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Exploitation of 3D face-centered cubic mesoporous silica as a carrier for a poorly water soluble drug: Influence of pore size on release rate
Materials Science and Engineering C 34 (2014) 78–85
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Exploitation of 3D face-centered cubic mesoporous silica as a carrier for a poorly water soluble drug: Influence of pore size on release rate Wenquan Zhu, Long Wan, Chen Zhang, Yikun Gao, Xin Zheng, Tongying Jiang, Siling Wang ⁎ Department of Pharmaceutics, Shenyang Pharmaceutical University, Shenyang, PR China
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Article history: Received 13 May 2013 Received in revised form 31 July 2013 Accepted 9 August 2013 Available online 17 August 2013 Keywords: 3D face-centered cubic mesoporous silica Drug delivery system Poorly water soluble drug Pore size Improved dissolution
a b s t r a c t The purposes of the present work were to explore the potential application of 3D face-centered cubic mesoporous silica (FMS) with pore size of 16.0 nm as a delivery system for poorly soluble drugs and investigate the effect of pore size on the dissolution rate. FMS with different pore sizes (16.0, 6.9 and 3.7 nm) was successfully synthesized by using Pluronic block co-polymer F127 as a template and adjusting the reaction temperatures. Celecoxib (CEL), which is a BCS class II drug, was used as a model drug and loaded into FMS with different pore sizes by the solvent deposition method at a drug–silica ratio of 1:4. Characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformation infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), nitrogen adsorption, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) was used to systematically investigate the drug loading process. The results obtained showed that CEL was in a non-crystalline state after incorporation of CEL into the pores of FMS-15 with pore size of 16.0 nm. In vitro dissolution was carried out to demonstrate the effects of FMS with different pore sizes on the release of CEL. The results obtained indicated that the dissolution rate of CEL from FMS-15 was significantly enhanced compared with pure CEL. This could be explained by supposing that CEL encountered less diffusion resistance and its crystallinity decreased due to the large pore size of 16.0 nm and the nanopore channels of FMS-15. Moreover, drug loading and pore size both play an important role in enhancing the dissolution properties for the poorly water-soluble drugs. As the pore size between 3.7 and 16.0 nm increased, the dissolution rate of CEL from FMS gradually increased. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the emergence of inorganic porous materials (such as porous silica, porous carbon, and composites) has opened up a new path for the development of drug delivery systems [1–5]. Compared with traditional pharmaceutical carriers, such as liposomes and polymer nanoparticles, inorganic carriers offer considerable advantages because of their pore structure, particle size, shape control, stability and surface functionalization [6–8]. Among these mesoporous materials, mesoporous silica offering different pore structures, a large pore size and pore volume, and high thermal and hydrothermal stability has been shown to be suitable for use as a drug delivery system [9,10]. There have been several studies of delivery systems involving mesoporous silica with 3D cubic, 2D hexagonal and 1D pore channel structures [11–13]. The 3D cubic mesoporous silica has several interesting features: including a large pore volume, interconnected pore channels, accessibility of pores from any direction, and a regular pore shape [14,15]. It has been reported that the potential advantages of 3D
⁎ Corresponding author at: 103 Wenhua Road, Shenyang, Liaoning Province 110016, PR China. Tel./fax: +86 24 23986348. E-mail address: [email protected] (S. Wang). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.08.014
cubic mesoporous silica in the control of drug release are superior to those of 2D hexagonal and 1D mesoporous silica [16]. The abovementioned advantages suggest that 3D cubic mesoporous silica is a very promising material for use as drug delivery system. Among the many factors that will play a role, the pore size is especially important for 3D cubic mesoporous silica together with its pore structure [17–19]. As far as we know, only two papers have been published using 3D cubic mesoporous silica as a drug delivery system for carbamazepine, oxcarbazepine, and carvedilol [16,20]. However, we were surprised that the pore sizes of the 3D cubic mesoporous silica reported in these studies were all lower than 10 nm. So, there were no studies of 3D cubic mesoporous silica with pore size (N 10 nm). To address this problem, we need a systematic study to explore the ability of 3D cubic mesoporous silica with pore size (N10 nm) to improve the dissolution rate of poorly water-soluble drugs. In recent years, 3D cubic mesoporous silica with different pore morphologies has been synthesized, such as face-centered cubic mesoporous silica and body-centered cubic mesoporous silica [21,22]. However, although face-centered cubic mesoporous silica had recently been synthesized, it had not been studied as a drug delivery system. Face-centered cubic mesoporous silica possesses a highly ordered face-centered cubic (space group Fm3m) mesostructure, which has a cubic unit cell with atoms located at each of the corners and the centers
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of all the cube faces [23]. In view of its advantages, we carried out a systematic study of face-centered cubic mesoporous silica as a drug delivery system. Celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor, is mainly used for the treatment of rheumatoid arthritis and acute pain [24,25]. Several studies have explored its application in the prophylactic treatment of tumors [26,27] and CEL is now seen as a promising drug for the treatment cancer diseases due to its potential application and, so, it was used as a model drug because of its poor water solubility. Mesoporous silica has been used in pharmaceutical applications as a drug carrier and its safety after oral administration is well known [28,29]. In the light of the above considerations, the purposes of our study were to evaluate the suitability of face-centered cubic (Fm3m) mesoporous silica (FMS) with pore size (N10 nm) as a carrier for CEL (a poorly water-soluble drug) and investigate the effect of its pore size on drug release. Face-centered cubic (Fm3m) mesoporous silica with pore size (16 nm) was successfully synthesized using a low temperature method with Pluronic block co-polymer F127 as a template, TEOS as a silica source and TMB as a pore enlargement agent. Notably, the synthetic process was simplified because the method involved modification of only one factor (temperature). The pore size of FMS was successfully enlarged to 16.0 nm by lowering the temperature and it could be easily regulated by controlling the synthesis temperature of the new method. This not only helped to explore its applications involving large pores, but also helped to investigate the effect of the pore size on the release properties of FMS. CEL was incorporated into FMS with different pore sizes by the solvent deposition method at a selected drug–silica ratio (1:4). The corresponding samples were characterized by TEM, FT-IR, TGA, BET, DSC and XRD investigations. The release behavior of CEL from FMS with different pore sizes was compared.
2. Materials and methods 2.1. Materials Pluronic block co-polymer F127 was kindly donated by BASF. Tetraethyl orthosilicate (TEOS), hydrochloric acid and potassium chloride were purchased from Yu Wang Reagent Company (Shandong, China). 1,3,5-trimethylbenzene (TMB) was obtained from SigmaAldrich (St. Louis, MO, USA). Celecoxib (purity N 99.0%) was kindly supplied from Shenyang Funing Pharmaceutical Co., Ltd. All other chemicals were used in accordance with the requirements of analytical/ spectroscopic/HPLC grade. Deionized water in all experiments was prepared by ion exchange.
2.2. Synthesis of FMS For the synthesis of 3D face-centered cubic mesoporous silica (FMS) with different pore sizes, 1.25 g of KCl and 0.5 g of F127 were mixed under gently stirring in 30.0 mL of HCl (2.0 M) at three different reaction temperatures (15, 18 and 20 °C), followed by the addition of 0.5 g of TMB under stirring. After 1 h of stirring, 2.0 g of TEOS was added drop by drop to the obtained solution under vigorous stirring. After another 24 h of stirring, the obtained suspension was homogenized by an ATS AH110D homogenizer (ATS Engineer Inc., Shanghai, China). Then, the mixture was placed into a Teflon-lined autoclave and carried out by hydrothermal reaction for 24 h at 130 °C which corresponds to the reaction temperature. The obtained mixture was filtered and dried at 60 °C in the air. The as-synthesized mesoporous silica was filtered and burn out at 600 °C for 5 h to remove surfactant completely. Finally, face-centered cubic mesoporous silica (FMS-15, FMS-18 and FMS-20) by controlling the different action temperatures (15, 18 and 20 °C), was obtained until the powers were dried at 60 °C for 48 h.
