Preparation and Characterization of Polymeric Micelles from Poly(D,L-lactide) and Methoxypolyethylen

TSINGHUA SCIENCE AND TECHNOLOGYISSN 1007-0214 20/20 pp493-496Volume 12, Number 4, August 2007Preparation and Characterization of Polymeric Micelles fromPoly(D, L-lactide) and Methoxypolyethylene Glycol BlockCopolymers as Potential Drug CarriersZHANG Jjianzheng (张建峥), JIANG Wei (姜维), ZHAO Xiuwen (赵秀文), WANG Yundong (王运东)”State Key Laboratory of Chemical Engineering, Department of Chemical Engineering,Tsinghua University, Beijing 10084, ChinaAbstract: Amphiphilic diblock copolymers composed of methoxy polyethylene glycol (MePEG) and poly(D,L-lactide) (PDLLA) were prepared for the preparation of polymeric micelles. The use of MePEG-PDLLA asdrug carriers has been reported in the open literature, but there are only few data on the application of a se-ries of MePEG-PDLLA copolymers with diferent lengths in the medical field. The shape of the polymeric mi-celles is also important in drug delivery. Studies on in vitro drug release profiles require a good sink condi-tion. The critical micelle concentration of a series of MePEG-PDLLA has a significant role in drug release. Toestimate their fesiblity as a drug carrier, polymeric micelles made of MePEG-PDLLA block copolymer wereprepared by the oil in water (OM) emulsion method. From dynamic light scattering (DL S) measurements,the size of the micelle formed was less than 200 nm. The critical micelle concentration of polymeric micelleswith various compositions was determined using pyrene as a fuorescence probe. The critical micelle con-centration decreased with increasing number of hydrophobic segments. MePEG-PDLLA micelles have aconsiderably low critical micelle concentration (0.4-0.5 μg/mL), which is apparently an advantage in utilizingthese micelles as drug carriers. The morphology of the polymeric micelles was observed using scanningelectron microscopy (SEM) and transmission electron microscopy (TEM). The micelles were found to benearly spherical. The yield of the polymeric micelles obtained from the OW method is as high as 85%.Key words: poly(D,L-lactide); methoxy polyethylene glycol; polymeric micelle; drug carrierpolymers to form micellar structures or can be physi-Introductioncally incorporated within the hydrophobic cores ofpolymeric micelleso. Although there are many possi-Recently, there has been increased interest in the appli-ble micelles that can exist in an aqueous environment,cation of biodegradable polymers for advanced devicesonly a limited number of copolymers are suitable asin the fields of tissue engineeringl'2 and drug deliv-drug delivery vehicles, due to the requirement of bio-ery!3,4] Amphiphilic diblock copolymer micelles arecompatibility of the copolymer.effective vehicles for the solubilization of hydrophobicPolyester-block- methoxypolyethylene glycol materi-drugs'!. Drugs can be covalently coupled to block co-als date back to the 1950s. when a US patent describedthe use of polvglvcol-polvacid ester for the treatmentRcceived: 205-11-15of text中国煤化工on!8) subsequentlySupported by the National Natural Science Foundation of China(No. 29836130)patente;YHC N M H Gspersible dnug de-*$ To whom crrespondence should be adresed.livery systems. Subsequenty, several studies haveE-mail: wangdqjtsinghua.cdu.cn; Tel: 86-10-62782748494Tsinghua Science and Technology, August 2007, 12(4): 493-496utilized polyester-block- -methoxypolyethylene glycol1.2 Micelle preparationcopolymers as micellar carriers of drugs. In a biode-gradable diblock copolymer, the polyester can beThe oil in water (O/W) emulsion method was used topoly(L-lactide) (PLLA), poly(D,L-lactide) (PDLLA),prepare the polymeric micelle solutions. Firstly, 50polyglycolide (PGA), polycaprolactone (PCL), or theirmg of a block copolymer was dissolved in 5 mL ofcopolymers. These polyesters have been proven to beacetone. Secondly, the solution was dripped into 50 mLsafe as implant materials and have been widely used aswater which was then stirred for 24 h followed bysurgical sutures, bone fracture fixation devices, andlyophilization.controlled drug delivery systems. Methoxypolyethyl-1.3 Micelle characterizationene glycol is water soluble and biocompatible.Polyester-methoxypolyethylene glycol diblock co-The average size and size distribution of polymericpolymers can be synthesized through a ring openingmicelles were determined by DLS measurement usingpolymerization!". It has been suggested that the hy-a light scattering spectrometer (Zetasizer 3000 HS,droxyl group is responsible for initiating the ring open-Malver Instruments, UK) at a wavelength of 633 nming polymerization' 0. In the polymerization reaction,and 25C. The intensity of the scattered light was de-the lactide ring opens and inserts into the initiation sitetected as a scattering angle of 90° . The critical micelleresulting in a growing polylactide chain from the hy-concentrations of MePEG-PDLLA copolymer micellesdroxyl group. Hence, the hydroxy group at one of thewas determined using pyrene as a fluorescence probe.methoxypolyethylene glycol chain ends acts as an ini-Polymeric micelles with the concentrations of MePEG-tiation site to allow the further growth of the polylac-PDLLA ranging from 1.0X10+ to 1 mg/mL weretide chain, resulting in a diblock copolymer of polylac-equilibrated with a saturated aqueous solution ctide