Synthesis and Properties of Star-Shaped Polylactide Attached to Poly

Biomacromolecules 2003, 4, 828-834

828

Synthesis and Properties of Star-Shaped Polylactide Attached to Poly(Amidoamine) Dendrimer Qing Cai, Youliang Zhao, Jianzhong Bei, Fu Xi, and Shenguo Wang* SKLPPC, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received February 21, 2003; Revised Manuscript Received March 11, 2003

Star-shaped polylactide was synthesized by bulk polymerization of lactide with poly(amidoamine) (PAMAM) dendrimer as initiator, which was marked as PAMAM-g-PLA for simplicity. The nonlinear architecture of PAMAM-g-PLA was confirmed by gel permeation chromatograph, nuclear magnetic resonance, and differential scanning calorimetry analysis. Unlike the linear polylactide (PLA) with similar molecular weight, PAMAM-g-PLA had a higher hydrophilicity and a faster degradation rate because of shortened polymer chains and increased polar terminal endgroups due to its branch structure. The highly branched structure significantly accelerated the release of water-soluble bovine serum albumin from PAMAM-g-PLA microspheres, whereas the linear PLA with similar molecular weight exhibited an initial time lag release. This star polymer may have potential applications for hydrophilic drug delivery in tissue engineering, including growth factor and antibodies to induce tissue regeneration, by adjusting the chain lengths of PLA branches. 1. Introduction Biodegradable polymers represent a class of biomaterials with growing importance, especially in the field of biomedical applications, such as drug delivery devices and tissue engineering.1-4 Biodegradable polyesters, e.g., polylactide (PLA) and its copolymer with glycolide (PLGA), have been widely used for controlled drug delivery systems, especially for the prolonged delivery of peptides and proteins.5-7 They are also among the few synthetic polymers used as cell scaffolds in tissue engineering, because of their excellent biocompatibility.8-11 Furthermore, drug delivery has become an important aspect of tissue engineering as a growing number of compounds, including growth factors and antibodies, needed to be delivered locally to induce tissue regeneration.12-18 However, drug release from PLA and PLGA is often not satisfactory. Such systems often show far from sustained release behavior and they are usually designated as “polyphasic” or “biphasic”.19,20 To overcome “polyphasic” drug release profiles, many approaches were tried. For example, continuous release of protein could be achieved by increasing the water-uptake of the polymer or ensuring that degradation commenced almost immediately. Blends or copolymer of hydrophilic poly(ethylene glycol) (PEG) with PLA and PLGA have been reported to show accelerated drug release.21,22 Continuous release also could be obtained by using low molecular weight PLA or PLGA with high glycolidyl unit content, because of their fast degradation rates.1,23 Star or comb-shaped aliphatic polyesters of lactide can be synthesized when polymerization of lactide is performed with multifunctional initiators, such as glycerol,24 sorbitol,25 * To whom correspondence should be addressed. Phone & Fax: +8610-6258-1241. E-mail: [email protected].

Figure 1. Molecular structure of the PAMAM dendrimer (G1.0).

poly(vinyl alcohol),26 or dextran.27 A primary feature of these materials is that they can have high molecular weight but relatively short PLA chains, which leads to higher hydrophilicity and faster degradation rates in comparison with linear PLA of similar molecular weight. These branched polyesters offer a potential to improve the “polyphasic” or “triphasic” release profiles.28,29 Poly(amidoamine) (PAMAM) dendrimers are the first complete dendrimer family to be synthesized, characterized, and commercialized. They differ from linear and randomly branched polymers in that one branch point is exactly added to one repeat unit.30,31 PAMAM dendrimers, as shown in Figure 1, are synthesized by the repetitive addition of a branching unit to an amine core (typically ammonia or ethylenediamine). PAMAM dendrimers have been determined to be nonimmunogenic and to exhibit low mammalian toxicity, especially when their surface contains anionic or neutral groups, such as carboxylic or hydroxylic functionalities.32 However, there were sparse literatures on using PAMAM dendrimers as initiators to form biodegradable starshaped PLA.

10.1021/bm034051a CCC: $25.00 © 2003 American Chemical Society Published on Web 04/04/2003

Synthesis and Properties of Star-Shaped Polylactide

Figure 2. 1H NMR spectrum of purified PAMAM(G1.0) in D2O and assignment of protons.