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2.3. Drug loading procedure CEL was respectively loaded into FMS samples (FMS-15, FMS-18 and FMS-20) by a solvent deposition method, which involved soaking equilibrium and solvent evaporation. In detail, CEL was dissolved in methanol to get a homogeneous solution (10 mg/mL), and then obtained solution was mixed with a selected amount of FMS samples to obtain a mixture at a certain proportion (1:4) (note: safer solvent such as ethanol can be used instead of methanol). Then, the mixture was brought to adsorption equilibrium under gently stirring for 24 h at room temperature in a closed container. Finally, the solvent was allowed to evaporate under gently stirring and then the precipitated powder was washed with ethanol to remove the drugs on the surface of carrier. The obtained powder was dried at 40 °C in air until no organic solvent residue was left. The drug-loaded samples were labeled FMSC15, FMSC-18 and FMSC-20, respectively. 2.4. Analysis of drug content Thermogravimetric analysis (TGA) was performed by a TGA-50 instrument (Shimadzu, Japan) at a heating rate of 10 °C/min under a nitrogen purge of 40 mL/min. The following equation was used to calculate the drug-loading rate. Drug-loading rate = weight of CEL in sample / weight of carrier in sample. All the drug content of samples was within 20 ± 5% of the theoretical value. 2.5. Characterization techniques 2.5.1. SEM study The morphology of the samples was characterized using a field emission scanning electron microscope (JEOL-6700, Japan). 2.5.2. TEM study The mesoporous structure of the samples was characterized using TEM (Tecnai G2 20, FEI, USA). 2.5.3. FT-IR study The functional groups and chemical bonding of samples were characterized using an FT-IR spectrometer (Bruker IFS 55, Switzerland). The scanning range of FT-IR spectroscopy was in the range of 400–4000 cm−1. 2.5.4. Nitrogen adsorption analysis Adsorption–desorption measurements were conducted on a surface area analyzer (SA3100, Beckman Coulter, USA). The carriers were degassed at 120 °C for 12 h, and the CEL-loaded samples were degassed at 50 °C for 24 h to remove physically adsorbed water before analysis. The BET (Brunauer–Emmett–Teller) method and BJH (Barrett–Joyner– Halenda) method were used to investigate the pore characteristics. 2.5.5. XRD and DSC analysis The crystalline characteristics of samples were described using X-ray diffractometer (PW3040/60 PANALYII CALB.V Netherlands). XRD patterns were obtained from 5° to 50° with a scan rate of 5°/min and a step size of 0.02°. DSC profiles of the samples were recorded from a DSC instrument (TA Instruments, Q1000, USA). The temperature range was 50–200 °C at a heating rate of 10 °C/min. 2.6. In vitro release profile study Dissolution studies were carried out using a USP II paddle method with a KC-8D dissolution instrument (KC-8D, Tianjin Guoming Medical Equipment Co., Ltd.). The dissolution media was phosphate buffer (pH 6.9). The dissolution procedure was as follows, dissolution media (900 mL) in basket was kept at 37 °C. The 200 mg samples (equivalent to 50 mg of CEL) were added with a stirring rate of 100 rpm and
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performed for 1 h. 5 mL samples were taken out at designated intervals (5, 10, 15, 20, 30, 45 and 60 min). The amount of CEL dissolved was determined by spectroscopy (UV-2000, Unico, USA) at 254 nm (n = 6). 3. Results and discussion 3.1. Preparation and characterization of FMS carrier In the present study, FMS-15, FMS-18 and FMS-20 were synthesized by a soft template method. The particle size and external morphology of them were characterized by SEM. As shown in Fig. 1B, C and D, the FMS15, FMS-18 and FMS-20 all exhibited the same standard hexagon and approximately 5–10 μm of particle size. The longitudinal section confirmed their three-dimensional structure. It is well known that the pore size is the most important factor regarding drug release in investigations of mesoporous silica as a carrier [30,31]. However, investigations of the pore size of mesoporous silica with a 3D cubic pore structure as a drug carrier has mainly focused on the range 2–10 nm due to difficulties in increasing the pore size of mesoporous silica [16,20]. There is a lack of information about 3D cubic mesoporous silica with a pore size greater than 10 nm in drug release studies. In our present work, FMS with a 3D face-centered cubic pore morphology and a pore size of 16.0 nm was prepared at a low synthesis temperature and using TMB as a pore enlargement agent. The synthetic process involves the following steps. First of all, KCl and F127 (template) were mixed in HCl (2.0 M) at three different reaction temperatures (15, 18 and 20 °C), followed by the addition of TMB (pore enlargement agent) and TEOS (silica source). Subsequently, the mixture underwent a hydrothermal reaction to form the mesoporous silica framework. Finally, FMS-15, FMS-18 and FMS-20 (15, 18 and 20 representing the reaction temperatures) were obtained by removing the template using burn out.
Fig. 1. SEM images of (A) CEL (B) FMS-15, (C) FMS-18, (D) FMS-20 and (E) FMSC-15.
In this study, the reaction temperature played the most important role in the regulation of pore size. Different reaction temperatures (15, 18 and 20 °C) were used in the synthesis of FMS with different pore sizes. Fig. 2 shows the TEM micrographs of FMS-15, FMS-18 and FMS20. It was found that FMS-15, FMS-18 and FMS-20 had different pore sizes (approximately 16.0, 6.9 and 3.7 nm, respectively) (Fig. 2A, B and C). As the temperature decreased from 20 to 15 °C, the pore size of FMS increased from 3.7 to 16 nm. FMS-15 with pore size of 16.0 nm was obtained when the reaction temperature reached 15 °C (Fig. 2C). As the reaction temperature decreased, the micelle bonds were weakened, allowing more pore enlargement agent TMB to be embedded in the micelle core, thus causing further enlargement [32]. In most cases, disordered mesoporous silica is obtained when the reaction temperature is reduced [33]. In order to solve this problem, it is necessary to add the right amount of KCl (1.25 g). Inorganic salts (such as KCl) can help to maintain the ordered mesoporous structure of FMS by improving the interaction between the silicate species and the nonionic pluronic block copolymer F127 [34]. As seen in Fig. 2A, B and C, the pores of FMS-15, FMS-18 and FMS-20 were in a highly ordered array which allowed effective control of the drug particle size and its release. To the best of our knowledge, no detailed investigations have been conducted using 3D face-centered cubic mesoporous silica with pore size of 16.0 nm as a carrier for drugs and there are no published reports of the effect of the pore size of 3D cubic mesoporous silica on drug release. 3.2. Drug loading and characterization In our work, CEL was incorporated into FMS samples (FMS-15, FMS18 and FMS-20) by the solvent deposition method. This method has been proved to be very effective for the synthesis of solid dispersions of poorly water-soluble drugs [35,36]. The drug loading process involves the two steps: the initial adsorption equilibrium and subsequent solvent evaporation. Methanol was selected as the loading solvent because of its volatility and because CEL was very soluble in it. The degree of drug loading was determined by TGA analysis to ensure a drug loading of 20%. The excellent drug loading characteristics of FMS with different pore sizes were confirmed by the results of SEM, TEM, FT-IR and TGA. As shown in Fig. 1A, B and E, the pure drug (celecoxib) was massive particles and present in a crystalline form (Fig. 1A). The FMS-15 exhibited the standard hexagon (Fig. 1B). There was almost no lump drugs remained on the surface of the carrier, indicating that drug not loaded into the pores was washed away (Fig. 1E). The morphology of the drug-loaded samples was studied by TEM. As seen in Fig. 2, in contrast to unloaded FMS samples, most of the pore channels of FMSC samples (FMSC-15, FMSC-18 and FMSC-20) were not as clear, indicating that a large number of the pore channels of FMSC samples had been filled with CEL. The inclusion of CEL into the pore channels of mesoporous silica was further to be confirmed by FT-IR. Celecoxib, a cyclooxygenase-2 (COX2) selective inhibitor is a non-steroid anti-inflammatory drug (NSAID). The molecular formula of celecoxib is C17H14F3N3O2S. Its molecular weight is 381.37. The chemical name is 4-[5-(4-methylphenyl)-3(trifluoromethyl)-1H-pyrazol-1-yl]-benzenesulfonamide. The benzenesulfonamide is a crucial functional group, which would play a crucial role in drug loading (Fig. 3). The FT-IR spectra of samples (CEL, FMS-15, their corresponding physical mixtures and FMSC-15) were shown in Fig. 4. The FMS-15 showed the wide adsorption bands in the range of 3750–3000 cm−1 due to the silanol groups. In addition, CEL showed a double sharp peak at 3342 and 3235 cm−1 and two single peaks at 1348 and 1164 cm−1, which corresponds to the amino groups and sulfonyl groups of sulfonamide in CEL, respectively. For the physical mixtures, the present peaks associated with both CEL and FMS-15 confirmed no interactions between them. However, a marked decrease of amino peaks was present in the spectrum of the FMSC-15, which may be due to the
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Fig. 2. TEM images of (A) FMS-20, (B) FMS-18, (C) FMS-15, (D) FMSC-20, (E) FMSC-18 and (F) FMSC-15.
