Preparation and in vitro release profiles of drug-eluting controlled biodegradable polymer coating stents

Colloids and Surfaces B: Biointerfaces 73 (2009) 199–206

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Preparation and in vitro release profiles of drug-eluting controlled biodegradable polymer coating stents Chang-Jiang Pan a , Jia-Ju Tang b , Ya-Jun Weng b , Jin Wang b , Nan Huang b,∗ a b

Key Laboratory of Biorheological Science and Technology (Chongqing University), Ministry of Education, chongqing, 400044, China Institute of Biomaterials & Surface Engineering, SouthWest Jiaotong University, Chengdu, 610031, China

a r t i c l e

i n f o

Article history: Received 17 March 2009 Received in revised form 15 May 2009 Accepted 19 May 2009 Available online 27 May 2009 Keywords: Drug-eluting stent Rapamycin Curcumin Emodin Heparin Release

a b s t r a c t In the present study, the different drug-eluting controlled biodegradable polymer coatings were fabricated on stainless steel stents. The coatings were not only uniform and smooth but also had excellent mechanical property. The drug release profiles of drug-eluting stents were studied in detail in this study. Depending on the drug type, different drug-eluting stents exhibited different drug release profile. There were two basic release profiles for different drug-eluting stents, i.e., two-phase release profile with burst release or linear release profile without burst release. Incorporating heparin in the rapamycin or curcumin eluting stents can improve the average drug release rate of both and the burst release of rapamycin. The average drug release rate increased with the increase of drug loading but was not proportional to increase of the ratio of drug/polymer. Fabricating the control release layer on rapamycin-eluting stent surface can prevent the burst release of rapamycin and prolong the release period of rapamycin. All results showed that the drug release profile of drug-eluting stents depends on many parameters including drug type, ratio of drug/polymer, and drug carrier properties. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The stent, first introduced in 1960s by Dotter [1], which is a temporary or permanent medical device designed to provide the scaffold to maintain or increase the lumen of blood vessel, has received extensive attention in recent years because of the high frequency of restenosis after percutaneous transluminal coronary angioplasty (PTCA). Metallic stent insertion, which can provide permanent support of diseased coronary artery, eliminates the acute recoil and remodeling of blood vessel, thus significantly reduces the restenosis of PTCA. Despite a high initial success, a series of negative biological reactions happened on stent surface after metallic stent implantation, rendering in stent restenosis (ISR) [2–6]. So far, in stent restenosis still remains the major drawback of stent implantation occurring to 20–30% of patients after insertion for 6 months [7]. In the past decades, many surface modifications, such as deposition of biocompatible organic or inorganic coatings, grafting bioactive molecules, loading drugs, etc., were developed to reduce in stent restenosis. Among all these methods, drug-eluting stent holds a great promise to treat the ISR because the loading drug can release a relative long time to provide a lasting treatment for injured or diseased blood vessel.

∗ Corresponding author. Tel.: +86 28 87600625; fax: +86 28 87600625. E-mail address: [email protected] (N. Huang). 0927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2009.05.016

In recent years, the drug-eluting stents (DES) are rapidly replacing the bare metallic stents in the daily practice of interventional cardiology because of their short-to-mid-term safety and efficacy in reducing the ISR. Heparin-coated stent represents the first step towards loading medications on stents [8]. The CypherTM stent (rapamycin-eluting stent) and TAXUSTM (paclitaxel-eluting stent) are now widely used in clinic to treat cardiovascular patients. However, both rapamycin and paclitaxel cannot enhance the endothelium healing of blood vessel because it can inhibit endothelial cell proliferation when preventing smooth muscle cell migration and growth. So, it is necessary to explore new drugs for developing the drug-eluting stents. Both curcumin (Fig. 1) and emodin (Fig. 1) also exhibit in vitro anti-proliferation and enhance endothelial cell growth to some degree confirmed by other researchers [9–14]. Heparin has good anti-thrombogenicity. So, local released these drugs or fabricating mixing drug-eluting stents may not only prevent thrombosis formation and smooth muscle cell proliferation but also enhance endothelium healing, leading to decreased in stent restenosis significantly. It is well known that the process of ISR after stent placement is the results of a complex interplay of several implant-induced biological processes [15]. The temporal drug release profile of any drug candidate should be tailored to be commensurate with the timing of the mechanism being targeted [16]. Also, the drug release profile of rapamycin and paclitaxel eluting stents is related to healing and hypersensitivity reactions [17]. So, the drug release profile of drugeluting stents is undoubtedly important because the drug release