In a previous paper, we had reported the successful synthesis of star-shaped PLA using PAMAM(G3.0) with terminal hydroxyl groups as initiator;33 however, it was shown that not only hydroxyl but also amine, even amide groups, can initiate the polymerization of lactide.34-36 Therefore, we reported here a systematic investigation of the synthesis and properties of highly branched polylactide initiated directly by amine terminated PAMAM dendrimer (PAMAM-g-PLA), as well as of the release of hydrophilic macromolecules from this polymer to evaluate its potential as protein delivery matrix. Because PAMAM dendrimers change from open, flexible scaffolding (generation ) 0-3) to semirigid container-type structures (generation ) 4-6 and above), the starburst dendrimer used in this work was generation 1 (G1.0) with eight terminal amines and a molecular weight of 1430. The flexibility of PAMAM(G1.0) facilitates the lactide monomers accessing to reaction sites. 2. Materials and Methods 2.1. Materials. Lactide was purchased from Purac (Netherlands) and purified by recrystallization from toluene twice. PAMAM dendrimer (G1.0) was synthesized and purified according to a literature method.37 Its 1H NMR spectrum is presented in Figure 2 with assignment of different protons, and the ratios of different protons were coincident with the theoretical values. PAMAM (G1.0) was rigorously dried under vacuum at 50 °C for 48 h before use. Stannous octoate (Sigma) was used as received. Toluene was dried over metallic sodium and distilled before use. Bovine serum albumin (BSA) from Sigma was used without further purification. All other reagents were of analytical grade and used without further purification. 2.2. Synthesis of PAMAM-g-PLA. Rigorously dried PAMAM, lactide (L- or D,L-) and stannous octoate were accurately weighed and introduced into a polymerization tube. The feeding ratio of lactide to PAMAM was 1000 to 1. After the system was purged with argon gas for three times, the tube was sealed under vacuum. Then the tube was immersed and kept in an oil bath thermostated at 150 °C for 60-72 h. The raw product was dissolved in chloroform, precipitated into petroleum ether and then purified by extracting with alcohol. The obtained yellowish polymer was dried under vacuum at room temperature to constant weight.

Biomacromolecules, Vol. 4, No. 3, 2003 829

Linear polylactide was synthesized in a similar way by replacing PAMAM with hexadecanol and the reaction was carried out at 130 °C for 24 h. 2.3. Characterization of PAMAM-g-PLA. Gel permeation chromatographic (GPC) analysis was performed at 35 °C with a Waters instrument equipped with Shodex KF-800 columns at a flow rate of 1 mL min-1. Chloroform served as solvent and a differential refractometer as detector. Polystyrene standards were used for calibration. 1H NMR spectra were recorded with a Bruker DMX 300 spectrometer, and D2O or CDCl3 was used as solvent. Differential scanning calorimeter (DSC) measurements were carried out at a heating rate of 10 °C min-1 with a DuPont Instrument Series 2100 thermal analyzer under N2 gas protection. The water sorption of PAMAM-g-PLA and linear PLA were measured by immersing their films with thickness about 0.2 mm in distilled water for 24 h at room temperature and calculated according to formula (Wwet - Wdry)/Wdry × 100. 2.4. Preparation of Microspheres. Blank or drug-loaded PAMAM-g-PLA and PLA microspheres were prepared by the (water-in-oil)-in-water emulsion technique at room temperature. In the case of drug-loaded microspheres preparation, a certain amount of BSA was first dissolved in 1 mL of distilled water as an internal aqueous phase and then the solution was dispersed into 10 mL of the polymer solution in dichloromethane (containing 0.05% of Span80) by ultrasonification. Then, the water-in-oil emulsion was intensively mixed with the external phase (200 mL of 1% poly(vinyl alcohol) aqueous solution containing 0.05% Tween60). After stirring for an hour and after evaporation of the solvent, the microspheres were collected by centrifugation and washed with distilled water and then lyophilized. The blank microspheres were prepared in a similar way except the internal aqueous phase contained no BSA. The morphology of the microspheres was observed with a Hitachi S-530 Scanning Electron Microscope (SEM) after gold-coating. BSA loading was determined photometrically at 278 nm after dissolution of the microspheres in tetrahydrofuran and extraction with water. 2.5. In Vitro Degradation of PAMAM-g-PLA Microspheres. 200 mg of blank microspheres of PAMAM-g-PLA and PLA were immersed in 10 mL of 0.1 M phosphate buffer solution (pH 7.4, PBS) at 37 °C with constant shaking. The buffer solution was renewed every week. At preset time intervals, the samples were recovered and dried under vacuum at room temperature to constant weight. Weight loss, molecular weight, and morphology change were determined. 2.6. In Vitro Release of BSA from the Microspheres. 100 mg of the BSA-loaded microspheres were immersed in 10 mL of 0.1M PBS (pH 7.4) and incubated at 37 °C with continuous orbital rotation at 50 cycles min-1. At predetermined time intervals, samples were collected by centrifugation (10 000 rpm), and the BSA concentration in the release medium was determined photometrically at 278 nm. 3. Results and Discussion 3.1. Synthesis and Characterization of PAMAM-gPLA. The mechanism of ring-opening polymerization of

830

Biomacromolecules, Vol. 4, No. 3, 2003

Cai et al.

