- Email: [email protected]
Solvent-free preparation of caprolactone oligomer microspheres
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599 www.elsevier.com/locate/jpcs
Solvent-free preparation of caprolactone oligomer microspheres Jisun Lee, Saemi Oh, Min Kyung Joo, Byeongmoon Jeong Department of Chemistry, Division of Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea Received 30 July 2007; accepted 17 September 2007
Abstract Caprolactone (Mn3000 Da, melting point 46 1C) oligomer microspheres were prepared by an organic solvent-free process, and were investigated as a delivery system of a hydrophobic model drug (estradiol). The drug was dissolved in a polycaprolactone/poly(ethylene glycol-b-caprolactone) mixture at 50 1C, which was above the melting points of both polymers. The mixture was homogenized in water for 10 min at 50 1C, then it was quenched in the ice bath for 10 min to harden the microsphere. The polycaprolactone of the poly(ethylene glycol-b-caprolactone) coaggregated into the microsphere, and poly(ethylene glycol) formed a shell layer of the microsphere that protects the microsphere from interparticle aggregation during the hardening process of the microsphere in water. The size of microsphere could be controlled by the amount of polycaprolactone relative to poly(ethylene glycol-b-caprolactone). Estradiol release from the microsphere was investigated. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymers; D. Microstructure; D. Surface properties
1. Introduction Biodegradable microspheres have been extensively studied as a delivery system of pharmaceutical agents for the last decade [1,2]. Recently, biodegradable oligomers with the molecular weight range of 1000–5000 Da have been applied as a short-term release system of the encapsulated drug, between 1 day to 1 week [3–5]. There has been intensive research to facilitate microsphere preparation, such as the spray drying method, the solvent evaporation method, the double emulsion method, and the supercritical fluid methods [6–9]. All above methods used organic solvents during the preparation of the microsphere. The organic solvents may cause harmful side effects for humans and deteriorate the pharmaceutical agents that are sensitive to the organic solvent. Therefore, US Pharmacopoeia limits the residual organic solvent to 500 ppm for methylene chloride and 50 ppm for chloroform, which are typical solvents to prepare microspheres [10]. Therefore, removing the residual organic solvent is a serious problem Corresponding author. Tel.: +82 2 3277 3411; fax: +82 2 3277 4192.
E-mail address: [email protected] (B. Jeong). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.09.016
in practical process. As an example, to remove residual methylene chloride in microspheres containing progesterone, high vacuum was applied for 3 days at 35–55 1C [11]. Here we are reporting a caprolactone oligomer microsphere preparation method without using any organic solvent. The concept of the current method was schematically shown in Fig. 1. The polycaprolactone (PCL) and monomethoxy poly(ethylene glycol-b-caprolactone) (mPEG–PCL) mixture is in a liquid state at 50 1C which is above their melting points. It acts as a solvent of a hydrophobic drug at this temperature. The mixture was added to water at 50 1C and was mixed by the homogenizer, then it was cooled down to harden the microspheres. The PEGs of the mPEG–PCL are exposed to the surface of the microsphere and stabilize the microsphere in water. The PCLs of the mPEG–PCL coaggregate with the PCL during the hardening process of the microsphere. Several molecular parameters were investigated in a series of preliminary studies. First, when the PCL molecular weight of mPEG–PCL (5000-PCL) was less than 500, a significant interface–diffusion between microparticles was observed during the hardening process, resulting in an irregular shape of the microspheres. The low molecular weight PCL of mPEG–PCL was not
ARTICLE IN PRESS J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599
1597
Quenching PCL / mPEG - PCL
Freeze Drying
Fig. 1. Solvent-free caprolactone oligomer microsphere preparation. After melting the PCL and mPEG–PCL at 50 1C, the mixture was added to water at 50 1C, followed by sonication by a homogenizer. The PCL of the mPEG–PCL coaggregates with the PCL homopolymer to form microsphere, whereas the PEG of the mPEG–PCL forms a shell layer of the microsphere. The system was quenched to ice bath to harden the microspheres. The microsphere was harvested by centrifugation, followed by the freeze-drying process.
