe systemic tox-icities and increasing the utilization of dr
ugs by improving their pharmacokinetics posed many challenges for drug delivery system (DDS)and drug development (Praneet,Tanasait,Amornrut,Theera-sak,&Auayporn,2007).Recently,several types of drug carrier,such as microspheres,liposomes,nanoparticles (Chang,Joseph,&Gardella,2005)and polymeric carriers,have been investigated as DDS,but non-selective scavenging of these carriers by the reticulo-endothelial system (RES)is a serious problem (Praneet et al.,2007).A polymeric micelle drug carrier system based on amphiphilic graft or block copolymers solves these problems by utilizing some pref-erable characteristics of polymeric micelles where the hydrophobic segments are segregated from the aqueous domain to form an in-ner core surrounded by a highly hydrated outer shells.The hydro-phobic core acts as a reservoir for poorly water-soluble drugs (Sui,Yin,Chen,Zhang,&Kong,2006),which keep a satisfactory aqueous stability irrespective of high contents of hydrophobic drug incorpo-rated into the inner core of the micelle (Fukashi et al.,1998).Fur-thermore,small polymeric micelles (<200nm)can avoid physical clearance by filtration in the lungs and in the spleen or excretion through the kidneys.As its unique core–shell structure and nano-size,it not only protects drugs from inactivation and pre-vents their sudden release in bloodstream in the physiological
environment,but also can reduce the drug toxicity and make them suitable as long-circulating drug carr
iers.Therefore,much interest has been focused on the polymeric micelles as DDS (Jindrich,2003;Prabaharan &Gong,2008;Praneet et al.,2007;Ye et al.,2008).Polymeric micelles used for DDS in intravenous administration must be of no danger to human body,so the segment used in graft or block copolymer should be non-toxic,biodegradable and bio-compatible (Praneet et al.,2007).Chitosan is an abundant,non-toxic and biocompatible natural polymer (Agnihotri &Aminabhavi,2004;Jayakumar,Prabaharan,Reis,&Mano,2005;Trong,Chia,&Wen,2002).So in recent years,the production of chitosan spheres for DDS was developed by some specific processing techniques,such as suspension cross-linking,spray–drying coagulation,emul-sification/solvent evaporation.However,it was insoluble in water and cannot form micelles in water.Therefore,to overcome the problems above,some modified chitosan e.g.N -octyl-O -sulfate chitosan (Zhang,Ping,Zhang,&Shen,2004)and (2-hydroxyl-3-butoxyl)-propylcarboxymethyl-chitosan (Sui et al.,2006),N -succi-nyl-chitosan (Zhu,Chen,&Yuan,2006)have been reported for the preparation of polymeric micelles.These polymeric micelles were based on chitosan derivatives modified by small molecules,but the work related to the modification with synthetic polymers was limiting (Praneet et al.,2007).Praneet,Tanasait,Amornrut,Theerasak,and Auayporn (2006)prepared an amphiphilic N -phthaloylchitosan-g-poly (ethylene glycol)methylether (PHCS-g-mPEG)in homogeneous phase using a key reaction intermediates (Li,Zhuang,Mu,Wang,&Fang,2004),N -phthaloylchitosan.Intro-duction of bulky phthaloyl groups into chitosan destroyed inter-
0144-8617/$-see front matter Ó2008Elsevier Ltd.All rights reserved.doi:10.1016/j.carbpol.2008.11.008
*Corresponding author.Tel.:+869318912387;fax:+869318912582.E-mail address:bianfl@lzu.edu (F.Bian).Carbohydrate Polymers 76(2009)
454–459
Contents lists available at ScienceDirect
Carbohydrate Polymers
j o u r n a l ho m e p a g e :w w w.e l s e v i er.c om/loc
ate/carbpol
and intra-hydrogen bonds of chitosan and resulted in its solublility in common organic solvents.The PHCS-g-mPEG self-assembled to form nano-size core–shell micelles(80–170nm),and the release of camptothecin(CPT)from the micelles was sustained.In drug deliv-ery system,this sustain effect not only can reduce drug toxicity, but also can reduce the side effects,to some extent,it is able to achieve controlled drug release.