and methoxypolyethylene glycol. Through poly-pyrene (5.0x10~ mol/L) for 24 h under shielded lightmer synthesis, the chain lengths of the hydrophobicpolyester block and the hydrophilic methoxypolyethyl-to load pyrene molecules into the polymeric micellecore. Fluorescence measurements were carried out uS-ene glycol block can be readily changed to giveing a F-2500 spectrophotometer at an excitation wave-diblock copolymers with a wide range of properties.In this work, we have synthesized various composi-length of 3333 nm and an emission wavelength of 392tons of diblock copolymer containing one block onm. The extent of pyrene solubilization was deter-MePEG and one block of PDLLA. Using these poly-mined as the function of MePEG-PDLLA. All experi-mers we have determined the critical micelle concen-ments were carried out at 25"C. The extermal and inter-tration of polymeric micelles and studied the size ofnal morphology of the polymeric micelles was ana-the polymeric micelles using a dynamic light scatteringlyzed by SEM. Samples were mounted on metal hold-(DLS) method. We have also investigated the yield ofers and vacuum-coated with a gold layer prior topolymeric micelles and examined the surface mor-examination.phology of the MePEG-PDLLA micelles using scan-ning electron microscopy (SEM) and transmission2 Results and Discussionelectron microscopy (TEM).2.1 Preparation of MePEG-PDLLA micelles1 Materials and MethodsThe MePEG-PDLLA micelles were obtained by theO/W emulsion method as mentioned in Section 1.21.1 MaterialsTheoretical value of number-average molecularDiblock copolymers of MePEG-PDLLA with MePEG'sweight for hydrophilic segments, MePEG is 5000,number-average molecular weight (M) of 5000 andwhile the number-average molecular weights meas-PDLLA ranging from 2500 to 100 000 were synthe-ured: segments, PDLLA,sized in our laboratory, Acetone and pyrene were pur-are中国煤化工449. One poblenchased locally (Weisi Corporation, China) and used asobse"TYHC N M H Gnicelle solution of-ten became unstable, especially for Mn values ofreceived without further purification.ZHANG Jianzheng (张建峥) et al: Preparation and Characeriation of Polymeric Micelles from...4953000PDLLA segments over 10 000 and temperaturesabove 25"C. The reason may be twofold. Firstly,2500-when the Mn value of PDLLA segments becomes lar-ger, it is harder to dissolve the block copolymers.Secondly, lowering temperature is preferable to disso-1500-lution of MePEG while for PDLLA the situation is1000the opposite.2.2 Critical micelle concentration of MePEG-PDLLA micelles350~370390410430450470 490Wavelength (nm)Pyrene solubilization has been used previously for theFig. 1 Fluorescence emission spectra (ler = 333 nm) ofdetermination of the critical micelle concentration inpyrene (5x10-7 mol/L) registered at 25C in the pres-block copolymer solutionsh.is. It is chosen as theence of various concentrations of MePEG-PDLLAfluorescent probe because of its photophysical andother properties. Pyrene is strongly hydrophobic, andits solubility in water is very low. It is preferentiallysolubilized into the interior of the hydrophobic regionsof micelles or similar macromolecular systems. Thefluorescence of the probe is sensitive to the change inthe micro-environment, which permits monitoring itsincorporation in polymeric micelles at concentrations0.exceeding the critical micelle concentration. Typicalemission spectra of pyrene and plots of fluorescence500010000 15000 20 000intensity are represented in Fig. 1. The intensity valuesLength of PDLLA scgmentsremained virtually constant below the critical micelleFig 2 Critical micelle concentration of MePEG-PDLLA versus length of PDLLA segmentsconcentration. Above the critical micelle concentration,the fluorescence increased susatanilly, reflecting in-corporation of pyrene in the hydrophobic core of theTable 1 Mean size of polymeric micellespolymeric micelles. The critical micelle concentrationLength of segmentsAverage sizePolydispersevalue decreases with the increasing PDLLA segmentMePEGPDLLA(nm)length as shown in Fig. 2. Studies on in vitro drug re-250019.20.31lease profiles require a good sink condition. Maintain-20.10.28ing a sink condition means keeping the drug concentra-I0 00023.80.23tion lower enough in the release medium not to affect20 00025.70.19the concentration gradient for drug releasell4l. So the3000029.10.20critical micelle concentration of a series of MePEG-40 00030.4PDLLA micelles plays a significant role in drug50 00032.60.16release.750000.15100 00035.42.3 Size of MePEG-PDLLA polymeric micellesThe size of the polymeric micelles of MePEG-PDLLA2.4 Morphology of MePEG-PDLLA blockcopolymeric micelleswas measured by the dynamic light scattering method.Table 1 lists the average size of the polymeric micellesThe morphology of the polymeric micelles was inves-of nine block copolymers. The average size of the mi-tigateq中国煤化工re 3 shows a SEMcelles range from 25 to 35 nm with the increase of theimageCNM H G'1 be seen that thehydrophobic segment (PDLLA).polymci ulnrtics alc opici ual allu uniform.496Tsinghua Science and Tehnology,August 20012(4); 493-496[4] Miller R H. Clloial Crriers for Contolled Dnug Deliv-ery and Targeing. Sutgat: CRC Pess, 1991.，5] Kataoka K, Kwon G s, Yokoyama M, et al. Block co-polymer micles as vchicles for drug delivery. J. 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