effectively coaggregated into the PCL microsphere and did not act as an effective surfactant to give the stability of the PCL microsphere in water. Second, mPEG–PCL (5000–1000) was fixed, and the molecular weight of the PCL was varied from 1000, 3000, and 6000 Da. The melting points of the PCLs with molecular weights of 1000, 3000, and 6000 Da were 40, 46, and 53 1C, respectively. When the PCL molecular weight was 1000, the microspheres also collapsed, and did not show spherical microsphere formation due to the soft nature of the low molecular weight PCL. Too high temperature is not desirable because some drugs might be deteriorated by the heat. The spherical microspheres were formed when the molecular weight of PCL was 3000 by the current process. Third, the cooling condition was varied between 20 and 0 1C to investigate the hardening process parameter at a given pair of PCL (MW ¼ 3000 Da) and mPEG–PCL (5000–1000). The interface–diffusion between microspheres during the hardening process of the microspheres decreased in the faster cooling process and showed a better spherical shape of the microspheres. Based on the above preliminary studies, the current research was focused on PCL (MW ¼ 3000 Da), mPEG– PCL (5000–1000), and quenching process to 0 1C for hardening of the microsphere. We varied the ratio of the amount of PCL to mPEG–PCL. Release profile of estradiol, as a model drug, from the microsphere was investigated. 2. Experimental section 2.1. Materials mPEGs with molecular weights of 5000 Da were purchased from Aldrich and used as received. Stannous
octoate, e-caprolactone, benzyl alcohol, and toluene were used as received from Aldrich.
2.2. mPEG–PCL diblock copolymer and PCL synthesis mPEG–PCL (5000–1000) diblock copolymers were synthesized by the ring-opening polymerization of e-caprolactone on the mPEGs [12,13]. To prepare the mPEG– PCL, mPEG (MW=5000 Da, 10.0 g) was dissolved in toluene (80 mL). Toluene was distilled off to a final volume of 20 mL to remove the water by azeotropic distillation. Stannous octoate (10 mL) and e-caprolactone (2.5 g) were added to the reaction mixture and stirred at 120 1C for 24 h. The reaction mixture was precipitated into diethyl ether and the residual solvent was removed under high vacuum at room temperature. The yield was about 80%. To prepare the polycaprolactone (MW=3000 Da), benzyl alcohol was used instead of mPEG as an initiator.
2.3. Characterization of mPEG–PCL The composition and molecular weight of the mPEG–PCL and PCL were investigated by 250 MHz NMR (9503DPX; Bruker) for 1H NMR (in CDCl3) and a gel permeation chromatography system (Waters 515) with a refractive index detector (Waters 410). Tetrahydrofuran was used as an eluting solvent. PEGs in the molecular weight range of 400–20000 Da were used as the molecular weight standards. Styragels HMW 6E and HR 4E columns (Waters) were used in series.
ARTICLE IN PRESS 1598
J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599
2.4. Microsphere preparation To prepare caprolactone oligomer microsphere (MS1), mPEG–PCL (5000–1000, 0.1 g) and PCL (MW=3000 Da, 0.3 g) were dissolved at 50 1C and slowly dropped to the deionized water (50 mL) at 50 1C. Then, the solution was sonicated by a homogenizer (Sonics, VCX 750) at 50 1C for 10 min, and then at 0 1C for 10 min. The microsphere was collected by centrifugation at 3000 rpm for 10 min and freeze-dried. In order to load estradiol, the drug (5.0, 10, or 15 mg) was dissolved in an mPEG–PCL and PCL mixture at the first step.
The size control of the microsphere is important because it determines the ratio of surface area to mass of the microsphere, and thus affects the release profile of the incorporated drug. The size of microsphere increased as the amount of PCL increased from 0.3 to 0.7 g at a fixed amount of mPEG–PCL (0.1 g). Fig. 2 shows the scanning electron microscopic images of the microspheres. The amount of PCL relative to mPEG–PCL is smallest for MS1, and MS3 used the largest amount of PCL. In addition, the encapsulation efficiency of the drug increased from 46.8% to 78.7%, and drug loading efficiency increased from 0.59 to 0.68 as the amount of PCL increased from 0.3 to 0.7 g (Table 1).