Fig. 3. Chemical structure of celecoxib
Fig. 4. FT-IR spectrum for crude CEL (A), the physical mixtures of CEL and FMS-15(B), FMSC-15 (C) and FMS-15 (D).
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3.3. Nitrogen adsorption analysis
Fig. 5. TGA profiles of CEL, FMS-15, FMS-18, FMS-20, FMSC-15, FMSC-18 and FMSC-20.
hydrogen bonding between the sulfonamide groups in the CEL and the silanol groups of the silica [37]. TGA measurements were used to measure the weight of CEL loaded in the FMSC. As showed in Fig. 5, FMS-15, FMS-18 and FMS-20 exhibited almost no weight loss due to the excellent thermal stability of mesoporous silica. The weight loss of FMS-15, FMS-18 and FMS-20 was 18.6%, 19.1% and 17.6%, respectively. However, there were some differences between the expected drug loading (20%) and the practical drug loading. The reason for this may be the loss during the washing process in drug loading. The results showed that the loss of CEL during the drug loading was less than 5%. Additionally, the volume occupation ratio of CEL in the mesoporous silica was evaluated from the amount of CEL measured by TGA. The amount of drug loaded into the FMSC-15, FMSC-18 and FMSC-20, estimated from the value of TGA was 18.6%, 19.1% and 17.6%, respectively. However, the theoretical volume occupation ratio of CEL loaded on FMSC-15, FMSC-18 and FMSC-20, estimated from the value of TGA is 9.7%, 12.2% and 14.3%, respectively, which was according to the following equation: Volume occupation ratio (%) = (wt.% (CEL) / M (molar mass)) × N (Avogadro's constant) × V (molecule volume) / V (pore volume). It was worth noting that the theoretical volume occupation ratio (9.7%, 12.2% and 14.3%) was all lower than the practical volume occupation ratio (24.6%, 33.3% and 78.8%). It was concluded that drug molecules inside the pores were all not ordered and may block the pore windows or pore channels. The difference (14.9%) between the theoretical volume occupation ratio (9.7%) and the practical volume occupation ratio (24.6%) of FMSC-15 was relatively smaller than that of FMSC-18 (21.1%) and FMSC-20 (64.5%), indicating uniform drug loading. This may facilitate the movement of the dissolution medium in the pore channels and the drug release. So the cumulative release percentage of FMSC-15 was the highest among them (Fig. 9).
The pore structure and BET characteristics of samples were provided by N2 adsorption measurements. As can be seen from Fig. 6A, B and C, the IV isotherms and a type H2 hysteresis loop of FMS samples, according to the IUPAC classification, confirmed their typical mesoporous structure. A relatively sharp increase at around P/P0 = 0.85 was seen in all the adsorption isotherm profiles of FMS samples, which confirmed their uniform pore structure. In addition, the pore size distribution of FMS samples all showed a single significantly sharp peak, which indicated that they had a uniform pore size. These results were also supported by the pore size analysis. Table 1 shows that FMS-15, FMS-18 and FMS-20 had a uniform pore size of 16.0, 6.9 and 3.7 nm, respectively. After drug loading, the type of isotherms and hysteresis loop for FMSC-18 and FMSC-20 remained almost unchanged compared with that of FMC18 and FMS-20, indicating that most pore channels were not affected by drug filling (Fig. 6B and C). However, the isotherm type and hysteresis loop shape of FMSC-15 were both different from that of FMS-15, which indicated that the original pore morphology of FMS-15 was not maintained (Fig. 6A). The data on the surface area (BET), pore volume (Vt) and the BJH pore diameter (WBJH) are given in Table 1. The surface area and pore volume of FMSC samples all exhibited a significant reduction compared with that of the corresponding FMS samples (unloaded drugs), which was due to the fact that the drug filling occupied the pore spaces and reduced the pore volume. In addition, the pore volume of FMS-15 was much larger than that of FMS-18 and FMS-20, which means that FMS-15 has a higher drug loading capacity than FMS-18 and FMS-20. This property confirms the potential advantages of using FMS-15 as a drug carrier. To investigate the relationship between the occupation volume and well-diffusion pathway, the volume occupation ratio of CEL loaded on FMSC-15, FMSC-18 and FMSC-20, estimated from the value of each pore volume is 24.6%, 33.3% and 78.8%, respectively. It was worth noting that the cumulative release amounts within 60 min followed the sequence: FMSC-15 N FMSC-18 N FMSC-20. The unoccupied pore space decreased on increasing the volume occupation ratio. It can be inferred that unoccupied pore space determined the amount of the dissolution medium into the pore channels. More dissolution medium moved into the pore channels and more drug molecules can be dissolved, when unoccupied pore space was bigger. So it was speculated that unoccupied pore space (FMSC-15 (75.4%) N FMSC-18 (66.7%) N FMSC-20 (21.2%)) may directly affect the cumulative release percentage (FMSC-15 N FMSC-18 N FMSC-20). 3.4. Solid state characterization by XRD and DSC The crystal properties of samples were evaluated by XRD analysis using a fixed angle ranging from 5° to 50°. As shown in the XRD patterns
Fig. 6. Adsorption–desorption isotherms and pore size distributions of (A) FMS-15 and FMSC-15, (B) FMS-18 and FMSC-18, and (C) FMS-20 and FMSC-20.
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Table 1 Surface area, pore volume and pore size of samples.
FMS-15 FMS-18 FMS-20 FMSC-15 FMSC-18 FMSC-20
Surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
252.15 219.76 428.09 170.90 154.61 68.10
0.81 0.66 0.52 0.61 0.44 0.11
16.01 6.93 3.78 12.33 6.28 3.88
(Fig. 7), two main intense and characteristic peaks of pure CEL were found at 16.1° and 21.6°. However, FMS-15, FMS-18 and FMS-20 all showed another two completely different diffraction peaks at 28.3° and 40.5°. After completion of the drug loading process, FMSC-15 and FMSC-18 did not show any characteristic CEL diffraction peaks, and the two diffraction peaks of FMS-15 and FMS-18 were still preserved. The drug is in a non-crystalline state if none of its characteristic peaks are present [38,39]. Accordingly, we can infer that the crystalline form of CEL in the pores of FMS-15 and FMS-18 was converted into a noncrystalline form. Also, the characteristic diffraction peaks of CEL in FMSC-20 were still present but weaker and the main diffraction peak of FMS-20 remained. This indicated that the crystalline form of CEL was not completely masked by FMS-20. This may be because a portion of the drug remained on the surface of FMS-20 and was not loaded into the pores of FMS-20. The crystalline characteristics of the samples were further confirmed using DSC which is also a sensitive method for detecting crystals. As is well known, the depression of the melting point is a standard means of determining whether a crystalline state is still present. As seen in Fig. 8, there was a single endothermic melting peak at 161.1 °C, in agreement with the melting point of pure CEL, while FMS-15, FMS-18 and FMS-20 did not show any diffraction peak at the same temperature. Furthermore, for FMSC-15 and FMSC-18, the inverted peak of CEL was not clearly present at the same temperature. In contrast, for FMSC-20, the endothermic melting peak was still present although it was weak. This indicated that the crystalline form of CEL was not covered up by FMS-20 and completely covered up by FMS-15 and FMS-18. These DSC results all agreed with the above XRD results. Together, the XRD and DSC results provided indirect evidence of the advantage of using FMS-15 with a large pore size as a drug carrier. As shown by both the XRD and DSC results, when CEL was loaded into
Fig. 8. DSC patterns of FMS-15, FMS-18, FMS-20, FMSC-15, FMSC-18, FMSC-20 and pure CEL.