Polyvinylpyrrolidone(PVP)has been used in the biomedical field due to its excellent biocompatibility,non-toxicity and physi-cal inertia(Haruhiko et al.,1999;Irene,Roberto,Etienne,&Emo, 2007).Recently,there has been a growing interest in grafting mod-ification of PVP for drug targeting carriers.Park et al,(2003)syn-thesized galactosylated chitosan graft polyvinylpyrrolidone (GCPVP)and then prepared the GCPVP/DNA complex which showed small sizes and narrow size distribution,and the results showed that the release of DNA from the GCPVP/DNA complex was dependent on both the concentration of chondroitin sulfate added and the molecular weight of chitosan.
Based on the unique properties of PHCS and PVP,we synthe-sized amphiphilic graft copolymer(PHCS-g-PVP)by a condensation reaction between the hydroxyl group of PHCS and carboxyl termi-nated group of PVP.Both nano-size blank micelles and prednisone acetate-loaded micelles were prepared by dialysis and in vitro re-lease of prednisone acetate from drug-loaded micelles was also studied.
2.Experimental
2.1.Materials and reagents
Chitosan{CS,degree of deacetylation=96%was determined by linear potentiometric titration(Jia&Li,2001),viscosity average molecular weight=18000,determined in0.1M acetate acid/ 0.2M NaCl aqueous solution at25±0.5°C by means of Ubbelohde Viscometer,according to the Mark–Houwink equation, K=1.81Â10À3,a=0.93(Li et al.,2004;Maghami and Roberts, 1988)},was purchased from Yuhuan Ocean Biochemical Co.Ltd. (Zhejiang).N-vinyl-2-pyrrolidone(NVP)was obtained from J&K Chemical Ltd and used after distillation under reduced pressure. 3-Mercaptopropionic acid(MPA)provied by Chenghui-Shuangda Chemical Co.Ltd.(Jinan)was used after distilled under reduced pressure.N,N-azobisisobutyronitrile(AIBN)provided by Shanghai Chemical Reagent Co.Ltd.wa
s used after recrystallization with methanol.N,N-dimethylformamide(DMF)and dimethyl sulfoxide (DMSO)were distilled under reduced pressure from calcium hy-dride.All other reagents and solvents were used without further purification.
2.2.Synthesis of carboxyl terminated PVP(PVP–COOH)
Carboxyl terminated PVP(PVP–COOH)was prepared by radical polymerization using MPA as a chain transfer agent(Haruhiko et al.,1999;Park et al.,2003).NVP(0.25mol),MPA(0.013mol) and AIBN(0.001mol)were dissolved in20ml ethanol.The solution was degassed by bubbling with nitrogen for30min.Polymeriza-tion was carried out at70°C for24h.After reaction,the polymer was precipitated into an excess of diethyl ether,then purified by repeated precipitation in diethyl ether.Finally the product was dried in a vacuum.The molecular weight was7200,measured by potentiometric titration with0.1M NaOH.
2.3.Preparation of N-phthaloylchitosan(PHCS)
N-phthaloylchitosan was synthesized as previously reported (Keisuke,Hiroyuki,Yuya,&Shimojoh,2002).To a solution of 5.6mmol of phthalic anhydride in definite N,N-dimethylformam-ide(DMF)containing5%(v/v)water was added(1.86mmol pyra-nose)of highly deac
etylated chitosan,and the mixture was heated in nitrogen at120°C with stirring.After8h of reaction, the resulting mixture solution was cooled to room temperature and poured into ice water.The precipitate was collected on afilter, washed fully with methanol at room temperature,and dried under vacuum at room temperature overnight.The degree of substitution of phthaloyl groups within PHCS was determined to be about1.12, calculated by elemental analysis.