Table 1. Physicochemical Characteristics of Purified PAMAM-g-PLA and PLA Polymers molecular weight (×10-4) sample

[M]/[I]a

PAMAM-g-PLLA PAMAM-g-PDLLA PLLA(3000) PLLA(70000) PDLLA(70000)

1000 1000 45 1000 1000

[η]b

(dL/g)

0.36 0.34 0.18 1.90 1.16

Mnc

Mnd

7.34 7.34 0.35 7.22 7.22

0.31g 0.33g 0.30

Mηe

Mwf

Mnf

polydispersityf

water sorptionh (%)

0.91 0.92 0.32 7.05 6.15

1.5 1.5 1.7 1.8 1.5

9.7 12.7

0.29 7.12 6.78

1.34 1.38 0.56 12.7 9.23

0.8 1.0

a [M]/[I] refers to the molar ratio of monomer to initiator. b Measured by Ubbelodhe viscometer at 30 °C using chloroform as solvent. c Theoretical number average molecular weight calculated according to yield and feeding dose. d Calculated from 1H NMR measurement. e Calculated from MarkHouwink formula ([η] ) kMR).39 f Measured by GPC and calculated using polystyrene as standards. g The data refers to the average molecular weight of each PLA branch. h Measured by immersing films with thickness about 0.2 mm in distilled water for 24 h at room temperature and calculated according to formula (Wwet - Wdry)/Wdry×100.

Figure 4. GPC profiles of purified PAMAM-g-PLLA at different polymerization times.

Figure 3. 1H NMR spectra of PAMAM-g-PLLA: (a) overall spectrum; (b) enlargement.

lactide initiated with stannous octoate has been reported in the literature.38 The polymerization may proceed via a coordinated insertion mechanism, and it has been stated that small amounts of water or hydrolyzed monomer present as impurities or compounds with hydroxyl are the real initiators. This concept has been the basis for the use of polyols in the synthesis of star-shaped polymers. Amine or amide groups were also shown to be able to initiate the lactide polymerization, but their reactivity was found weaker than hydroxyl groups. Therefore, the polymerization of lactide initiated by amine terminated PAMAM(G1.0) needed to be performed at a higher temperature (e.g., 150 °C), instead of 130 °C as lactide polymerization initiated by hexadecanol, and the resulted PAMAM-g-PLA polymers were yellowish because of amine groups liable to be oxidized under heating. All of the obtained PAMAM-g-PLA and linear PLA polymers exhibited yields above 95%, and a typical 1HNMR spectrum of the purified PAMAM-g-PLA with assignment is presented in Figure 3. It could be seen that peaks b, g, and f were attributed to PLA, which were assigned as methyl protons, methylidyne protons in the chain, or methylidyne

protons at the end of the chain adjacent to the terminal hydroxyl, respectively. Besides, several new signals could be detected, including peaks a, c, d, and e, which indicated the introduction of PAMAM. Peak c was assigned to the molecular structure of -NH-CO- adjacent to lactidyl units, and it could be extinguished by trifluoroacetyl anhydride. The presence of peak c confirmed the initiation of lactide polymerization by amine groups; but it also demonstrated that all of the amine groups could not initiate two PLA chains to propagate. Peaks d and e also could be extinguished by trifluoroacetyl anhydride, and it was considered that they attributed to the terminal hydroxyl of PLA chain and amide of PAMAM, respectively. However, it was difficult to detect and assign the various methylene protons in PAMAM because of PAMAM’s poor solubility in CDCl3, which made the signals unobvious. According to the assignment, the number average molecular weight (Mn) of PLA branches could be estimated by dividing the integration of peak g by that of peak f, because the integration of peak f closely related to the number of PLA chains. The Mn of all of the obtained PAMAM-g-PLA and PLA were calculated and are presented in Table 1. With the GPC technique, the molecular weight and molecular weight distribution (polydispersity) of PAMAMg-PLA at different polymerization times were measured, and the results are shown in Figure 4. A GPC profile of multipeaks was observed for the reaction product of 12 h, when the yield was only about 65%. The peaks moved to higher molecular weight and collapsed to monomodal peak with prolonged reaction time. Kim40 had reported that not all OH groups of Pentaerythritol initiated lactide polymerization at low monomer/initiator ratio (e.g., [M]/[I] below 32) due to steric hindrance, whereas no unreacted Pentaerythritol hydroxyl groups were detected if the [M]/[I] was