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Fig. 1. The chemical structures of the drugs applied in this study.

rate may affect the subsequent neointimal formation after stent implantation [18]. In this study, using PLGA as drug carrier, the aforementioned drugs (rapamycin, curcumin, emodin, heparin) were chosen as the model drugs to investigate the release profile of drug-eluting stents developed in our lab. The rapamycin and curcumin-eluting stents were in combination with heparin to improve their blood compatibility. The effect of heparin on rapamycin and curcumin release profile was investigated in this work. In order to overcome the burst release of rapamycin and prolong its released time, the controlled released layer (PLGA with a molecular weight of 20,000 Da) was fabricated onto the rapamycin-eluting stent and the release profile was also examined. 2. Materials and methods 2.1. Materials, drugs, and reagents The bare 316LVM stainless steel stents were produced by laser cutting followed by electrochemical polishing. The PLGA resins were purchased from Sichuan Zhuxin biomaterials Co, Ltd. Table 1 summarizes the details of the polymers used in this study. Rapamycin (high-performance liquid chromatography (HPLC) purity, >98.3%) was purchased from Institute of Microorganism, Fujian Province, China. Curcumin, emodin and heparin were

Table 1 Details of the polymers used in this study. Polymers PLGA 85/15 PLGA 75/25 a

Reported by supplier.

M (Da)a 95,800 20,000

DLLA/GA 85/15 75/25

obtained from Shanghai YouSi biotechnology Co., Ltd, China. The purity of curcumin and emodin was above 98% (HPLC grade) and the titer of heparin was above 150 IU/mg. The reagents (methanol, water, acetonitrile, glacial acetic acid) purchased from Chengdu KeLong Chemical Co., Ltd were high-performance liquid chromatography (HPLC) grade. All other reagents in this study were analytical grade if not specified. 2.2. Fabrication of the drug-eluting stents The preparation of drug-eluting stents was based on the method described in our previous work [19]. The stainless steel stents were cleaned carefully by acetone, ethanol and water in sequence, and then kept in vacuum to dry for 48 h. Spraying solutions were prepared by dissolving polymer (PLGA) and drug in the corresponding solvents at room temperature. The solution was then sprayed onto the cleaned stents. All details of the drug-eluting stents are summarized in Table 2. The drug loading were calculated from stent coating weight and the drug weight percent in stent coating. The coating weight was obtained from the difference of the pre- and post-sprayed stent weight. 2.3. Preparation of bi-layer drug-eluting stent The schematic structure of the coating of bi-layer drug-eluting stent was shown in Fig. 2. The bi-layer drug-eluting stent was prepared using the same method described in Section 2.2. After sprayed the inner coating on stent surface and dried in vacuum, the outer layer was sprayed onto the inner layer surface using the same method. The inner coating contained about 316 ␮g rapamycin and the outer coating had about 30 ␮g rapamycin. The control drugeluting stents, which consisted of only inner coating and out layer coating, were also prepared by the same method.

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Table 2 The prepared drug-eluting stents and corresponding informations* . Samples

Drug

Drug/Polymer(w/w)

Total drug (␮g)

Drug/mm−2 (␮g/mm−2 )

R15 R25 R40 C10 C20 C35 E10 E20 E30 H10 H20 H30 RH37 RH55 RH73 CH37 CH55 CH73

Rapamycin Rapamycin Rapamycin Curcumin Curcumin Curcumin Emodin Emodin Emodin Heparin Heparin Heparin Rapamycin/Heparin = 3/7 Rapamycin/Heparin = 5/5 Rapamycin/Heparin = 7/3 Curcumin/Heparin = 3/7 Curcumin/Heparin = 5/5 Curcumin/Heparin = 7/3