2.4.Synthesis of PHCS-g-PVP copolymer
The whole grafting procedure of PHCS-g-PVP was showed in Scheme1.PHCS(0.40mol)was stirred with PVP–COOH(1mol) in20ml DMF solution.1-Hydroxybenzotrizole(HOBt)(3mol) was added as catalyst and stirred at room temperature until fully dissolved.Then DCC(3mol)was added.After48h reaction at room temperature,the copolymer was dialyzed against distilled water, and then the precipitate was collected,washed fully with ethanol, and dried under vacuum at room temperature overnight to obtain white particles.The graft content(G%)was292.3%,calculated as follows:G%=(W gÀW0)/W0Â100,where W g and W0are the weight of graft copolymers and PHCS,respectively.
2.5.Preparation of polymeric micelles
Polymeric micelles of PHCS-g-PVP were prepared by the dialysis method(Praneet et al.,2006;Yokoya
ma et al.,1999).Distilled water was added slowly(1drop minÀ1)into DMSO solution of graft copolymer(1mg/ml)under vigorous stirring until slightly turbid. Then the solution was put into a dialysis bag(MWCO=14,000) and dialyzed against distilled water atÀ4°C for48h,with the dis-tilled water being changed every12h.The micelles solution was purified by ultrafiltration using afiltration membrane of0.45l m.
2.6.Measurement of critical micelle concentration
Critical micelle concentration(CMC)of PHCS-g-PVP was deter-mined byfluorescence spectroscopy(Praneet et al.,2006;Zhang et al.,2004).Aliquots of pyrene solutions in diethyl ether(5l l) was placed into each of a series of tubes and the diethyl ether was evaporated,4ml aqueous polymer solutions at different con-centrations(1–0.42Â10À6mg/ml)were added to each tube con-taining the pyrene residue([py]=6Â10À7M),then the mixture was sonicated for30min and stored overnight at room tempera-ture to reach the dissolution equilibrium of pyrene in the aqueous phase.An excitation spectrum was measured at336nm,and emis-sion spectra were recorded ranging from350to550nm.From the pyrene emission spectra,the intensity ratios(I1/I3)of thefirst band (374nm)to the third band(385nm)were analyzed as a function of polymer concentration.The critical micelle concentration(CMC) value was determined at the onset of a decrease in the plot of the polymer conce
ntration versus ratio of I1/I3.
2.7.Drug loading
The incorporation of prednisone acetate into polymeric micelles was carried out by a dialysis method(Praneet et al.,2006).5mg of copolymer and prednisone acetate(5mg)were dissolved in2ml DMSO in a glass tube.The mixture was stirred at room tempera-ture until completely dissolved,then distilled water was added slowly(1drop minÀ1)into the mixture under vigorous stirring un-til slightly turbid,the mixture was placed in a dialysis bag (MWCO=14000),dialyzed against distilled water over night.
F.Bian et al./Carbohydrate Polymers76(2009)454–459455
Drug-loaded micelles were purified byfiltration with a0.45l m pore-sized microfiltration membrane.
Prednisone acetate concentration was measured at242nm with a UV–Vis spectrometer and was calculated based on the stan-dard curve:c(mg/L)=A/0.0231,where A is the UV absorbance at 242nm(Wei,Zheng,Zhou,Cheng,&Zhuo,2006).
The loading capacity was calculated from the formula:
Loading capacityð%Þ¼
M0
M0þM p
reaction between pvp and aminoÂ100
where M o is the amount of drug-loaded in the polymeric micelles, and M P is the amount of copolymer.M o was calculated by subtract-ing the amount of unloaded drug from the initial feed drug amount. The amount of unloaded drug was analyzed by measuring the absorbance at242nm of the dialysisfluid.The amount of predni-sone acetate incorporated into polymeric micelles was about 4.03mg,and the loading capacity was44.6wt%.
2.8.In vitro drug release
Drug release from prednisone acetate-loaded micelles was mea-sured using a dialysis bag(MWCO=14000)as described previously (Praneet et al.,2006).3ml prednisone acetate-loaded micelles were placed in a dialysis bag and immersed in the medium(PBS,0.1M, pH=7.4),which kept at37°C.At certain time intervals,3ml ali-quots of the medium were withdrawn and the same volume of fr
esh medium was added.The amount of prednisone acetate released from micelles was measured using UV–Vis spectrometer at 242nm.All experiments were performed in triplicate.