FMS-15, FMSC-18 and FMS-20, the CEL (in the pores of FMS-15 and FMSC-18) was in a non-crystalline state, while in the pores of FMS-20 it was still in the original crystalline state. It was concluded that FMS15 was responsible for the non-crystalline state of CEL due to the fact that nanopores of mesoporous materials can retain a drug in the noncrystalline state by space confinement [40–42]. Furthermore, more drug loading by FMS-15 was produced by the large pore size. When the same amount of CEL was loaded into FMS-15 and FMS-20, all drugs were completely loaded into the pores of FMS-15, while some of the drug remained on the surface of FMS-20, leading to the disappearance of the crystalline form for FMSC-15 and the presence of the crystalline form for FMSC-20. The larger the pore size, the greater the degree of drug loading. It was also concluded that CEL molecules can be effectively prevented from growing into a crystalline form when they were confined to the nanopores of FMS-15. 3.5. In vitro drug dissolution Dissolution tests were carried out in order to study the application of FMS-15 with a larger pore size (about 16.0 nm) and a 3D face-centered cubic mesoporous structure as a drug carrier and to investigate the effect of different pore sizes of FMS samples on the dissolution behavior of CEL. The in vitro drug release of CEL from FMSC-15, FMSC-18 and FMSC-20 can be mainly divided into two stages: burst release (0–10 min) and prolonged release (10–60 min). The burst release phenomenon is present in many drug release processes which use mesoporous materials as drug carriers [28,39]. The reason for the initial burst release was mainly
Fig. 7. XRD patterns of FMS-15, FMS-18, FMS-20, FMSC-15, FMSC-18, FMSC-20 and pure CEL.
Fig. 9. Dissolution profiles of FMSC-15, FMSC-18, FMSC-20 and pure CEL.
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attributed to the presence of drug in the outer pores of mesoporous materials. As shown in Fig. 9, all the release rates of CEL from FMSC samples were much faster than that of CEL during the initial stage. The dissolution rate of CEL within 10 min was only about 13% due to its poor dissolution as a BCS class II drug. However, the cumulative release of CEL from FMSC15, FMSC-18 and FMSC-20 was 63%, 46% and 58%, respectively. The most likely reason for the improvement in dissolution rate may be that the spatial confinement within the nanosized pores of FMS reduced the degree of CEL crystallization or maintained CEL in a non-crystalline state, since it is well known that if a drug is in a non-crystalline state this effectively increases their dissolution rate [43,44]. Furthermore, the burst release for FMSC-20 was more significant than that of FMSC-15 and FMSC18 within the 5 min. This may be due to the fact that the amount of drug remaining in the external pores with FMS-20 was greater than that of FMS-15 and FMS-18, which was caused by the smaller pore size of FMS-20 compared with that of FMS-15 and FMS-18. This was also confirmed by the results of XRD and DSC analysis. The excess drug present in the external pores of FMS-20 resulted in the presence of the crystalline form for CEL, which cannot be covered up by FMS-20 (Figs. 7 and 8). Additionally, the cumulative release amounts within 60 min followed the sequence: FMSC-15 N FMSC-18 N FMSC-20. This was consistent with their pore size: FMS-15 (16.0 nm) N FMS-18 (6.9 nm) N FMS-20 (3.7 nm). So, it was concluded that the release behavior of CEL was mainly influenced by the pore size of the FMS samples. The cumulative release amounts of CEL increased on increasing the pore size (3.6 nm– 16.0 nm). This could be explained as follows: One reason may be that drugs in the larger pores encountered less diffusion resistance due to the spacious pore channels and moved more easily into the dissolution medium. For example, the pore size of FMS-15 (16.0 nm) was much larger than that of FMS-18 (6.9 nm) and FMS-20 (3.7 nm). The cumulative release amounts of FMSC-15 was the fastest of all, which could be attributed to the fact that the CEL molecules in the pores of FMS-15 with relatively pore size of 16.0 nm had more opportunity to escape from pores and enter the dissolution medium. Another reason may be that the diffusion-controlled process could be used to scribe the drug release process. When the dissolution medium penetrated into the pore channels of the carriers, the drug molecules would dissolve in the dissolution medium and escape from the pores along the pore channels filled with the solvent. The pore channels with larger pore volume would be filled with the more dissolution medium, which would dissolve more drug molecules. So the drug molecules that stayed in the larger pores would have more chance to escape from the carriers. Therefore, it was predicted that FMS-15 with a face-centered cubic pore structure could significantly increase the release rate of poorly water-soluble drugs by taking advantage of its large pores and open pore structure. Many forms of mesoporous silica with a large pore size have been used to load proteins [45]. However, to the best of our knowledge, there is almost no information about the use of 3D face-centered cubic mesoporous silica with a large pore size (up to over 10 nm) as a carrier for loading poorly water-soluble drugs, which was the main reason for using FMS-15 as a carrier for the poorly water-soluble drug CEL in our study. FMS-15 has the following advantages as a carrier for poorly water-soluble drugs: Firstly, FMS-15 with a large pore size (about 16.0 nm) could significantly improve the drug release rate due to the fact that the diffusion resistance of drugs in the larger pore sizes was reduced. Secondly, FMS-15 with an open pore structure (face-centered pore structure) and threedimensional interconnected pore structure (cubic pore structure) could facilitate drugs transfer and reduce the diffusion resistance of drugs. Thirdly, FMS-15 with a nanopore structure could effectively reduce drug crystallinity or inhibit formation of the crystalline form. Highly ordered crystals could not be formed due to the spatial confinement within nanosized pores of FMS-15. As is well known, changing the drug crystal form is an effective way of improving the dissolution rate of poorly water-soluble drugs [46,47]. It was concluded that FMS-15 with a large pore size (about 16.0 nm) and face-centered cubic pore structure was especially suitable as a carrier for poorly water-soluble drug CEL.