Figure 6. Degradation behaviors of different polymer microspheres under pH 7.4 at 37 °C: (a) weight average molecular weight (Mw, measured by GPC) changing with time; (b) weight loss changing with time. Figure 5. Thermal behavior of purified PAMAM-g-PLLA and PLLA(3000): (a) PAMAM-g-PLLA, second run; (b) PLLA(3000), first run; (c) PLLA(3000), second run.

increased. In our case, a possible explanation for the appearance of multi GPC peaks at low monomers conversion could be the formation of oligomers with different molecular weights at different initiating sites (amine or amide groups) due to steric hindrance. With the chain propagation, the difference between different PLA chains decreased, and monomodal GPC profiles were obtained. The polydispersity of the PAMAM-g-PLA was estimated as 1.5 according to polystyrene standards as shown in Table 1. The yield of lactide initiated by PAMAM could reach above 95% by reacting at 150 °C for 72 h, then the theoretical Mn of the resulted PAMAM-g-PLA was calculated to be about 7 × 104 according to the feeding dose. In comparison with linear PLA with similar Mn, it was noted that the inherent viscosity of PAMAM-g-PLA was much smaller than that of corresponding linear PLA. Normally, branched polymers should have smaller hydrodynamic volume (Vd) than linear polymers of similar molecular weight; therefore, the significant difference in inherent viscosity between PAMAM-g-PLA and the corresponding linear PLA demonstrated that the PAMAM-g-PLA should have a highly branched structure. On the other hand, the molecular weight of PAMAM-g-PLA measured by GPC was also much lower than that of corresponding linear PLA. One explanation for this was their different molecular structure, and the other reason might be that the hydrophobic polystyrene standards were not an appropriate choice for spherical hydrophilic PAMAM conjugates. From Table 1, the average Mn of each PLA branch was estimated to be about 3000 from 1H NMR. There were 28 in total of initiating sites in the G1.0 PAMAM dendrimer; that is, amine could initiate lactide polymerization twice and amide once. The overall molecular weight of PAMAM-gPLA could not match to the theoretical value if it was calculated by timing 3000 with 28. As analyzed above, however, not all of the amine groups could initiate lactide polymerization twice because of the steric hindrance, and then it could be inferred that only about 20 of the total (28) initiating sites carried PLA branches. Also, it was interesting to find that it was consistent with the initiation of one PLA chain per primary amine and per amide. The PAMAM-g-PLLA was also subjected to DSC measurement to evaluate its thermal behavior. As shown in Figure 5, the linear PLLA(3000) had a crystallization temperature (Tc) at 103.5 °C and a melting temperature (Tm)

at 149.4 °C. According to its melting enthalpy (∆Hm ) -48.2J/g) and the melting enthalpy of 100% crystallized PLLA (-93J/g),41 the degree of crystallinity of linear PLLA(3000) was estimated as 51.8%. However, no crystallization could be detected for PAMAM-g-PLLA although its PLLA chains also had a molecular weight of 3000. Only a glass transition attributed to PLA chains was observed around 52.1 °C. This difference should result from their different molecular architectures. In the highly branched PAMAM-gPLLA, PLLA chains were attached to a PAMAM core; therefore, chain movements were hindered and their crystallizability was weakened, whereas no such steric hindrance existed in linear PLLA to diminish its crystallizability. 3.2. In Vitro Degradation of PAMAM-g-PLA Microspheres. 3.2.1. Molecular Weight Change and Weight Loss. The degradation behaviors of blank PAMAM-g-PLA and linear PLA microspheres were investigated by immersing them in pH 7.4 PBS and thermostated at 37 °C. Their molecular weights were monitored by GPC, and weight loss was measured and profiled as functions of time (Figure 6). The degradation rate of PDLLA was obviously faster than that of the corresponding PLLA polymer, which was due to the different morphology between PDLLA and PLLA, and that PDLLA was an amorphous and PLLA(70000) was a semicrystalline polymer with a Tm at 165.4 °C and a degree of crystallinity about 24.2%. On the other hand, the PAMAM-g-PDLLA also showed a faster degradation rate than PAMAM-g-PLLA, although both were amorphous before hydrolysis. Evaluated by DSC for the hydrolyzed specimens, however, the original amorphous PAMAM-gPLLA was found changing into a weak crystalline state with degradation, which led to the slower degradation rate of PAMAM-g-PLLA microspheres than that of PAMAM-gPDLLA. A Tm around 82 °C was detected, and the degree of crystallinity was estimated only about 3.7% for the PAMAM-g-PLLA microspheres after being hydrolyzed for 14 weeks. Besides, the Tg of PAMAM-g-PLLA was found decreasing to 35 °C compared to the original 52 °C because of hydrolysis. On the other hand, the weight loss of PAMAM-g-PLAs was higher than that of corresponding linear PLAs, although the molecular weight of PAMAM-g-PLAs decreased slower. It is normal that Mw decreases faster for linear PLA than for star PLA, because an ester cleavage along a PLA segment decreases little the overall Mw of a star PLA, whereas it will decrease by half the Mw of linear PLA. Different from the change in Mw, however, the weight loss for the linear

832


Cai et al.