The cumulative drug release was calculated from the formula cumulative drug release(%)=(M t/M o)Â100.Where M t is the amount drug release from micelles at time t,and M o is the amount of drug-loaded in PHCS-g-PVP polymeric micelles.
2.9.Characterization
Fourier-transform infrared(FT-IR)transmission spectra were obtained from samples in KBr pellets using a Bruker IFS66v/S FT-IR spectrophotometer.1H NMR(nuclear magnetic resonance)spec-tra were recorded on an AV-300M NMR spectrometer.The mor-phology of the copolymers aggregates was studied by H-600, Japan transmission electron micrograph(TEM).Dynamic light scat-tering(DLS)measurements were carried out with a BI-200SM, Brookhaven instrument.UV–Vis measurements were carried out at25°C with a Perkin–Elmer LS55(America)spectrophotometer.
3.Results and discussion
3.1.Preparation and characterization of graft copolymers
Carboxyl terminated PVP was prepared by radical polymeriza-tion using3-Mercaptopropionic acid as chain transfer agents.Fig.1a shows the FT-IR of PVP–COOH.The strong signals at 1673.9cmÀ1and1288.3cmÀ1attributed to the C@O stretching vibration(Amide I)and C–N stretching vibration in the PVP ring (Irene et al.,2007).The stretching vibration of O–H appears at 3464.3cmÀ1.As indicated in1H NMR of PVP–COOH(Fig.2a),the SCH2C H2peaks appear at1.4–1.7ppm and the peaks of CH2C@O appear at2.2–2.4ppm.The strong signals at3.2and3.5–3.6ppm were due to the protons of CHC H2and C H CH2in PVP ring(Irene et al.,2007).The above results proved the synthesis of PVP–COOH.
N-phthaloylchitosan was synthesized by chitosan reacting with excess phthalic anhydride.The FT-IR spectrum of N-phthaloylchi-tosan was shown in Fig.1c,compared with that of original chitosan (Fig.1b),PHCS showed the characteristic absorptions peaks at 1776.1cmÀ1and1712.5cmÀ1referring to the carbonyl anhydride, and the absorptions peak at721.3cmÀ1belonging to the aromatic ring(Keisuke et al.,2002;Li,Zhuang,Mu,Wang,&Fang,2008).The 1H NMR(Fig.2b)of N-phthaloylchitosan,showed mainly two parts of broad peaks:2.8–5.0ppm belonged to the chitosan backbone hydrogens and7.8–8.0ppm assigned to the phthaloyl group(Pra-neet et al.,2007).These results verified that the PHCS was synthesized.
Graft copolymer(PHCS-g-PVP)was synthesized in homoge-neous phase by a condensation reaction between the hydroxyl group of PHCS and carboxyl terminated group of PVP.Compared to the FT-IR spectrum of PHCS,the obtained PHCS-g-PVP(Fig.1d) presented new absorption peaks at1186.2cmÀ1and1089.5 cmÀ1,which referring to the characteristic dissymmetrical and symmetry stretching vibration peaks of C–O–C of the ester
group, Fig.1.FT-IR spectra of(a)PVP–COOH,(b)CS,(c)PHCS,and(d)PHCS-g-PVP.
456 F.Bian et al./Carbohydrate Polymers76(2009)454–459
the peaks at 1643.5cm À1and 2929.9cm À1belonging to the car-bonyl group and C–H (–CH 2CH 2),respectively.The evidence im-plied the successful introduction of the PVP chains on PHCS.Compared with PHCS,the 1H NMR spectrum (Fig.2c)of graft copolymer showed new proton peaks at 1.4–1.7ppm and 2.2–2.4ppm assigned to C–H (SCH 2C H 2)and CH 2C @O of PVP graft chain,respectively.This further confirmed the PVP were success-fully grafted onto chitosan chains.