2.5. Scanning electron microscope The images of the microsphere were photographed by a field emission scanning electron microscope (FE-SEM, JEOL JSM35 CF). The microspheres were coated with gold for 6 min to obtain the scanning electron microscopic image. 2.6. In-Vitro drug release Five milligrams of the microsphere was put in the conical tube and 2.0 mL of release medium (PBS containing 0.1 wt.% Tween 20) was added. The vial was shaken at 100 strokes/min. in a 37 1C water bath. At a given time interval of 1 or 2 h, the tube was centrifuged at 3,800 rpm for 10 min and 1.0 mL of the medium was taken for HPLC and 1.0 mL of the fresh medium was replaced. The released amount of the drug was assayed by high-performance liquid chromatography (Waters 996/Photodiode Array Detector) [14]. 3. Results and discussion The mPEG–PCL was prepared by ring-opening polymerization of caprolactone in the presence of mPEG at 120 1C for 24 h, using stannous octoate as a catalyst [9,10]. 1 H NMR spectra show an ethylene glycol unit at 3.6 ppm, a methylene peak next to the carbonyl group of caprolactone unit at 2.2 ppm. The number average molecular weight (Mn) was 5000–740 by 1H NMR spectra and 6000 Da by gel permeation chromatography. The polydispersity index defined by Mw/Mn of the mPEG–PCL determined by the gel permeation chromatography was 1.3. mPEG–PCL (5000–1000) indicates this polymer. PCL was prepared using benzyl alcohol instead of mPEG as an initiator. The molecular weight of PCL was 3000 Da and polydispersity index was 1.2. The PCL (MW=3000 Da) and mPEG–PCL (5000–1000) have melting points of 46 and 43 1C, respectively, thus the mPEG and PCL mixture is in a liquid state at 50 1C. It acts as a solvent of a hydrophobic drug (estradiol) at 50 1C. The mixture was added to the warm water (50 1C) and mixed by a homogenizer, then quenched in an ice bath to harden the microspheres.
Fig. 2. Scanning electron microscopic images of microspheres: effect of amount of PCL relative to mPEG–PCL. The mPEG–PCL (5000–1000) was fixed at 0.1 g and PCL (MW ¼ 3000 Da) was varied between 0.3 (MS1), 0.5 (MS2), and 0.7 g (MS3). The scale bar is 10 mm.
ARTICLE IN PRESS J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599
4. Conclusions
Table 1 List of microspheres
MS1 MS2 MS3
PCL (g)
Drug loaded (mg)
e.e. (%)
l.e. (%)
0.3 0.5 0.7
5 5 5
46.8 51.2 78.7
0.59 0.64 0.68
*mPEG–PCL (5000–1000) (0.1 g) was used and the amount of PCL (MW ¼ 3000 Da) was varied between 0.3, 0.5, and 0.7. *e.e. (encapsulation efficiency) ¼ 100 drug in microsphere (mg)/drug used in preparation of microsphere (mg). *l.e. (loading efficiency) ¼ 100 drug in microsphere (mg)/microsphere (mg).
Cummulative Released Amount (%)
100 5 mg 10 mg 15 mg
80
1599
Caprolactone oligomer microspheres were prepared by coaggregation of melted PCL and mPEG–PCL in the absence of any organic solvent. The molecular weight of PCL and mPEG–PCL and cooling condition were important parameters to prepare the spherical-shaped PCL microsphere. The optimal result was obtained with PCL (MW ¼ 3000 Da), mPEG–PCL (5000–1000), and a quenching process to 0 1C. As the amount of PCL relative to mPEG–PCL increased, the size of microsphere, the encapsulation efficiency, and the loading efficiency of the hydrophobic drug increased. The model hydrophobic drug, estradiol, was released from the microsphere and the release rate was dependent on the loaded amount of the drug. Acknowledgements This work was supported by the SRC program of MOST/KOSEF through the center for Intelligent NanoBio Materials at Ewha Womans University (Grant: R11-2005008-0000-0) and Basic Research Promotion Fund (KRF2006-005-J04003).
60
40
References 20
0 0
4
8
12
16
20
24
Time (hours) Fig. 3. Model drug release profile from microspheres: Effect of drug loading. The mPEG–PCL (5000–1000) (0.1 g) and PCL (MW ¼ 3000 Da) (0.3 g) were used, and the loaded amount of model drug was varied between 5, 10, and 15 mg.
The amount of loaded drug affected the release profile of the drug from the microsphere (Fig. 3). When the incorporated amount of the drug increased from 5 to 15 mg, the percent cumulative amount (the released amount at time t (Mt) divided by the initial loaded amount (D0)) of the drug decreased, and the release profile approached the zero-order release. The loaded hydrophobic drug might tighten the microsphere core by the hydrophobic interactions, and the diffusion rate of the drug would decrease.