15/85 25/75 40/60 10/90 20/80 35/65 10/90 20/80 30/70 10/90 20/80 30/70 35/65 35/65 35/65 35/65 35/65 35/65

150 250 400 140 280 490 100 200 300 100 200 300 105(R),245(H) 175(R),175(H) 245(R),105(H) 147(C),343(H) 245(C),245(H) 343(C),147(H)

1.25 2.08 3.33 1.17 2.33 4.08 0.83 1.67 2.50 0.83 1.67 2.50 0.88(R), 2.04(H) 1.46(R),1.46(H) 2.04(R),0.88(R) 1.225(C),2.86(H) 2.04(C),2.04(H) 2.86(C),1.225(H)

*

The drug carrier of all drug-eluting stents in this table is PLGA 95,800.

2.5. Drug release and measurement

Fig. 2. Diagrams of bi-layer rapamycin-eluting stent. The outer layer is PLGA 20,000 coating containing 30 ␮g rapamycin. The inner layer is PLGA 95,800 coating containing 316 ␮g rapamycin.

2.4. Surface morphology of the drug-eluting stents The surface morphologies of the drug-eluting stents were examined by optical microscopy (OM, Motic three-dimensional microscopy, Chengdu, China) and scanning electron microscopy (SEM, Quanta200, Philipis). Some OM and SEM images are shown in Fig. 3. As seen, the coatings on stent surfaces are very uniform and smooth. No webbings and “bridges” could be observed between the stent struts. According to our previous study [19–21], the stent coating can withstand the strains imparted during mounting onto angioplasty balloon and expansion process. No delamination and destruction of the PLGA coating on the stent were found based on the OM and SEM images.

Considering the solubility of different drugs, different release media were used. The selection of the release medium was based on adequate solubility while trying to use as much buffer as was permissible. For heparin, PBS buffer (pH = 7.4) was used because the drug has sufficient solubility in the buffer. For rapamycin, curcumin and emodin, a mixture of PBS buffer and methanol (VPBS :Vmethanol = 90:10, pH = 7.4) was used for the release medium [22]. The stent sample was incubated in 2.0 ml capped tube containing 1.5 ml release medium at 37 ◦ C. The capped tube was being shaken for about 120 rev min−1 . At selected time, the incubation medium was completely removed for analysis and replaced with the fresh medium. Detection and quantitative analysis of drugs (rapamycin, curcumin, emodin) were performed using HPLC System (Waters 2695 separations module and Waters 2487 dual  absorbance detector), equipped with a 4.6–250 mm ZORBAX Eclipse C-18 column (particle size: 5 ␮m). The released heparin was measured using the Azure A method described elsewhere [23]. This method is based on the measurement of the metachromatic activity of heparin with the dye, Azure A. Azure A was dissolved in PBS buffer and 100 ␮l of this solution was added to 100 ␮l of the heparin sample in 96-well culture plate.

Fig. 3. The OM and SEM images of some drug-eluting stents. (a) bare stainless steel stent; (b) and (h),curcumin-eluting stent; (c), emodin-eluting stent; (d)and (e) rapamycineluting stent; (f) heparin-eluting stent; (g) curcumin-heparin eluting stent. All images show uniform and smooth coatings on bare stainless steel stents.