Figure 7. Comparison of morphology change of various microspheres prepared from different polymers between before (upper) and after (lower) degradation: (a) PLLA(70000), (b) PAMAM-g-PLLA, (c) PDLLA(70000), (d) PAMAM-g-PDLLA, (e) PLLA(70000), 14 weeks, (f) PAMAMg-PLLA, 2 weeks, (g) PDLLA(70000), 14 weeks, (h) PAMAM-g-PDLLA, 2 weeks.

polymer was much smaller than that for the star polymer. The faster weight loss of PAMAM-g-PLA than that of the corresponding linear PLA was considered closely relating to the much shorter PLA chains in the star polymer. It has been reported that PLA segments could dissolve in water if their molecular weight was below 1000,42 and the bulk degradation mechanism of this aliphatic polyester (accompanying with slow weight loss) could change into surface degradation (with faster weight loss), if the PLA chains were short enough.29 3.2.2. Morphology Change. As shown in Figure 7, the smooth surface of the PAMAM-g-PLA microspheres had become rough and uneven after only two weeks degradation in vitro. However, no significant morphology change was observed for either PLLA(70000) or PDLLA(70000) microspheres even after 14 weeks of degradation. On one hand, it was their different degradation rates causing this difference. On the other hand, the nonlinear structure of the starlike graft PLA was expected to effectively promote the permeability of water into the polymer matrix because of an increase of hydrophilicity resulting from polar terminal endgroups. Therefore, the spherical shape was destroyed by the uptake of water. 3.2.3. Degradation Mechanism. Aliphatic polyesters could be degraded by ester bond and chain scission into water-soluble oligomers. This process could occur randomly along the polyester chain. For the degradation of PAMAMg-PLA, the same degradation mechanism seemed to occur. From the 1H NMR analysis (Figure 8), peak c that was assigned to the hydrogen atom of -NH-CO- groups adjacent to lactidyl units could be seen weakened and broadened with degradation and almost disappeared after 30 weeks of degradation. Moreover, the average Mn of PLLA chains were calculated to have decreased from the original 3100 to 1800, 1000, and 600 respectively for 8, 14, and 30 weeks of degradation. As stated above, PLA oligomers dissolved in water if their molecular weight was lower than 1000, and then it was coincident to find that the onset of weight loss was between week 10∼14 as shown in Figure 6. From these data, it was concluded that the accelerated weight loss was a result of PLA chains being cleaved from the PAMAM cores and dissolving in water, or on the other hand, it was caused by hydrophilic PAMAM cores carrying some lactidyl residues to dissolve in water. These observa-

Figure 8. Change of 1H NMR spectra of PAMAM-g-PLLA with degradation time: (a) 0 week, (b) 8 weeks, (c) 14 weeks, (d) 30 weeks. Table 2. Preparation of the Microspheres of the Star-Shaped PAMAM-PLA and Linear PLA Containing BSA sample PAMAM-g-PLLA PAMAM-g-PDLLA PLLA(70000) PDLLA(70000)

drug loading (%) encapsulation (%) size (µm) 6.4 5.8 7.2 6.1

45.5 40.7 78.1 65.4

2∼30 2∼30 5∼50 5∼50

tions confirmed that PAMAM-g-PLA was degraded in two steps. First, ester groups were hydrolyzed and PLA chains were cleaved randomly, and then short PLA chains were detached from PAMAM cores. Basically, this degradation process was similar to the reported degradation mechanism for brush-like branched polyesters, such as PLA or PLGA grafting to dextran backbone.28 3.3. In Vitro Release of BSA. BSA-loaded microspheres of different materials were prepared with the W/O/W emulsion method. Drug loading was about 6%. As shown in Table 2, the encapsulation efficiency was lower for PAMAM-g-PLA microspheres than for PLA microspheres. SEM observation showed that all of the BSA-loaded microspheres possessed nonporous and smooth surfaces, and the average size was found to be below 50 µm. The in vitro release of BSA from microspheres prepared from PAMAM-g-PLAs was significantly quickened than that from microspheres of corresponding linear PLAs, as shown in Figure 9. Unlike the linear PLA’s, which only demon-