[1] P.B. O’Donnell, J.W. Mcginity, Adv. Drug Del. Rev. 28 (1997) 25. [2] H. Tamber, P. Johansen, H.P. Merkle, B. Gander, Adv. Drug Deliv. Rev. 57 (2005) 357. [3] R.T. Liggins, H.M. Burt, Int. J. Pharm. 222 (2001) 19. [4] L. Yu, H. Zhang, S.X. Cheng, R.X. Zhou, H. Li, J. Biomed. Mater. Res. B. 77B (2006) 39. [5] S. Sharifpoor, B. Amsden, Eur. J. Pharm. Biopharm. 65 (2007) 336. [6] P.A. Burke, L.A. Klumb, J.D. Herberger, X.C. Nguyen, R.A. Harrell, M. Zordich, Pharm. Res. 21 (2004) 500. [7] F. Quaglia, L. Ostacolo, G. Nese, G. De Rosa, M.I. La Rotonda, R. Palumbo, G. Maglio, Macromol. Biosci. 5 (2005) 345. [8] C.G. Oster, T. Kissel, J. Microencapsul. 22 (2005) 235. [9] F. Ruchatz, P. Kleinebudde, B.W. Muller, J. Pharm. Sci. 86 (1997) 101. [10] The United States Pharmacopeia, Organic Volatile Impurities, 23 revision, Rockville, MD, USA, 1995, pp. 1746. [11] Y. Aso, S. Yoshioka, Y.P. Li, T. Terao, J. Controlled Rel. 31 (1994) 33. [12] H. Fu, L. Yu, H. Zhang, X. Zhang, S. Cheng, R. Zhuo, J. Biomed. Mater. Res. A. 81A (2007) 186. [13] B. Jeong, D.S. Lee, J.I. Sohn, Y.H. Bae, S.W. Kim, J. Polym. Sci. Polym. Chem. 37 (1999) 751. [14] L. Havlikova, L. Novakova, L. Matysova, J. Sicha, P. Solich, J. Chromatogr. A 1119 (2006) 216.
Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599 www.elsevier.com/locate/jpcs
Solvent-free preparation of caprolactone oligomer microspheres Jisun Lee, Saemi Oh, Min Kyung Joo, Byeongmoon Jeong Department of Chemistry, Division of Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea Received 30 July 2007; accepted 17 September 2007
Abstract Caprolactone (Mn3000 Da, melting point 46 1C) oligomer microspheres were prepared by an organic solvent-free process, and were investigated as a delivery system of a hydrophobic model drug (estradiol). The drug was dissolved in a polycaprolactone/poly(ethylene glycol-b-caprolactone) mixture at 50 1C, which was above the melting points of both polymers. The mixture was homogenized in water for 10 min at 50 1C, then it was quenched in the ice bath for 10 min to harden the microsphere. The polycaprolactone of the poly(ethylene glycol-b-caprolactone) coaggregated into the microsphere, and poly(ethylene glycol) formed a shell layer of the microsphere that protects the microsphere from interparticle aggregation during the hardening process of the microsphere in water. The size of microsphere could be controlled by the amount of polycaprolactone relative to poly(ethylene glycol-b-caprolactone). Estradiol release from the microsphere was investigated. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymers; D. Microstructure; D. Surface properties
1. Introduction Biodegradable microspheres have been extensively studied as a delivery system of pharmaceutical agents for the last decade [1,2]. Recently, biodegradable oligomers with the molecular weight range of 1000–5000 Da have been applied as a short-term release system of the encapsulated drug, between 1 day to 1 week [3–5]. There has been intensive research to facilitate microsphere preparation, such as the spray drying method, the solvent evaporation method, the double emulsion method, and the supercritical fluid methods [6–9]. All above methods used organic solvents during the preparation of the microsphere. The organic solvents may cause harmful side effects for humans and deteriorate the pharmaceutical agents that are sensitive to the organic solvent. Therefore, US Pharmacopoeia limits the residual organic solvent to 500 ppm for methylene chloride and 50 ppm for chloroform, which are typical solvents to prepare microspheres [10]. Therefore, removing the residual organic solvent is a serious problem Corresponding author. Tel.: +82 2 3277 3411; fax: +82 2 3277 4192.