3.2.Preparation and characterizations of polymeric micelles
In general,the formation of self-assembled micelles occurs as a result of two forces (Sui et al.,2006).One is an attractive force that result in the self-assembly of molecules,while the other repulsi
ve force prevents unlimited growth of the micelles to a distinct mac-roscopic phase.Amphiphilic copolymer can self-assembly when its solution was dialysized in a poor solvent for either hydrophobic or hydrophilic segment.In our experiment,the self-assembled mi-celles were prepared by dialysis of a DMSO solution of the PHCS-g-PVP copolymers against distilled water.
The CMC of polymeric micelles were characterized by fluores-cence spectroscopy using pyrene as a fluorescent probe.The plot of the intensity ratio I 1/I 3of the pyrene excitation spectra against the polymer concentration is shown in Fig.3.As seen from the Fig.3,below the CMC,the fluorescence intensity in I 1/I 3was nearly a constant,it indicated that the copolymers exist as single chains,but when the copolymer concentration reached a value which above the CMC,a dramatic decrease in I 1/I 3was observed,indicat-ing the formation of micelles and dissolution of pyrene into the hydrophobic core of micelles.Compared to low molecular weight surfactants micelles (e.g.2.3mg/ml for sodium dodecyl sulfate in water)(Nagasaki,Okada,&Scholz,1998),the CMC of PHCS-g-PVP with 0.83mg/L is lower.The polymeric micelles with lower CMC will be suitable as drug targeting devices since they are stable in an aqueous environment and cannot easily dissociate on extre-mely diluted by blood in intravenous administration,and can pro-long circulation in the bloodstream.
The morphology of polymeric micelles was measured by TEM (Fig.4a).Fig.4a showed that self-assemb
led micelles are well dis-persed as individual nano-size micelle with regularly spherical
shape,and the size of the nano-size micelles were around 40–100nm in diameter.Fig.5a showed that PHCS-g-PVP micelles ex-hibit a unimodal size distribution with an average diameter of 89.8nm measured by DLS.These are consistent with the TEM results.
3.3.Prednisone acetate incorporation into PHCS-g-PVP micelles As the inner core of the polymeric micelles is hydrophobic,hydrophobic drug molecules can be physically incorporated and stabilized by hydrophobic interactions.Prednisone acetate is a good anti-inflammation and anti-allergic drug,but it is water-insoluble,here it was used as a model drug to be entrapped into the hydrophobic core of amphiphilic graft copolymer micelles.The prednisone acetate was successfully loaded into the inner core of polymeric micelles with a loading capacity of 4
4.6wt%.
The TEM photograph (Fig.4b)showed that the size of drug-loaded polymeric micelles were approximately 118–200nm,and it were much bigger than the blank micelles (Fig.4a),indicating that the dissolution of the prednisone acetate into the micelles made the micelles much bigger.These are consistent with the DLS results (Fig.5b).The drug-load micelles showed an average size of 143.3nm.T
herefore,if the polymeric micelles were used as DDS,its nano-size can reduce non-selective system scavenging by the reticuloendothelial.3.4.Drug release measurement
In vitro release of prednisone acetate from drug-loaded micelles was evaluated in PBS (pH =7.4,0.1M)at 310K.The cumulative drug release pattern of the polymeric micelles is showed in Fig.6.As seen from the curve,the drug release rate was very stable,no initial burst of release appeared.After 69h incubation at 37°C,only 26%drug was gradually released.The stability and low release rate probably attributed to two factors.On the one hand,the inter-action between the drug and inner core of micelles is stronger than that between drug and solvent (Zhao,Wang,Winnik,Riess,&Crou-cher,1990),on the other hand,since the hydroxyl in PHCS and the carbonyl group of PVP can form intermolecular H-bond with the carbonyl groups and hydroxyl of prednisone acetate,which is prone to stabilize the structure of micelles.