Furthermore, the dissolution rate of CEL could be controlled by regulating the pore size of 3D face-centered cubic mesoporous silica. The optimization of mesopore size in order to enhance the dissolution properties for the poorly water-soluble drugs in this study should be based on uniform drug loading without pore blocking. However, uniform drug loading for all samples (FMS-15, FMS-18 and FMS-20) was not to be obtained by the same drug loading method. This may be because the pore channels of FMS-18 and FMS-20 with a smaller pore size be much easier to be blocked, compared with that of FMS-15. In the case of FMS-15, the pore diameter after the loading of CEL was changed from 16.0 to12.3 nm (Fig. 6). However, pore diameter of FMS-18 and FMS-20 (6.9 nm and 3.8 nm, respectively) after the loading of CEL (6.3 nm and 3.9 nm, respectively) was almost unchanged (Fig. 6). Moreover, isotherm type and hysteresis loop shape of FMSC15 were both different from that of FMS-15. The type of isotherms and hysteresis loop for FMSC-18 and FMSC-20 remained almost unchanged compared with that of FMC-18 and FMS-20. The above results probably indicated that CEL may not be uniformly introduced into the mesopore of FMSC-18 and FMSC-20, compared with that of FMSC-15. This may cause nonuniform distribution of drugs in the pore channels or the blocked pore windows. Especially, the burst release for FMSC-20 was more significant than that of FMSC-15 and FMSC-18 within the 5 min. Most drugs may accumulate at the pore window and part of drugs may be loaded into the pore channels due to irregular pore filling for FMSC-20. So drugs at the pore window diffused rapidly to the dissolution medium, which resulted in significant burst release of FMSC-20. Moreover, irregular pore filling for FMSC-20 hampered the drug release, resulting in 74.4% of cumulative release. So pore filling may also have a direct relationship with the drug release. In comparison, the cumulative release percentage of FMSC-15 within 60 min was higher than that of FMSC-18 and FMSC-20. This may be attributed to the more regular pore filling of FMS-15 than that of FMSC-18 and FMSC-20. The regular pore filling could facilitate the well-diffusion pathway of the dissolution medium. In conclusion, uniform drug loading and pore size both play an important role in enhancing the dissolution properties for the poorly water-soluble drugs. 4. Conclusion The mesoporous silica (FMS-15) with a large pore size (16.0 nm) and faced-centered cubic structure was successfully synthesized and then the rapid release of CEL (a poorly water-soluble drug) from FMS15 was carried out in our present study. This was the first report of the use of 3D face-centered cubic mesoporous silica with large pore sizes (up to over 10 nm) as a drug carrier. Characterization using SEM, TEM, BET, FT-IR, TGA, XRD and DSC demonstrated the successful incorporation of CEL into the FMS samples (FMS-15, FMS-18 and FMS-20). A great reduction in crystallinity, partial or total crystalline-amorphous transformation was observed for CEL after it was loaded into FMS samples. The marked crystallinity loss or disappearance was probably due to spatial confinement of nanopores. Moreover, we further investigated the effect of pore size on the release of CEL from FMS samples. The results obtained showed that the cumulative release amounts within 60 min of CEL from FMS samples increased on increasing the pore size (3.7–16.0 nm) of the FMS samples. This may be due to the fact that less diffusion resistance was encountered for drugs in the larger pores and more dissolution medium penetrated into pore channels would dissolve more drug molecules. In addition, the results of in vitro dissolution tests also showed that CEL released from FMS samples all exhibited a significant improvement of dissolution rate. The following factors appear to be responsible for the dissolution improvement: the reduction in crystalline form and formation of the amorphous form, nanopore size and the interconnected cubic pore channels of the FMS samples, Moreover, uniform drug loading and pore size both play an important role in enhancing the dissolution properties for the poorly watersoluble drugs. It was worth noting that FMSC-15 had the fastest
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dissolution rate among all the FMSC samples, which was due to its special face-centered cubic pore structure, large pore size of 16.0 nm and uniform drug loading. We believe that our study will open up new opportunities for using 3D cubic mesoporous silica with large pores as delivery systems for poorly water-soluble drugs. Acknowledgment This work was supported by the National Basic Research Program of China (973 Program) (No. 2009CB930300) and the National Natural Science Foundation of China (No. 81273449). References [1] F. Chen, et al., Chitosan enclosed mesoporous silica nanoparticles as drug nanocarriers: sensitive response to the narrow pH range, Microporous Mesoporous Mater. 150 (2012) 83–89. [2] C.X. Lin, et al., Periodic mesoporous silica and organosilica with controlled morphologies as carriers for drug release, Microporous Mesoporous Mater. 117 (2009) 213–219. [3] T. Heikkila, et al., Mesoporous silica material TUD-1 as a drug delivery system, Int. J. Pharm. 331 (2007) 133–138. [4] Y. Zhu, et al., PEGylated hollow mesoporous silica nanoparticles as potential drug delivery vehicles, Microporous Mesoporous Mater. 141 (2011) 199–206. [5] X. Li, et al., Preparation of mesoporous calcium doped silica spheres with narrow size dispersion and their drug loading and degradation behavior, Microporous Mesoporous Mater. 102 (2007) 151–158. [6] M. Vialpando, et al., Risk assessment of premature drug release during wet granulation of ordered mesoporous silica loaded with poorly soluble compounds itraconazole, fenofibrate, naproxen, and ibuprofen, Eur. J. Pharm. Biopharm. 81 (2012) 190–198. [7] I. Slowing, et al., Mesoporous silica nanoparticles as controlled release drug delivery and gene transfection carriers, Adv. Drug Deliv. Rev. 60 (2008) 1278–1288. [8] M. Liu, et al., A novel liposome-encapsulated hemoglobin/silica nanoparticle as an oxygen carrier, Int. J. Pharm. 427 (2012) 354–357. [9] H. Peng, et al., A pH-responsive nano-carrier with mesoporous silica nanoparticles cores and poly(acrylic acid) shell-layers: fabrication, characterization and properties for controlled release of salidroside, Int. J. Pharm. 446 (2013) 153–159. [10] J. Liu, et al., Magnetic silica spheres with large nanopores for nucleic acid adsorption and cellular uptake, Biomaterials 33 (2012) 970–978. [11] S. Wang, Ordered mesoporous materials for drug delivery, Microporous Mesoporous Mater. 117 (2009) 1–9. [12] Y. Zhou, et al., 3D net-linked mesoporous silica monolith: new environmental adsorbent and catalyst, Catal. Today 166 (2011) 39–46. [13] Y. Hu, et al., Facile synthesis of 3D cubic mesoporous silica microspheres with a controllable pore size and their application for improved delivery of a water-insoluble drug, J. Colloid Interface Sci. 363 (2011) 410–417. [14] W. Na, et al., Large pore 3D cubic mesoporous silica HOM-5 for enzyme immobilization, Mater. Lett. 62 (2008) 3707–3709. [15] J. Huang, et al., Large-pore cubic Ia-3d mesoporous silicas: synthesis, modification and catalytic applications, J. Mol. Catal. A Chem. 271 (2007) 200–208. [16] Y. Hu, et al., 3D cubic mesoporous silica microsphere as a carrier for poorly soluble drug carvedilol, Microporous Mesoporous Mater. 147 (2012) 94–101. [17] S.K. Das, et al., Effects of surface acidity and pore size of mesoporous alumina on degree of loading and controlled release of ibuprofen, Microporous Mesoporous Mater. 118 (2009) 267–272. [18] F. Qu, et al., Controlled release of Captopril by regulating the pore size and morphology of ordered mesoporous silica, Microporous Mesoporous Mater. 92 (2006) 1–9. [19] Y.P. Wang, et al., Control of pore size in mesoporous silica templated by a multiarm hyperbranched copolyether in water and cosolvent, Microporous Mesoporous Mater. 120 (2009) 447–453. [20] M.J.K. Thomas, et al., Inclusion of poorly soluble drugs in highly ordered mesoporous silica nanoparticles, Int. J. Pharm. 387 (2010) 272–277.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Exploitation of 3D face-centered cubic mesoporous silica as a carrier for a poorly water soluble drug: Influence of pore size on release rate Wenquan Zhu, Long Wan, Chen Zhang, Yikun Gao, Xin Zheng, Tongying Jiang, Siling Wang ⁎ Department of Pharmaceutics, Shenyang Pharmaceutical University, Shenyang, PR China
a r t i c l e
i n f o