E-mail address: [email protected] (B. Jeong). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.09.016
in practical process. As an example, to remove residual methylene chloride in microspheres containing progesterone, high vacuum was applied for 3 days at 35–55 1C [11]. Here we are reporting a caprolactone oligomer microsphere preparation method without using any organic solvent. The concept of the current method was schematically shown in Fig. 1. The polycaprolactone (PCL) and monomethoxy poly(ethylene glycol-b-caprolactone) (mPEG–PCL) mixture is in a liquid state at 50 1C which is above their melting points. It acts as a solvent of a hydrophobic drug at this temperature. The mixture was added to water at 50 1C and was mixed by the homogenizer, then it was cooled down to harden the microspheres. The PEGs of the mPEG–PCL are exposed to the surface of the microsphere and stabilize the microsphere in water. The PCLs of the mPEG–PCL coaggregate with the PCL during the hardening process of the microsphere. Several molecular parameters were investigated in a series of preliminary studies. First, when the PCL molecular weight of mPEG–PCL (5000-PCL) was less than 500, a significant interface–diffusion between microparticles was observed during the hardening process, resulting in an irregular shape of the microspheres. The low molecular weight PCL of mPEG–PCL was not
ARTICLE IN PRESS J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599
1597
Quenching PCL / mPEG - PCL
Freeze Drying
Fig. 1. Solvent-free caprolactone oligomer microsphere preparation. After melting the PCL and mPEG–PCL at 50 1C, the mixture was added to water at 50 1C, followed by sonication by a homogenizer. The PCL of the mPEG–PCL coaggregates with the PCL homopolymer to form microsphere, whereas the PEG of the mPEG–PCL forms a shell layer of the microsphere. The system was quenched to ice bath to harden the microspheres. The microsphere was harvested by centrifugation, followed by the freeze-drying process.
effectively coaggregated into the PCL microsphere and did not act as an effective surfactant to give the stability of the PCL microsphere in water. Second, mPEG–PCL (5000–1000) was fixed, and the molecular weight of the PCL was varied from 1000, 3000, and 6000 Da. The melting points of the PCLs with molecular weights of 1000, 3000, and 6000 Da were 40, 46, and 53 1C, respectively. When the PCL molecular weight was 1000, the microspheres also collapsed, and did not show spherical microsphere formation due to the soft nature of the low molecular weight PCL. Too high temperature is not desirable because some drugs might be deteriorated by the heat. The spherical microspheres were formed when the molecular weight of PCL was 3000 by the current process. Third, the cooling condition was varied between 20 and 0 1C to investigate the hardening process parameter at a given pair of PCL (MW ¼ 3000 Da) and mPEG–PCL (5000–1000). The interface–diffusion between microspheres during the hardening process of the microspheres decreased in the faster cooling process and showed a better spherical shape of the microspheres. Based on the above preliminary studies, the current research was focused on PCL (MW ¼ 3000 Da), mPEG– PCL (5000–1000), and quenching process to 0 1C for hardening of the microsphere. We varied the ratio of the amount of PCL to mPEG–PCL. Release profile of estradiol, as a model drug, from the microsphere was investigated. 2. Experimental section 2.1. Materials mPEGs with molecular weights of 5000 Da were purchased from Aldrich and used as received. Stannous
octoate, e-caprolactone, benzyl alcohol, and toluene were used as received from Aldrich.
2.2. mPEG–PCL diblock copolymer and PCL synthesis mPEG–PCL (5000–1000) diblock copolymers were synthesized by the ring-opening polymerization of e-caprolactone on the mPEGs [12,13]. To prepare the mPEG– PCL, mPEG (MW=5000 Da, 10.0 g) was dissolved in toluene (80 mL). Toluene was distilled off to a final volume of 20 mL to remove the water by azeotropic distillation. Stannous octoate (10 mL) and e-caprolactone (2.5 g) were added to the reaction mixture and stirred at 120 1C for 24 h. The reaction mixture was precipitated into diethyl ether and the residual solvent was removed under high vacuum at room temperature. The yield was about 80%. To prepare the polycaprolactone (MW=3000 Da), benzyl alcohol was used instead of mPEG as an initiator.