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For determination of the calibration curve, using a heparin preparation of known specific activity (␮g ml−1 ), the same procedure was performed. The optical density of the solution was measured with a spectrophotometer at 590 nm, and the extinction was related to the heparin concentration. For bi-layer drug-eluting stent, rapamycin/heparin drug-eluting stent, curcumin/heparin drug-eluting stent, a mixture of PBS buffer and methanol (VPBS :Vmethanol = 90:10, pH = 7.4) was used as the release medium. The drug measurement and analysis were based on the above methods. 2.6. In vitro biodegradation experiment The loss weight rate of the PLGA film was investigated according to the method elsewhere [24]. Briefly, the PLGA films with the same weight and thickness were individually incubated in 20 ml of PBS medium at 37 ◦ C and PH7.4. The medium was shaken and changed every week. The loss weight rate was calculated based on the following equations. Each reported value was average of data of triplicate specimens. W0 − Wd × 100% Loss weight rate : WL = W0 Where Wd is the weight of dried polymer film and W0 is the initial weight of PLGA film. 2.7. Statistical analysis All released data were expressed as the average value of three same stent samples. 3. Results and discussions The release kinetics of drug from drug-eluting stent is generally governed by three parts of parameters including the coating (coating thickness, materials properties, ratio of drug/polymer, et al.), drug (for example, drug properties, drug diffusion coefficient, etc.) and process parameters. Among all these factors, the ratio of drug/polymer, coating thickness and drug diffusion coefficient may be the most important factors which affect the drug release profile. In general, there are two dominant mechanisms, one is the diffusion-controlled release mechanism and another is the degradation-controlled mechanism in which the drug release depends on the biodegradable behavior of polymer [25]. In most cases, the degradation-controlled mechanism contains diffusioncontrolled release mechanism. The drug release data obtained from the in vitro release study can be used to analyze the drug release rate, using the drug release equations given below [26]: 1 Mt = k·t2 M0

(1)

(Where Mt /M0 is the fraction of drug released, ‘t’ is the release time and ‘k’ is a constant characteristic of the system) or a linear relationship: Mt =k·t M0

(2)

The value of diffusion coefficient, D, can be calculated according to the following function: 1

k = 4(D/l2 ) 2

(3)

Where, “l” is the thickness of film. In our study, the thickness of the stent coating was determined by scanning electron microscopy (The detailed method was not shown).

Fig. 4. The weight loss curve of PLGA 95,800 and PLGA 20,000.

According to the results reported by Frank Alexis, et al., the drug release profiles from stents and films were very similar [22]. So, the above functions were applied to calculate the release kinetics parameters in the present study. Eq. (3) is valid for release of less than 60% of initial load [26]. For curcumin and emodin, Eqs. (3) and (2) were applied to calculate D value because the release curves of these two drugs exhibited approximate linear relation with time. For rapamycin and heparin, Eqs. (3) and (1) were used to calculate D value. 3.1. In vitro biodegradation Generally speaking, the drug release profile from the biodegradable matrix can be divided into three phases (early burst release, diffusion phase and degradation-controlled phase). So, the degradable profile of the polymer carrier is important for drug release profile. It is well know that the degradation of aliphatic polyesters proceeds through hydrolysis of the ester bonds [27]. The weight loss rate represents a good method to investigate the biodegradation process of the polymer materials. As can be seen from Fig. 4, the weight loss curve of PLGA 95,800 appeared linear throughout most of the degradation period, corresponding to an approximately constant mass loss rate. After degrading 86 days, the weight loss percent achieved about 60.7%. The PLGA 20,000 degraded obviously faster than PLGA 95,800, as shown in weight loss curve. It reached 80% weight loss after two weeks. 3.2. Drug release from the drug-eluting stents Fig. 5 plots the drug release profile of different drug-eluting stents with different ratio of drug-to-polymer. It can be seen that the different drug exhibits the different release profile. Rapamycin had an obvious two-phase release profile with a burst release of about two days. After the burst release, there was a nearly linear sustained and slower release period, exhibiting approximate zero-order release model at this stage. The burst release amount increased with the increase of ratio of rapamycin/PLGA. The sequence of the burst release amount was R40 (about 20.10 wt%) > R25 (about 6.19 wt%) > R15 (about 4.72 wt%) within 0–2 days, however, the cumulative released percent was 9.2 wt% for R15 and 27 wt% for R40 within 18 days. The immediate dissolution of rapamycin at stent surface in release medium brings on the burst release. It is easy to understand that the burst release goes up with the increase of drug loading because larger drug loading contributes to more drugs on stent surface. However, for curcumin, emodin and heparin eluting stents, Fig. 5 shows that the drug release amount

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Fig. 5. Drug release profile of different drug-eluting stents with different ratio of drug-to-polymer. (a) curcumin-eluting stents, (b) rapamcyin-eluting stents, (c) emodin-eluting stents, (d) heparin-eluting stents. Table 3 The average drug release rate of different drug-eluting stents. Samples