Article history: Received 13 May 2013 Received in revised form 31 July 2013 Accepted 9 August 2013 Available online 17 August 2013 Keywords: 3D face-centered cubic mesoporous silica Drug delivery system Poorly water soluble drug Pore size Improved dissolution
a b s t r a c t The purposes of the present work were to explore the potential application of 3D face-centered cubic mesoporous silica (FMS) with pore size of 16.0 nm as a delivery system for poorly soluble drugs and investigate the effect of pore size on the dissolution rate. FMS with different pore sizes (16.0, 6.9 and 3.7 nm) was successfully synthesized by using Pluronic block co-polymer F127 as a template and adjusting the reaction temperatures. Celecoxib (CEL), which is a BCS class II drug, was used as a model drug and loaded into FMS with different pore sizes by the solvent deposition method at a drug–silica ratio of 1:4. Characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformation infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), nitrogen adsorption, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) was used to systematically investigate the drug loading process. The results obtained showed that CEL was in a non-crystalline state after incorporation of CEL into the pores of FMS-15 with pore size of 16.0 nm. In vitro dissolution was carried out to demonstrate the effects of FMS with different pore sizes on the release of CEL. The results obtained indicated that the dissolution rate of CEL from FMS-15 was significantly enhanced compared with pure CEL. This could be explained by supposing that CEL encountered less diffusion resistance and its crystallinity decreased due to the large pore size of 16.0 nm and the nanopore channels of FMS-15. Moreover, drug loading and pore size both play an important role in enhancing the dissolution properties for the poorly water-soluble drugs. As the pore size between 3.7 and 16.0 nm increased, the dissolution rate of CEL from FMS gradually increased. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the emergence of inorganic porous materials (such as porous silica, porous carbon, and composites) has opened up a new path for the development of drug delivery systems [1–5]. Compared with traditional pharmaceutical carriers, such as liposomes and polymer nanoparticles, inorganic carriers offer considerable advantages because of their pore structure, particle size, shape control, stability and surface functionalization [6–8]. Among these mesoporous materials, mesoporous silica offering different pore structures, a large pore size and pore volume, and high thermal and hydrothermal stability has been shown to be suitable for use as a drug delivery system [9,10]. There have been several studies of delivery systems involving mesoporous silica with 3D cubic, 2D hexagonal and 1D pore channel structures [11–13]. The 3D cubic mesoporous silica has several interesting features: including a large pore volume, interconnected pore channels, accessibility of pores from any direction, and a regular pore shape [14,15]. It has been reported that the potential advantages of 3D
⁎ Corresponding author at: 103 Wenhua Road, Shenyang, Liaoning Province 110016, PR China. Tel./fax: +86 24 23986348. E-mail address: [email protected] (S. Wang). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.08.014
cubic mesoporous silica in the control of drug release are superior to those of 2D hexagonal and 1D mesoporous silica [16]. The abovementioned advantages suggest that 3D cubic mesoporous silica is a very promising material for use as drug delivery system. Among the many factors that will play a role, the pore size is especially important for 3D cubic mesoporous silica together with its pore structure [17–19]. As far as we know, only two papers have been published using 3D cubic mesoporous silica as a drug delivery system for carbamazepine, oxcarbazepine, and carvedilol [16,20]. However, we were surprised that the pore sizes of the 3D cubic mesoporous silica reported in these studies were all lower than 10 nm. So, there were no studies of 3D cubic mesoporous silica with pore size (N 10 nm). To address this problem, we need a systematic study to explore the ability of 3D cubic mesoporous silica with pore size (N10 nm) to improve the dissolution rate of poorly water-soluble drugs. In recent years, 3D cubic mesoporous silica with different pore morphologies has been synthesized, such as face-centered cubic mesoporous silica and body-centered cubic mesoporous silica [21,22]. However, although face-centered cubic mesoporous silica had recently been synthesized, it had not been studied as a drug delivery system. Face-centered cubic mesoporous silica possesses a highly ordered face-centered cubic (space group Fm3m) mesostructure, which has a cubic unit cell with atoms located at each of the corners and the centers
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of all the cube faces [23]. In view of its advantages, we carried out a systematic study of face-centered cubic mesoporous silica as a drug delivery system. Celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor, is mainly used for the treatment of rheumatoid arthritis and acute pain [24,25]. Several studies have explored its application in the prophylactic treatment of tumors [26,27] and CEL is now seen as a promising drug for the treatment cancer diseases due to its potential application and, so, it was used as a model drug because of its poor water solubility. Mesoporous silica has been used in pharmaceutical applications as a drug carrier and its safety after oral administration is well known [28,29]. In the light of the above considerations, the purposes of our study were to evaluate the suitability of face-centered cubic (Fm3m) mesoporous silica (FMS) with pore size (N10 nm) as a carrier for CEL (a poorly water-soluble drug) and investigate the effect of its pore size on drug release. Face-centered cubic (Fm3m) mesoporous silica with pore size (16 nm) was successfully synthesized using a low temperature method with Pluronic block co-polymer F127 as a template, TEOS as a silica source and TMB as a pore enlargement agent. Notably, the synthetic process was simplified because the method involved modification of only one factor (temperature). The pore size of FMS was successfully enlarged to 16.0 nm by lowering the temperature and it could be easily regulated by controlling the synthesis temperature of the new method. This not only helped to explore its applications involving large pores, but also helped to investigate the effect of the pore size on the release properties of FMS. CEL was incorporated into FMS with different pore sizes by the solvent deposition method at a selected drug–silica ratio (1:4). The corresponding samples were characterized by TEM, FT-IR, TGA, BET, DSC and XRD investigations. The release behavior of CEL from FMS with different pore sizes was compared.
2. Materials and methods 2.1. Materials Pluronic block co-polymer F127 was kindly donated by BASF. Tetraethyl orthosilicate (TEOS), hydrochloric acid and potassium chloride were purchased from Yu Wang Reagent Company (Shandong, China). 1,3,5-trimethylbenzene (TMB) was obtained from SigmaAldrich (St. Louis, MO, USA). Celecoxib (purity N 99.0%) was kindly supplied from Shenyang Funing Pharmaceutical Co., Ltd. All other chemicals were used in accordance with the requirements of analytical/ spectroscopic/HPLC grade. Deionized water in all experiments was prepared by ion exchange.
2.2. Synthesis of FMS For the synthesis of 3D face-centered cubic mesoporous silica (FMS) with different pore sizes, 1.25 g of KCl and 0.5 g of F127 were mixed under gently stirring in 30.0 mL of HCl (2.0 M) at three different reaction temperatures (15, 18 and 20 °C), followed by the addition of 0.5 g of TMB under stirring. After 1 h of stirring, 2.0 g of TEOS was added drop by drop to the obtained solution under vigorous stirring. After another 24 h of stirring, the obtained suspension was homogenized by an ATS AH110D homogenizer (ATS Engineer Inc., Shanghai, China). Then, the mixture was placed into a Teflon-lined autoclave and carried out by hydrothermal reaction for 24 h at 130 °C which corresponds to the reaction temperature. The obtained mixture was filtered and dried at 60 °C in the air. The as-synthesized mesoporous silica was filtered and burn out at 600 °C for 5 h to remove surfactant completely. Finally, face-centered cubic mesoporous silica (FMS-15, FMS-18 and FMS-20) by controlling the different action temperatures (15, 18 and 20 °C), was obtained until the powers were dried at 60 °C for 48 h.