In many reported papers on DDS (Sui et al.,2006;Zhang et al.,2004),showed an initial rapid burst of release and then were slowly released.This phenomenon results in large amounts
of
Fig.2.1H NMR spectra of (a)PVP–COOH in D 2O,(b)PHCS in DMSO-d 6,and (c)PHCS-g-PVP in DMSO-d 6.
F.Bian et al./Carbohydrate Polymers 76(2009)454–459457
drugs being wasted before getting to the targeted sites.In addition,the micellar solution used in intrave
nous administration were of-ten extensively diluted by blood,so they are easily deformable and disassemble which result in the faster leakage of loaded drugs (Ye et al.,2008).Different from other studies,the drug release rate is stable in our drug release experiment and no sudden release ap-peared,these not only can reduce the number of injections a pa-tient must endure,but also can improve the utilization of drugs.
4.Conclusion
A novel non-toxic,biocompatible and biodegradable graft copolymer (PHCS-g-PVP)was successfully synthesized in homoge-neous system.In aqueous solution,the spherical polymeric mi-celles with nano-size and narrow distribution were formed based on amphiphilic graft copolymer.An anti-inflammation drug,pred-nisone acetate was incorporated into the micelles with loading capacity around 44.6wt%.In vitro tests,release of prednisone ace-tate from the micelles was sustained.Taking the advantage of the nano-size and stable drug release rate,the nanoscope polymeric micelles might be useful as drug carriers to achieve controlled drug release.References
Agnihotri,S.A.,&Aminabhavi,T.M.(2004).Controlled release of clozapine through
chitosan microparticles prepared by a novel method.Journal of Controlled Release,96,245–259.
Chang,S.H.,Joseph,A.,&Gardella,J.(2005).Surface chemistry of biodegradable
polymer for drug delivery systems.Chemical Reviews,105,4205–4232.
Fukashi,K.,Kiyotaka,S.,Takao,A.,Masayuki,Y.,Yasuhsa,S.,&Teruo,O.(1998).
Preparation and characterization of thermally responsive block copolymer micelles comprising poly (N -isopropylacryla-b-DL -lactide).Journal of Controlled Release,55,87–98.
Haruhiko,K.,Yasuo,T.,Shin-ichi,T.,Tetsunari,K.,Yoshihisa,K.,Yoko,Y.,et al.
(1999).Molecular design of conjugated tumor necrosis factor-a:Synthesis and characteristics of polyvinyl pyrrolidone modified tumor necrosis factor-a.Biochemical and Biophysical Research Communications,257,448–453.
Irene,B.,Roberto,S.,Etienne,S.,&Emo,C.(2007).Hydroxylend-capped macromers
of N -vinyl-2-pyrrolidinone as precursors of amphiphilic block copolymers.European Polymer Journal,43,4628–4638.
Jayakumar,R.,Prabaharan,M.,Reis,R.L.,&Mano,J.F.(2005).Graft copolymerized
chitosan-present status and applications.Carbohydrate Polymers,65,1–17.
Jia,Z.S.,&Li,Q.B.(2001).Determination of the deacetylation degree of chitosan by
two a brupt change potential titration method.Chemistry World,5,240–241.Jindrich,K.(2003).Smart and genetically engineered biomaterials and drug delivery
systems.European Journal of Pharmaceutical Sciences,20,1–16.
Keisuke,K.,Hiroyuki,I.,Yuya,M.,&Shimojoh,M.H.(2002).Chemoselective
protection of the amion groups of chitosan by controlled phthaloylation:Facile preparation of a precursor useful for chemical modifications.Biomacromolecules,3,1.
Li,G.,Zhuang,Y.L.,Mu,Q.,Wang,M.Z.,&Fang,Y.E.(2004).Rapid N -phthaloylation
of chitosan by microwave irradiation.Carbohydrate Polymers,57,97–100.
Li,G.,Zhuang,Y.L.,Mu,Q.,Wang,M.Z.,&Fang,Y. E.(2008).Preparation,
characterization and aggregation behavior of amphiphilic chitosan derivative having poly (L -lactic acid)
side chains.Carbohydrate Polymers,72,60–66.