Release rate (␮g/day)

Release period

Sample

Release rate(␮g/day)

Release period (day)

C10 C20 C35 E10 E20 E30

5.25 7.37 8.3 4.3 4.9 7.5

0–18 0–18 0–18 0–18 0–18 0–18

R15 R25 R40 H10 H20 H30

0.37 0.53 1.69 1.20 1.23 1.41

2–18 2–18 2–18 0–7 0–7 0–7

increases nearly linearly with the increase of time. It is speculated that replacing the releasing medium every day established a significant concentration gradient of drug between the drug-eluting stent and the release medium, and thus led to nearly constant drug diffusion [28]. The average drug release rate of different drug-eluting stent is shown in Table 3. Despite of similar drug loading amount, different drug exhibits the different release rate. The sequence of average drug release rate at stable stage from stent coating is curcumin>emodin > heparin > rapamycin (Table 3). Generally speaking, the diffusion-controlled mechanism is the major release kinetics profile for drug dispersed in carrier matrix of nonbiodegradable polymer or of biodegradable polymer with large molecular weight. Rapamycin (Fig. 1) is a natural macrocyclic lactone produced by Streptomyces hygroscopicus [29]. The hydrophobicity of rapamycin could lead to a stronger interaction with the PLGA as compared to the other two drugs (curcumin and emodin), rendering its slower release. On the other side, the larger molecule volume of rapamycin may be another reason for its slow release rate because diffusion of larger molecule is relative difficult. Heparin is highly soluble in water; its rapid dissolution creates voids

which lead to enhanced dissolution, rendering its faster release rate than that of rapamycin. In addition, it can be seen that the heparin release rate has a slowly increased tendency after six days; it is speculated that the swelling of water in PLGA coating can enhance the diffusion of heparin. The diffusion coefficients of four drug-eluting stents were calculated. Table 4 presents the diffusion coefficients, D, of the drug from different drug-eluting stents. The calculated diffusion coefficients appear to follow the trend of molecular size, rapamycin (FW

Table 4 Results from kinetics of drug release from different drug-eluting stents* . Samples

R15

C20

E20

H20

Period (days) Rate, k (%/day) Diffusion coefficient, D (cm2 /s) R2

2–18 0.238 1.29e−13 0.9574

0–18 2.647 1.59e−11 0.9882

0–18 2.442 1.35e−11 0.9870

0–7 0.497 5.62e−13 0.9236

* The data showed in Fig. 5 is used for calculating the D values. For curcumin and emodin, Eqs. (3) and (2) are applied to calculate D value. For rapamycin and heparin, Eqs. (3) and (1) are used to calculate D value.

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C.-J. Pan et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 199–206 Table 6 The average drug release rate of rapamycin/heparin stents. Samples

RH37 RH55 RH73

Fig. 6. The drug release profile of rapamycin/heparin eluting stents. In this graph, R, H represents rapamycin and heparin, respectively. The number between RH and R or H represents the ratio of rapamycin/heparin (w/w) in stent coating.

914 g mol−1 ) has larger molecular weight compared to curcumin (FW 368 g mol−1 ) and emodin (FW 268 g mol−1 ). However, regardless of higher molecular weight of herarin than that of rapamucin, the calculated D value of heparin is higher than that of rapamycin, this is because heparin is a soluble drug. In addition, as shown in Fig. 5 and Table 3, with the increase of drug-to-polymer ratio, the average release rate increases. The larger concentration gradient between the drug loaded coating and the release medium leads to the faster diffusion rate according to the Fick diffusion-controlled mechanism. However, the enhancement is not proportionally linear with the increasing drug loading (drug-topolymer ratio). It suggests that the drug release rate is determined by not only the properties of drugs but also other factors such as film thickness, diffusion coefficient, et al. 3.3. Effect of heparin on rapamycin release In order to overcome the problem of acute and subacute thrombosis formation, it is hypothesized that incorporating heparin in rapamycin-loaded coating could enhance the blood compatibility of rapamycin-eluting stent. The rapamycin/heparin eluting stents with three different ratios of rapamycin/heparin (the weight ratio was 3/7, 5/5 and 7/3, respectively) were prepared. Fig. 6 plots the drug release kinetics curve of these drug-eluting stents. Combined with the data shown in Fig. 5, it is observed that rapamycin also has similar release kinetics with a burst release period of about two days. The burst release amount of rapamycin is shown in Table 5. The rapamycin weight percent of R25 corresponds to that of RH73 (24.5%), however, the burst release percent of R25 (6.19%) is significantly less than that of RH73 (15.36%). Regardless of the higher rapamycin weight percent in R15 (15%) than that of RH37 (10.5%), the burst release percent of the former (4.72%) was also less than that of the latter (9.31%). According to the foregoing analysis results in section 3.2, the average release rate of heparin was higher than that of rapamycin. The remaining voids after heparin released can enhance water uptake and then make the coating loose, rendering Table 5 The bust release amount of rapamycin from rapamycin and rapamycin/heparin eluting stents. Samples