2.3. Characterization of mPEG–PCL The composition and molecular weight of the mPEG–PCL and PCL were investigated by 250 MHz NMR (9503DPX; Bruker) for 1H NMR (in CDCl3) and a gel permeation chromatography system (Waters 515) with a refractive index detector (Waters 410). Tetrahydrofuran was used as an eluting solvent. PEGs in the molecular weight range of 400–20000 Da were used as the molecular weight standards. Styragels HMW 6E and HR 4E columns (Waters) were used in series.
ARTICLE IN PRESS 1598
J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599
2.4. Microsphere preparation To prepare caprolactone oligomer microsphere (MS1), mPEG–PCL (5000–1000, 0.1 g) and PCL (MW=3000 Da, 0.3 g) were dissolved at 50 1C and slowly dropped to the deionized water (50 mL) at 50 1C. Then, the solution was sonicated by a homogenizer (Sonics, VCX 750) at 50 1C for 10 min, and then at 0 1C for 10 min. The microsphere was collected by centrifugation at 3000 rpm for 10 min and freeze-dried. In order to load estradiol, the drug (5.0, 10, or 15 mg) was dissolved in an mPEG–PCL and PCL mixture at the first step.
The size control of the microsphere is important because it determines the ratio of surface area to mass of the microsphere, and thus affects the release profile of the incorporated drug. The size of microsphere increased as the amount of PCL increased from 0.3 to 0.7 g at a fixed amount of mPEG–PCL (0.1 g). Fig. 2 shows the scanning electron microscopic images of the microspheres. The amount of PCL relative to mPEG–PCL is smallest for MS1, and MS3 used the largest amount of PCL. In addition, the encapsulation efficiency of the drug increased from 46.8% to 78.7%, and drug loading efficiency increased from 0.59 to 0.68 as the amount of PCL increased from 0.3 to 0.7 g (Table 1).
2.5. Scanning electron microscope The images of the microsphere were photographed by a field emission scanning electron microscope (FE-SEM, JEOL JSM35 CF). The microspheres were coated with gold for 6 min to obtain the scanning electron microscopic image. 2.6. In-Vitro drug release Five milligrams of the microsphere was put in the conical tube and 2.0 mL of release medium (PBS containing 0.1 wt.% Tween 20) was added. The vial was shaken at 100 strokes/min. in a 37 1C water bath. At a given time interval of 1 or 2 h, the tube was centrifuged at 3,800 rpm for 10 min and 1.0 mL of the medium was taken for HPLC and 1.0 mL of the fresh medium was replaced. The released amount of the drug was assayed by high-performance liquid chromatography (Waters 996/Photodiode Array Detector) [14]. 3. Results and discussion The mPEG–PCL was prepared by ring-opening polymerization of caprolactone in the presence of mPEG at 120 1C for 24 h, using stannous octoate as a catalyst [9,10]. 1 H NMR spectra show an ethylene glycol unit at 3.6 ppm, a methylene peak next to the carbonyl group of caprolactone unit at 2.2 ppm. The number average molecular weight (Mn) was 5000–740 by 1H NMR spectra and 6000 Da by gel permeation chromatography. The polydispersity index defined by Mw/Mn of the mPEG–PCL determined by the gel permeation chromatography was 1.3. mPEG–PCL (5000–1000) indicates this polymer. PCL was prepared using benzyl alcohol instead of mPEG as an initiator. The molecular weight of PCL was 3000 Da and polydispersity index was 1.2. The PCL (MW=3000 Da) and mPEG–PCL (5000–1000) have melting points of 46 and 43 1C, respectively, thus the mPEG and PCL mixture is in a liquid state at 50 1C. It acts as a solvent of a hydrophobic drug (estradiol) at 50 1C. The mixture was added to the warm water (50 1C) and mixed by a homogenizer, then quenched in an ice bath to harden the microspheres.
Fig. 2. Scanning electron microscopic images of microspheres: effect of amount of PCL relative to mPEG–PCL. The mPEG–PCL (5000–1000) was fixed at 0.1 g and PCL (MW ¼ 3000 Da) was varied between 0.3 (MS1), 0.5 (MS2), and 0.7 g (MS3). The scale bar is 10 mm.