Maghami,G.G.,&Roberts,G.A.F.(1988).Evaluation of the viscometric constants
for chitosan.Macromolecule Chemistry,189,195–200.
Nagasaki,Y.,Okada,T.,&Scholz,C.(1998).The reactive polymeric micelle based on
an aldehyde-ended poly (ethylene glycol)/poly (lactide)block copolymer.Macromolecules,31,1473–1479.
Park,I.K.,Ihm,J.E.,Park,Y.H.,Choi,Y.J.,Kim,S.I.,Kim,W.J.,Akaike,T.,&Cho,C.S.
(2003).Galactosylated chitosan (GC)-graft-poly (vinyl pyrrolidone)(PVP)ashe-patocyte-targeting DNA carrier.Preparation and physicochemical characterization of GC-graft-PVP/DNA complex.Journal of Controlled Release,86,
349–359.
Fig.4.Transmission electron micrograph (a)blank micelles (Â50,000),and (b)Prednisone acetate-loaded micelles (Â50,000).
458 F.Bian et al./Carbohydrate Polymers 76(2009)454–459
Prabaharan,M.,&Gong,S.(2008).Novel thiolated carboxymethyl chitosan-g-b-cyclo-dextrin as mucoadhesive hydrophobic drug delivery carriers.
Carbohydrate Polymers,73,117–125.
Praneet,O.,Tanasait,N.,Amornrut, C.,Theerasak,R.,&Auayporn, A.(2006).
Incorporation of camptothecin into N-phthaloyl chitosan-g-mPEG self-assembly micellar system.European Journal of Pharmaceutics and Biopharmaceutics,64,269–276.
Praneet,O.,Tanasait,N.,Amornrut, C.,Theerasak,R.,&Auayporn, A.(2007).
N-Phthaloylchitosan-g-mPEG design for all-trans retinoicacid-loaded polymeric micelles.European Journal of Pharmaceutical Sciences,30, 424–431.
Sui,W.P.,Yin,C.Q.,Chen,Y.J.,Zhang,Z.G.,&Kong,X.Z.(2006).Self-assembly of an amphiphilic derivative of chitosan and micellar solubilization of puerarin.
Colloids and Surfaces B:Biointerfaces,48,13–16.
Trong,M.D.,Chia,F.K.,&Wen,Y.C.(2002).Synthesis and properties of chitosan modified poly(vinylacetate).Journal of Applied Polymer Science,86,3057–3063.Wei,H.,Zheng,X.Z.,Zhou,Y.,Cheng,S.X.,&Zhuo,R.X.(2006).Self-assembled thermoresponsive micelles of poly(N-isopropylacrylacrylamide-b-methylmethacrylate).Biomaterials,27,2028–2034.
Ye,Y.Q.,Yang,F.L.,Hu,F.Q.,Du,Y.Z.,Yuan,H.,&Yu,H.Y.(2008).Core-modified chitosan-based polymeric micelles for controlled release of doxorubicin.
International Journal of Pharmaceutics,352,294–301.
Yokoyama,M.,Okano,T.,Sakurai,S.Y.,Fukushima,S.,Okamoto,K.,&Kataoka,K.
(1999).Selective delivery of adriamycin to a solid tumor using a polymeric micelle carrier system.Journal Drug Target,7,171–186.
Zhang,C.,Ping,Q.N.,Zhang,H.J.,&Shen,J.(2004).Preparation of N-alkyl-O-sulfate chitosan derivatives and micellar.Carbohydrate Polymers,54,137–141. Zhao,C.,Wang,Y.,Winnik,M.A.,Riess,G.,&Croucher,M.D.(1990).Fluorescence probe technique used to study micelle formation in water-soluble block co-polymer.Langmuir,6,514–516.
Zhu,A.P.,Chen,T.,&Yuan,L.H.(2006).Synthesis and characterization of N-succinyl-chitosan and its self-assembly of nanospheres.Carbohydrate Polymers, 66,274–279.
F.Bian et al./Carbohydrate Polymers76(2009)454–459459
版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。
发表评论