R15

R25

R40

RH37

RH55

RH73

Rapamycin/coating (%) Release amount (␮g) Release percent (%)

15 7.10 4.72

25 15.47 6.19

40 80.41 20.10

10.5 9.78 9.31

17.5 23.60 13.48

24.5 37.63 15.36

Release rate (␮g/day) Rapamycin (2–18)

Heparin (0–7)

0.40 0.51 1.19

3.43 2.61 1.95

the higher diffusion rate of rapamycin. So, it can be concluded that incorporating heparin in rapamycin-loaded coating can enhance the burst release of rapamycin. After the burst release, it can be seen from Fig. 6 that there is a sustained and slow release period. Table 6 shows the average release rate of rapamycin and heparin at the stable phase. Compared with the data in Table 3, it can be obtained that the average drug release rate of the stents with similar rapamycin loading is different. For example, the release rate of RH73 (about 24.5% rapamycin) is higher than that of R25 (25% rapamycin). The rapamycin weight percent of RH37 (10.5% rapamycin) is less than R15 (15% rapamycin), however, the rapamycin release rate of the former is higher than that of the latter. Taking into all these results, it is indicated that incorporating heparin in the rapamycin-eluting coating can improve the release rate of rapamycin. This is because that the higher dissolution of heparin in water leads to form many voids on stent coating surface which make the PLGA coating loose, resulting in higher diffusion rate of rapamycin. The release kinetics parameters of rapamycin of the R25 and RH73 stents are shown in Table 7. It can be seen that the D value of RH73 is larger than R25, suggesting that incorporating heparin in rapamycin-loading coating would improve the diffusion coefficient of rapamycin, rendering its faster release rate. 3.4. Effect of heparin on curcumin release Curcumin, as a Chinese herbal drug, has many good pharmaceutical activities such as anti-coagulation, anti-proliferation, anti-inflammatory and others [30–32]. It caught our attention to develop a curcumin-eluting stents. In our previous study [19], the physical and anti-coagulation characterisitics were studied in detail. In this study, the effect of heparin on curcumin release kinetics was investigated. Fig. 7 shows the curcumin and heparin release kinetics curves of the curcumin/heparin eluting stents. Regardless of different drug loading amount, the release profile of curcumin and heparin did not change significantly compared with the curves shown in Fig. 5. So, it can be concluded that the drug release profile may be mainly determined by the properties of drug itself when the same drug carrier is used. The average drug release rate is shown in Table 8; it is obvious that the release rate of curcumin has a little bit change after incorporating heparin in stent coating. It is concluded that the release rate of heparin is obviously less than that of curcumin, rendering unconspicuous change of relase rate of curcumin. The drug release kinetics parameters of curcumin of CH37 and C10 are shown in Table 9. After loading heparin, the diffusion coefficient of curcumin increased a little bit from 3.203 e−11 cm2 s−1 (C10) to 4.028 e−11 cm2 s−1 (CH37). The changes of release kinetics parameters are consistent with the drug release rate. Table 7 Results from kinetics of rapamycin release from R25 and RH73. Samples

R25

RH73

Period (days) Rate, k (%/day) Diffusion coefficient, D (cm2 /s) R2

2–18 0.167 6.322e−14 0.9279

2–18 0.406 3.753e−13 0.9231

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Fig. 7. The drug release profile of curcumin/heparin eluting stents. In this graph, C, H represents curcumin and heparin, respectively. The number betwee CH and C or H represents the ratio of curcumin/heparin (w/w) in stent coating. Table 8 The average drug release rate of curcumin/heparin eluting stents.