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2.3. Drug loading procedure CEL was respectively loaded into FMS samples (FMS-15, FMS-18 and FMS-20) by a solvent deposition method, which involved soaking equilibrium and solvent evaporation. In detail, CEL was dissolved in methanol to get a homogeneous solution (10 mg/mL), and then obtained solution was mixed with a selected amount of FMS samples to obtain a mixture at a certain proportion (1:4) (note: safer solvent such as ethanol can be used instead of methanol). Then, the mixture was brought to adsorption equilibrium under gently stirring for 24 h at room temperature in a closed container. Finally, the solvent was allowed to evaporate under gently stirring and then the precipitated powder was washed with ethanol to remove the drugs on the surface of carrier. The obtained powder was dried at 40 °C in air until no organic solvent residue was left. The drug-loaded samples were labeled FMSC15, FMSC-18 and FMSC-20, respectively. 2.4. Analysis of drug content Thermogravimetric analysis (TGA) was performed by a TGA-50 instrument (Shimadzu, Japan) at a heating rate of 10 °C/min under a nitrogen purge of 40 mL/min. The following equation was used to calculate the drug-loading rate. Drug-loading rate = weight of CEL in sample / weight of carrier in sample. All the drug content of samples was within 20 ± 5% of the theoretical value. 2.5. Characterization techniques 2.5.1. SEM study The morphology of the samples was characterized using a field emission scanning electron microscope (JEOL-6700, Japan). 2.5.2. TEM study The mesoporous structure of the samples was characterized using TEM (Tecnai G2 20, FEI, USA). 2.5.3. FT-IR study The functional groups and chemical bonding of samples were characterized using an FT-IR spectrometer (Bruker IFS 55, Switzerland). The scanning range of FT-IR spectroscopy was in the range of 400–4000 cm−1. 2.5.4. Nitrogen adsorption analysis Adsorption–desorption measurements were conducted on a surface area analyzer (SA3100, Beckman Coulter, USA). The carriers were degassed at 120 °C for 12 h, and the CEL-loaded samples were degassed at 50 °C for 24 h to remove physically adsorbed water before analysis. The BET (Brunauer–Emmett–Teller) method and BJH (Barrett–Joyner– Halenda) method were used to investigate the pore characteristics. 2.5.5. XRD and DSC analysis The crystalline characteristics of samples were described using X-ray diffractometer (PW3040/60 PANALYII CALB.V Netherlands). XRD patterns were obtained from 5° to 50° with a scan rate of 5°/min and a step size of 0.02°. DSC profiles of the samples were recorded from a DSC instrument (TA Instruments, Q1000, USA). The temperature range was 50–200 °C at a heating rate of 10 °C/min. 2.6. In vitro release profile study Dissolution studies were carried out using a USP II paddle method with a KC-8D dissolution instrument (KC-8D, Tianjin Guoming Medical Equipment Co., Ltd.). The dissolution media was phosphate buffer (pH 6.9). The dissolution procedure was as follows, dissolution media (900 mL) in basket was kept at 37 °C. The 200 mg samples (equivalent to 50 mg of CEL) were added with a stirring rate of 100 rpm and
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performed for 1 h. 5 mL samples were taken out at designated intervals (5, 10, 15, 20, 30, 45 and 60 min). The amount of CEL dissolved was determined by spectroscopy (UV-2000, Unico, USA) at 254 nm (n = 6). 3. Results and discussion 3.1. Preparation and characterization of FMS carrier In the present study, FMS-15, FMS-18 and FMS-20 were synthesized by a soft template method. The particle size and external morphology of them were characterized by SEM. As shown in Fig. 1B, C and D, the FMS15, FMS-18 and FMS-20 all exhibited the same standard hexagon and approximately 5–10 μm of particle size. The longitudinal section confirmed their three-dimensional structure. It is well known that the pore size is the most important factor regarding drug release in investigations of mesoporous silica as a carrier [30,31]. However, investigations of the pore size of mesoporous silica with a 3D cubic pore structure as a drug carrier has mainly focused on the range 2–10 nm due to difficulties in increasing the pore size of mesoporous silica [16,20]. There is a lack of information about 3D cubic mesoporous silica with a pore size greater than 10 nm in drug release studies. In our present work, FMS with a 3D face-centered cubic pore morphology and a pore size of 16.0 nm was prepared at a low synthesis temperature and using TMB as a pore enlargement agent. The synthetic process involves the following steps. First of all, KCl and F127 (template) were mixed in HCl (2.0 M) at three different reaction temperatures (15, 18 and 20 °C), followed by the addition of TMB (pore enlargement agent) and TEOS (silica source). Subsequently, the mixture underwent a hydrothermal reaction to form the mesoporous silica framework. Finally, FMS-15, FMS-18 and FMS-20 (15, 18 and 20 representing the reaction temperatures) were obtained by removing the template using burn out.
Fig. 1. SEM images of (A) CEL (B) FMS-15, (C) FMS-18, (D) FMS-20 and (E) FMSC-15.
In this study, the reaction temperature played the most important role in the regulation of pore size. Different reaction temperatures (15, 18 and 20 °C) were used in the synthesis of FMS with different pore sizes. Fig. 2 shows the TEM micrographs of FMS-15, FMS-18 and FMS20. It was found that FMS-15, FMS-18 and FMS-20 had different pore sizes (approximately 16.0, 6.9 and 3.7 nm, respectively) (Fig. 2A, B and C). As the temperature decreased from 20 to 15 °C, the pore size of FMS increased from 3.7 to 16 nm. FMS-15 with pore size of 16.0 nm was obtained when the reaction temperature reached 15 °C (Fig. 2C). As the reaction temperature decreased, the micelle bonds were weakened, allowing more pore enlargement agent TMB to be embedded in the micelle core, thus causing further enlargement [32]. In most cases, disordered mesoporous silica is obtained when the reaction temperature is reduced [33]. In order to solve this problem, it is necessary to add the right amount of KCl (1.25 g). Inorganic salts (such as KCl) can help to maintain the ordered mesoporous structure of FMS by improving the interaction between the silicate species and the nonionic pluronic block copolymer F127 [34]. As seen in Fig. 2A, B and C, the pores of FMS-15, FMS-18 and FMS-20 were in a highly ordered array which allowed effective control of the drug particle size and its release. To the best of our knowledge, no detailed investigations have been conducted using 3D face-centered cubic mesoporous silica with pore size of 16.0 nm as a carrier for drugs and there are no published reports of the effect of the pore size of 3D cubic mesoporous silica on drug release. 3.2. Drug loading and characterization In our work, CEL was incorporated into FMS samples (FMS-15, FMS18 and FMS-20) by the solvent deposition method. This method has been proved to be very effective for the synthesis of solid dispersions of poorly water-soluble drugs [35,36]. The drug loading process involves the two steps: the initial adsorption equilibrium and subsequent solvent evaporation. Methanol was selected as the loading solvent because of its volatility and because CEL was very soluble in it. The degree of drug loading was determined by TGA analysis to ensure a drug loading of 20%. The excellent drug loading characteristics of FMS with different pore sizes were confirmed by the results of SEM, TEM, FT-IR and TGA. As shown in Fig. 1A, B and E, the pure drug (celecoxib) was massive particles and present in a crystalline form (Fig. 1A). The FMS-15 exhibited the standard hexagon (Fig. 1B). There was almost no lump drugs remained on the surface of the carrier, indicating that drug not loaded into the pores was washed away (Fig. 1E). The morphology of the drug-loaded samples was studied by TEM. As seen in Fig. 2, in contrast to unloaded FMS samples, most of the pore channels of FMSC samples (FMSC-15, FMSC-18 and FMSC-20) were not as clear, indicating that a large number of the pore channels of FMSC samples had been filled with CEL. The inclusion of CEL into the pore channels of mesoporous silica was further to be confirmed by FT-IR. Celecoxib, a cyclooxygenase-2 (COX2) selective inhibitor is a non-steroid anti-inflammatory drug (NSAID). The molecular formula of celecoxib is C17H14F3N3O2S. Its molecular weight is 381.37. The chemical name is 4-[5-(4-methylphenyl)-3(trifluoromethyl)-1H-pyrazol-1-yl]-benzenesulfonamide. The benzenesulfonamide is a crucial functional group, which would play a crucial role in drug loading (Fig. 3). The FT-IR spectra of samples (CEL, FMS-15, their corresponding physical mixtures and FMSC-15) were shown in Fig. 4. The FMS-15 showed the wide adsorption bands in the range of 3750–3000 cm−1 due to the silanol groups. In addition, CEL showed a double sharp peak at 3342 and 3235 cm−1 and two single peaks at 1348 and 1164 cm−1, which corresponds to the amino groups and sulfonyl groups of sulfonamide in CEL, respectively. For the physical mixtures, the present peaks associated with both CEL and FMS-15 confirmed no interactions between them. However, a marked decrease of amino peaks was present in the spectrum of the FMSC-15, which may be due to the
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Fig. 2. TEM images of (A) FMS-20, (B) FMS-18, (C) FMS-15, (D) FMSC-20, (E) FMSC-18 and (F) FMSC-15.