4. Conclusions

Figure 9. Release profiles of BSA from microspheres prepared from different polymers.

strated 10 or 20% release of BSA during a period of 20 days, both the PAMAM-g-PLA microspheres showed fast release of BSA, and 60∼80% of encapsulated BSA was released within 1∼2 days. Drug release from PLA or PLGA is generally controlled by both drug diffusion and polymer erosion. Usually, an initial phase can be observed that the release of protein occurs predominantly by diffusion of the drug through aqueous pores generated in the dosage form.43 The aqueous channels are generated by leakage of the drug at or near the surface of the delivery matrix or by osmotic force. However, the drug within the body of the delivery matrix cannot be released until the degradation of the polymer, which is associated with generation of micropores in the degrading and enhanced water uptake.44,45 As shown in Figure 6, the degradation of linear PLLA(70000) and PDLLA(70000) was slow that no significant weight loss was detected within several weeks, and their hydrophilicities were also poor. Thus, the release of hydrophilic BSA from them only could continue until their significant degradation occurred after the initial burst release. For the highly branched PAMAM-gPLA, however, their hydrophilicity was improved for the shortened PLA chains in comparison with linear PLA of similar molecular weight and for the increased density of hydrophilic terminal groups and its peculiar location at the outer layer of molecules. These effects increased the drug release rate from PAMAM-g-PLA. Furthermore, the sizes of PAMAM-g-PLA microspheres were found to be smaller than those of corresponding linear PLA as presented in Table 2, which would also lead to faster drug release rate from PAMAM-g-PLA microspheres because of their larger surface areas. Although the drug release profiles from PAMAM-g-PLA microspheres were not so satisfactory because of large initial burst release, it was believed that this large initial burst release could be reduced by increasing the length of the PLA chains attached to PAMAM cores. As reported in the literature,46 Breitenbach and co-workers had obtained a sustained release of BSA from brush-like polyester grafted dextran copolymers by modulating the molecular structure. Therefore, the release profile from starburst polyester grafted PAMAM also could be tailored by adjusting the length of the PLA chains or by changing the branch composition, and then the highly branched copolymers might be a potential matrix for realizing sustained drug release.

It had been proved that PAMAM was a potential initiator for lactide polymerization to form a kind of starburst polylactide. With similar molecular weights, PAMAM-gPLA had a faster degradation rate than linear PLA because of its shortened polymer chains. The in vitro release of BSA from microspheres prepared from PAMAM-g-PLA was significantly accelerated in comparison with that from linear PLA microspheres because of the increased hydrophilicity. Therefore, this star-shaped PLA could be a potential biomaterial for the controlled release of proteins and peptides in tissue engineering. To obtain satisfactory and continuous release profiles from this kind of star polyester, a further investigation of the polymer chains on the in vitro release of BSA is underway. Acknowledgment. This projected is supported by National Basic Science Research and Development Grants (973) (Project Nos. G1999054305 and G1999054306). References and Notes (1) Herrero-Vanrell, R.; Refojo, M. F. Biodegradable microspheres for vitreoretinal drug delivery. AdV. Drug DeliVery ReV. 2001, 52, 5-16. (2) Saltzman, W. M. Cell interactions with polymers. In Principles of tissue engineering; Lanza, R. P., Langer, R., Chick, W. L., Eds.; R. G. Landes Inc.: 1997; pp 225-246. (3) Freed, L. E.; Vunjak-Novakovic, G.; Langer, R. Biodegradable polymer scaffolds for tissue engineering. Bio/Technology 1994, 12, 689-693. (4) Wang, S. G. Synthesis and biomedical applications of biodegradable polymer. Chem. (Chin.) 1997, 2, 45-47. (5) Li, S. M.; Vert, M. Biodegradable polymers: polyesters. In Proceedings of “Global Chinese Symposium on Biomaterials and Controlled Release”; Taipei, Taiwan, 1999; pp 333-355. (6) Richard, L. D. Clinical applications and update on the poly(R-hydroxy acids). In Biomedical applications of synthetic biodegradable polymers; Hollinger, J. O., Ed.; CRC Press: Boca Raton, FL, 1995; pp 17-31. (7) Cai, Q.; Bei, J. Z.; Wang, S. G. Study on the biocompatibility and degradation behavior of poly(L-lactide-co-glycolide) in vitro and in vivo. J. Funct. Polym. (Chin.) 2000, 13, 249-254. (8) Anderson, J. M.; Shive, M. S. Biodegradation and biocompatibility of PLA and PLGA microspheres. AdV. Drug DeliVery ReV. 1997, 28, 5-24. (9) Athanasiou, K. A.; Niederauer, G. G.; Agrawal, C. M. Sterilization, toxicity, biocompatibility and clinical applications of polylactic acid/ polyglycolic acid copolymers. Biomaterials 1996, 17, 93-102. (10) Wang, C. Y.; Yan, Q. J.; Hou, W. P.; Guo, X. M.; Zhao, Q.; Duan, C. M.; Shao, C. X.; Ye, B. L.; Bei, J. Z.; Wang, S. G.; Shi, G. X.; Cai, Q. Construction of tissue-engineered urinary bladder using biodegradable polymer matrixes as cell scaffolds. Chin. J. Biomed. Eng. (Engl. Ed.) 2001, 10, 70-73. (11) Hutmacher, D. W. Scaffolds in tissue engineering bone and cartilage. Biomaterials 2000, 21, 2529-2534. (12) Hubbel, J. A. Biomaterials in tissue engineering. Biotechnology (NY) 1995, 13, 565-576. (13) Soppimath, K. S.; Aminabhavi, T. M.; Kulkarni, A. R.; Rudzinski, W. E. Biodegradable polymeric nanoparticles as drug delivery devices. J. Control. Release 2001, 70, 1-20. (14) Tabata, Y. The importance of drug delivery systems in tissue engineering. Pharm. Sci. Technol. Today 2000, 3, 80-89. (15) Sheridan, M. H.; Shea, L. D.; Peters, M. C.; Mooney, D. J. Bioabsorbable polymer scaffolds for tissue engineering capable of sustained growth factor delivery. J. Control. Release 2000, 64, 91102. (16) Baldwin, S. P.; Saltzman, W. M. Materials for protein delivery in tissue engineering. AdV. Drug DeliVery ReV. 1998, 33, 71-86. (17) Whang, K.; Goldstick, T. K.; Healy, K. E. A biodegradable polymer scaffold for delivery of osteotropic factors. Biomaterials 2000, 21, 2545-2551. (18) Wang, S. G.; Hou, J. W.; Bei, J. Z.; Zhao, Y. Q. Tissue engineering and peripheral nerve regeneration (III) - sciatic nerve regeneration with PDLLA nerve guide. Sci. Chin. (Ser. B) 2001, 44, 419-426.