205

Fig. 8. The rapamycin release profile of bi-layer and control drug-eluting stents. The inner layer represents the controll drug-eluting stent which only contains PLGA 95,800 coating with 316 ␮g rapamycin. The out layer represents the control drugeluting stent which is only PLGA 20,000 coating containing 40 ␮g rapamycin.

can not only prevent the burst release of rapamycin but also prolong the release period of rapamycin.

Samples

Release rate (␮g/day) Curcumin (0–18)

Heparin (0–7)

CH37 CH55 CH73

6.94 6.94 7.24

3.63 2.75 2.31

3.5. Effect of the coating on rapamycin release To prevent the burst release of rapamycin, the PLGA 20,000 coating containing rapamcyin was fabricated onto rapamycin-eluting stent surface. It is expected that this bi-layer configuration has slower release kinetics than from R40 alone, and sustains the release of rapamycin over a longer period. The release profile is depicted in Fig. 8. It is obvious that the outer layer fabricated on R40 can significantly inhibit the burst release of rapamycin. Based on the foregoing analysis, the rapamycin release before two days was 4.72% for R15 and 9.31% for RH37; however, the released percent of rapamycin from bi-layer stent was only 0.8% (about 2.78 ␮g) within two days. This suggests that the existing top-coating systems may in fact reduce the burst release of rapamycin. In addition, it can be seen from Fig. 8 that the release profile of bi-layer rapamycineluting stent can be divided into three stages. The first stage was 0–8 days with a slow release rate. The second stage was 8–15 days, the release rate of rapamycin increased. It can be seen from Fig. 4 that the PLGA 20,000 mostly degrades during 0–15 days, thus it is concluded that the biodegradation of out layer causes the faster release of rapamycin. Subsequently, the release rate of rapamycin became slower at the third stage after 15 days. The release amount of R40 exceeded 100 ␮g; however, the bi-layer stent is only about 48 ␮g in the whole measurement period. As a result, toxic side effects caused by high dose release of drug due to burst release seem to be effectively inhibited. It is important to ensure the patient is receiving a safe and beneficial dosage of the drug. So, the top-coating system

4. Conclusions The release kinetics of single drug eluting stents (rapamycin, curcumin, emodin, heparin) and mixing drug eluting stents (curcumin/heparin, rapamycin/heparin) was investigated in this study. The release of rapamycin (including rapamycin and rapamycin/heparin eluting stents) exhibits two-stage profile with a burst release of about two days, followed by slower and sustainedreleased period. Incorporating heparin in rapamycin-loaded PLGA coating does not change the basic release profile; however, it enhances the burst release amount of rapamcyin and the average release rate of rapamycin in the stable stage. Incorporating heparin in curcumin-loaded coating does not remarkably change the release rate of curcumin. The release curves of curcumin and emodin have an approximate zero-order release behavior regardless of different drug loading. The average release rate increases with the increase of drug loading. Fabricating out layer prepared from PLGA 20,000 onto single rapamycin-eluting stent surface can eliminate the burst release of rapamycin and prolong the release period. All results show that the drug release profile of drug-eluting stents depends on many parameters including drug type, ratio of drug-polymer, and drug carrier properties (for example, biodegradable or nonbiodegradable). Acknowledgements This work is financially supported by the key basic research program No. 2005CB623904, NSFC-RGC 30318006# of Natural Science Fund of China and the innovation fund of Southwest Jiaotong University in China. References

Table 9 Results from kinetics of curcumin release from C10 and CH37. Samples

C10

CH37

Period (days) Rate, k (%/day) Diffusion coefficient, D (cm2 /s) R2

0–18 3.755 3.203e−11 0.9964

0–18 4.211 4.028e−11 0.9413

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