ARTICLE IN PRESS J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1596–1599
4. Conclusions
Table 1 List of microspheres
MS1 MS2 MS3
PCL (g)
Drug loaded (mg)
e.e. (%)
l.e. (%)
0.3 0.5 0.7
5 5 5
46.8 51.2 78.7
0.59 0.64 0.68
*mPEG–PCL (5000–1000) (0.1 g) was used and the amount of PCL (MW ¼ 3000 Da) was varied between 0.3, 0.5, and 0.7. *e.e. (encapsulation efficiency) ¼ 100 drug in microsphere (mg)/drug used in preparation of microsphere (mg). *l.e. (loading efficiency) ¼ 100 drug in microsphere (mg)/microsphere (mg).
Cummulative Released Amount (%)
100 5 mg 10 mg 15 mg
80
1599
Caprolactone oligomer microspheres were prepared by coaggregation of melted PCL and mPEG–PCL in the absence of any organic solvent. The molecular weight of PCL and mPEG–PCL and cooling condition were important parameters to prepare the spherical-shaped PCL microsphere. The optimal result was obtained with PCL (MW ¼ 3000 Da), mPEG–PCL (5000–1000), and a quenching process to 0 1C. As the amount of PCL relative to mPEG–PCL increased, the size of microsphere, the encapsulation efficiency, and the loading efficiency of the hydrophobic drug increased. The model hydrophobic drug, estradiol, was released from the microsphere and the release rate was dependent on the loaded amount of the drug. Acknowledgements This work was supported by the SRC program of MOST/KOSEF through the center for Intelligent NanoBio Materials at Ewha Womans University (Grant: R11-2005008-0000-0) and Basic Research Promotion Fund (KRF2006-005-J04003).
60
40
References 20
0 0
4
8
12
16
20
24
Time (hours) Fig. 3. Model drug release profile from microspheres: Effect of drug loading. The mPEG–PCL (5000–1000) (0.1 g) and PCL (MW ¼ 3000 Da) (0.3 g) were used, and the loaded amount of model drug was varied between 5, 10, and 15 mg.
The amount of loaded drug affected the release profile of the drug from the microsphere (Fig. 3). When the incorporated amount of the drug increased from 5 to 15 mg, the percent cumulative amount (the released amount at time t (Mt) divided by the initial loaded amount (D0)) of the drug decreased, and the release profile approached the zero-order release. The loaded hydrophobic drug might tighten the microsphere core by the hydrophobic interactions, and the diffusion rate of the drug would decrease.
[1] P.B. O’Donnell, J.W. Mcginity, Adv. Drug Del. Rev. 28 (1997) 25. [2] H. Tamber, P. Johansen, H.P. Merkle, B. Gander, Adv. Drug Deliv. Rev. 57 (2005) 357. [3] R.T. Liggins, H.M. Burt, Int. J. Pharm. 222 (2001) 19. [4] L. Yu, H. Zhang, S.X. Cheng, R.X. Zhou, H. Li, J. Biomed. Mater. Res. B. 77B (2006) 39. [5] S. Sharifpoor, B. Amsden, Eur. J. Pharm. Biopharm. 65 (2007) 336. [6] P.A. Burke, L.A. Klumb, J.D. Herberger, X.C. Nguyen, R.A. Harrell, M. Zordich, Pharm. Res. 21 (2004) 500. [7] F. Quaglia, L. Ostacolo, G. Nese, G. De Rosa, M.I. La Rotonda, R. Palumbo, G. Maglio, Macromol. Biosci. 5 (2005) 345. [8] C.G. Oster, T. Kissel, J. Microencapsul. 22 (2005) 235. [9] F. Ruchatz, P. Kleinebudde, B.W. Muller, J. Pharm. Sci. 86 (1997) 101. [10] The United States Pharmacopeia, Organic Volatile Impurities, 23 revision, Rockville, MD, USA, 1995, pp. 1746. [11] Y. Aso, S. Yoshioka, Y.P. Li, T. Terao, J. Controlled Rel. 31 (1994) 33. [12] H. Fu, L. Yu, H. Zhang, X. Zhang, S. Cheng, R. Zhuo, J. Biomed. Mater. Res. A. 81A (2007) 186. [13] B. Jeong, D.S. Lee, J.I. Sohn, Y.H. Bae, S.W. Kim, J. Polym. Sci. Polym. Chem. 37 (1999) 751. [14] L. Havlikova, L. Novakova, L. Matysova, J. Sicha, P. Solich, J. Chromatogr. A 1119 (2006) 216.