834


(19) Sanders, L. M.; Kent, J. S.; McRea, G. I.; Vickery, B. H.; Tice, T. R.; Lewis, D. H. Controlled release of a luteinizing hormone-releasing hormone analogue from poly(d, l-lactide-co-glycolide) microspheres. J Pharm. Sci. 1984, 73, 1294-1297. (20) Miyajima, M.; Koshika, A.; Okada, J.; Ikeda, M. Effect of polymer/ basic drug interaction on the two-stage diffusion-controlled release from a poly(L-lactic acid) matrix. J. Control. Release 1999, 61, 295304. (21) Cleek, R. L.; Ting, K. C.; Eskin, S. G.; Mikos, A. G. Microparticles of poly(DL-lactic-co-glycolic acid)/poly(ethylene glycol) blends for controlled drug delivery. J. Control. Release 1997, 48, 259-268. (22) Li, Y. X.; Kissel, T. Synthesis and properties of biodegradable ABA triblock copolymers consisting of poly(L-lactic acid) or poly(L-lacticgo-glycolic acid) A-blocks attached to central poly(oxyethylene) B-blocks. J. Control. Release 1993, 27, 247-257. (23) Herrero-Vanrell, R.; Refojo, M. F. Biodegradable microspheres for vitreoretinal drug delivery. AdV. Drug DeliVery ReV. 2001, 52, 5-16. (24) Arvanitoyannis, I.; Nakayama, A.; Kawasaki, N.; Yamamoto, N. Novel star-shaped polylactide with glycerol using stannous octoate or tetraphenyl tin as catalyst: 1. Synthesis, characterization and study of their biodegradability. Polymer 1995, 36, 2947-2956. (25) Arvanitoyannis, I.; Nakayama, A.; Psomiadou, E.; Kawasaki, N.; Yamamoto, N. Synthesis and degradability of a novel aliphatic polyester based on L-lactide and sorbitol. Polymer 1996, 37, 651660. (26) Breitenbach, A.; Kissel, T. Biodegradable comb polyesters: Part 1. Synthesis, characterizarion and structural analysis of poly(lactide) and poly(lactide-co-glycolide) grafted onto water-soluble poly(vinyl alcohol) as backbone. Polymer 1998, 39, 3261-3271. (27) Li, Y. X.; Nothnagel, J.; Kissel, T. Biodegradable brush-like graft polymers from poly(D, L-lactide) or poly(D, L-lactide-co-glycolide) and charge-modified, hydrophilic dextrans as backbone - Synthesis, characterization and in vitro degradation properties. Polymer 1997, 38, 6197-6206. (28) Li, Y. X.; Volland, C.; Kissel, T. Biodegradable brush-like graft polymers from poly(D, L-lactide) or poly(D, L-lactide-co-glycolide) and charge-modified, hydrophilic dextrans as backbone - in-vitro degradation and controlled released of hydrophilic macromolecules. Polymer 1998, 39, 3087-3097. (29) Breitenbach, A.; Pistel, K. F.; Kissel, T. Biodegradable comb polyesters: Part II. Erosion and release properties of poly(vinyl alcohol)-g-poly(lactic-go-glycolic acid). Polymer 2000, 41, 47814792. (30) Tomalia, D. A.; Esfand, R. Edndrons, dendrimers and dendrigrafts. Chem. Ind. 1997, 11, 416-420. (31) Tomalia, D. A.; Dvornic, P. R. Dendritic polymers: divergent synthesis. In Polymeric materials encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp 1814-1830. (32) Roberts, J. C.; Bhalgat, M. K.; Zera, R. T. Preliminary biological evaluation of polyamidoamine (PAMAM) starburst dendrimers. J. Biomed. Mater. Res. 1996, 30, 53-65.

Cai et al. (33) Zhao, Y. L.; Cai, Q.; Jiang, J.; Shuai, X. T.; Bei, J. Z.; Chen, C. F.; Xi, F. Synthesis and thermal properties of novel star-shaped poly(L-lactide)s with starburst PAMAM-OH dendrimer macroiniator. Polymer 2002, 43, 5819-5825. (34) Brode, G. L.; Koleske, J. V. Lactone polymerization and polymer properties. J. Macromol. Sci. -Chem. 1972, A6, 1109-1144. (35) Saotome, K.; Kodaira, Y. Polymerization of δ-valerolactone and preparation of a thermostable derivative from its polyester. Makromol. Chem. 1965, 84, 41-52. (36) Dubois, Ph.; Dege´e, Ph.; Je´roˆme, R.; Teyssie´, Ph. Macromolecular engineering of polylactones and polylactides, 8. Ring-opening polymerization of -carpolactone initiated by primary amines and trialkylaluminum. Macromolecules 1992, 25, 2614-2618. (37) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. A new class of polymers: starburstdendritie macromolecules. Polym. J. 1985, 17, 117-132. (38) Kricheldorf, H. R.; Kreiser-Saunders, I.; Stricher, A. Polylactones 48. SnOct2-Initiated polymerizations of lactide: A mechanistic study. Macromolecules 2000, 33, 702-709. (39) Schindler, A.; Harper, P. Polylactide II. Viscosity-Molecular weight relationships and unperturbed chain Dimensions. J. Polym. Sci. Polym. Chem. Ed. 1979, 17, 2593-2599. (40) Kim, S. H.; Han, Y. K.; Kim, Y. H.; Hong, S. I. Multifunctinal initiation of lactide polymerization by stannous octoate/Pentaerythritol. Makromol. Chem. 1992, 193, 1623-1631. (41) Tsuji, H.; Mizuno, A.; Ikada, Y. Enhanced crystallization of poly(L-lactide-co- -caprolactone) during storage at room temperature. J. Appl. Polym. Sci. 2000, 76, 947-953. (42) Park, T. G. Degradation of poly(lactic-co-glycolic acid) microspheres: effect of copolymer composition. Biomaterials 1995, 16, 1123-303. (43) Siegel, R. A.; Langer, R. Mechanistic studies of macromolecular drug release from macroporous polymers. II: Model for slow kinetics of drug release. J. Control. Release 1991, 14, 153-167. (44) Shah, S. S.; Cha, Y.; Pitt, C. G. Poly(glycolic acid-co-DL-lactic acid): diffusion or degradation controlled drug delivery? J. Control. Release 1992, 18, 261-270. (45) Kissel, T.; Li, Y. X.; Volland, C.; Go¨rich, S.; Koneberg, R. Parenteral protein delivery systems using biodegradable polyesters of ABA block structure, containing hydrophobic poly(lactide-co-glycolide) A blocks and hydrophilic poly(ethylene oxide) B blocks. J. Control. Release 1996, 39, 315-326. (46) Breitenbach, A.; Li, Y. X.; Kissel, T. Branched biodegradable polyesters for parenteral drug delivery systems. J. Conrol. Release 2000, 64, 167-178.

BM034051A

Synthesis and Properties of Star-Shaped Polylactide Attached to Poly

Recommend Documents