Fig. 3. Chemical structure of celecoxib
Fig. 4. FT-IR spectrum for crude CEL (A), the physical mixtures of CEL and FMS-15(B), FMSC-15 (C) and FMS-15 (D).
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3.3. Nitrogen adsorption analysis
Fig. 5. TGA profiles of CEL, FMS-15, FMS-18, FMS-20, FMSC-15, FMSC-18 and FMSC-20.
hydrogen bonding between the sulfonamide groups in the CEL and the silanol groups of the silica [37]. TGA measurements were used to measure the weight of CEL loaded in the FMSC. As showed in Fig. 5, FMS-15, FMS-18 and FMS-20 exhibited almost no weight loss due to the excellent thermal stability of mesoporous silica. The weight loss of FMS-15, FMS-18 and FMS-20 was 18.6%, 19.1% and 17.6%, respectively. However, there were some differences between the expected drug loading (20%) and the practical drug loading. The reason for this may be the loss during the washing process in drug loading. The results showed that the loss of CEL during the drug loading was less than 5%. Additionally, the volume occupation ratio of CEL in the mesoporous silica was evaluated from the amount of CEL measured by TGA. The amount of drug loaded into the FMSC-15, FMSC-18 and FMSC-20, estimated from the value of TGA was 18.6%, 19.1% and 17.6%, respectively. However, the theoretical volume occupation ratio of CEL loaded on FMSC-15, FMSC-18 and FMSC-20, estimated from the value of TGA is 9.7%, 12.2% and 14.3%, respectively, which was according to the following equation: Volume occupation ratio (%) = (wt.% (CEL) / M (molar mass)) × N (Avogadro's constant) × V (molecule volume) / V (pore volume). It was worth noting that the theoretical volume occupation ratio (9.7%, 12.2% and 14.3%) was all lower than the practical volume occupation ratio (24.6%, 33.3% and 78.8%). It was concluded that drug molecules inside the pores were all not ordered and may block the pore windows or pore channels. The difference (14.9%) between the theoretical volume occupation ratio (9.7%) and the practical volume occupation ratio (24.6%) of FMSC-15 was relatively smaller than that of FMSC-18 (21.1%) and FMSC-20 (64.5%), indicating uniform drug loading. This may facilitate the movement of the dissolution medium in the pore channels and the drug release. So the cumulative release percentage of FMSC-15 was the highest among them (Fig. 9).
The pore structure and BET characteristics of samples were provided by N2 adsorption measurements. As can be seen from Fig. 6A, B and C, the IV isotherms and a type H2 hysteresis loop of FMS samples, according to the IUPAC classification, confirmed their typical mesoporous structure. A relatively sharp increase at around P/P0 = 0.85 was seen in all the adsorption isotherm profiles of FMS samples, which confirmed their uniform pore structure. In addition, the pore size distribution of FMS samples all showed a single significantly sharp peak, which indicated that they had a uniform pore size. These results were also supported by the pore size analysis. Table 1 shows that FMS-15, FMS-18 and FMS-20 had a uniform pore size of 16.0, 6.9 and 3.7 nm, respectively. After drug loading, the type of isotherms and hysteresis loop for FMSC-18 and FMSC-20 remained almost unchanged compared with that of FMC18 and FMS-20, indicating that most pore channels were not affected by drug filling (Fig. 6B and C). However, the isotherm type and hysteresis loop shape of FMSC-15 were both different from that of FMS-15, which indicated that the original pore morphology of FMS-15 was not maintained (Fig. 6A). The data on the surface area (BET), pore volume (Vt) and the BJH pore diameter (WBJH) are given in Table 1. The surface area and pore volume of FMSC samples all exhibited a significant reduction compared with that of the corresponding FMS samples (unloaded drugs), which was due to the fact that the drug filling occupied the pore spaces and reduced the pore volume. In addition, the pore volume of FMS-15 was much larger than that of FMS-18 and FMS-20, which means that FMS-15 has a higher drug loading capacity than FMS-18 and FMS-20. This property confirms the potential advantages of using FMS-15 as a drug carrier. To investigate the relationship between the occupation volume and well-diffusion pathway, the volume occupation ratio of CEL loaded on FMSC-15, FMSC-18 and FMSC-20, estimated from the value of each pore volume is 24.6%, 33.3% and 78.8%, respectively. It was worth noting that the cumulative release amounts within 60 min followed the sequence: FMSC-15 N FMSC-18 N FMSC-20. The unoccupied pore space decreased on increasing the volume occupation ratio. It can be inferred that unoccupied pore space determined the amount of the dissolution medium into the pore channels. More dissolution medium moved into the pore channels and more drug molecules can be dissolved, when unoccupied pore space was bigger. So it was speculated that unoccupied pore space (FMSC-15 (75.4%) N FMSC-18 (66.7%) N FMSC-20 (21.2%)) may directly affect the cumulative release percentage (FMSC-15 N FMSC-18 N FMSC-20). 3.4. Solid state characterization by XRD and DSC The crystal properties of samples were evaluated by XRD analysis using a fixed angle ranging from 5° to 50°. As shown in the XRD patterns
Fig. 6. Adsorption–desorption isotherms and pore size distributions of (A) FMS-15 and FMSC-15, (B) FMS-18 and FMSC-18, and (C) FMS-20 and FMSC-20.
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Table 1 Surface area, pore volume and pore size of samples.
FMS-15 FMS-18 FMS-20 FMSC-15 FMSC-18 FMSC-20
Surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
252.15 219.76 428.09 170.90 154.61 68.10
0.81 0.66 0.52 0.61 0.44 0.11
16.01 6.93 3.78 12.33 6.28 3.88
(Fig. 7), two main intense and characteristic peaks of pure CEL were found at 16.1° and 21.6°. However, FMS-15, FMS-18 and FMS-20 all showed another two completely different diffraction peaks at 28.3° and 40.5°. After completion of the drug loading process, FMSC-15 and FMSC-18 did not show any characteristic CEL diffraction peaks, and the two diffraction peaks of FMS-15 and FMS-18 were still preserved. The drug is in a non-crystalline state if none of its characteristic peaks are present [38,39]. Accordingly, we can infer that the crystalline form of CEL in the pores of FMS-15 and FMS-18 was converted into a noncrystalline form. Also, the characteristic diffraction peaks of CEL in FMSC-20 were still present but weaker and the main diffraction peak of FMS-20 remained. This indicated that the crystalline form of CEL was not completely masked by FMS-20. This may be because a portion of the drug remained on the surface of FMS-20 and was not loaded into the pores of FMS-20. The crystalline characteristics of the samples were further confirmed using DSC which is also a sensitive method for detecting crystals. As is well known, the depression of the melting point is a standard means of determining whether a crystalline state is still present. As seen in Fig. 8, there was a single endothermic melting peak at 161.1 °C, in agreement with the melting point of pure CEL, while FMS-15, FMS-18 and FMS-20 did not show any diffraction peak at the same temperature. Furthermore, for FMSC-15 and FMSC-18, the inverted peak of CEL was not clearly present at the same temperature. In contrast, for FMSC-20, the endothermic melting peak was still present although it was weak. This indicated that the crystalline form of CEL was not covered up by FMS-20 and completely covered up by FMS-15 and FMS-18. These DSC results all agreed with the above XRD results. Together, the XRD and DSC results provided indirect evidence of the advantage of using FMS-15 with a large pore size as a drug carrier. As shown by both the XRD and DSC results, when CEL was loaded into
Fig. 8. DSC patterns of FMS-15, FMS-18, FMS-20, FMSC-15, FMSC-18, FMSC-20 and pure CEL.
FMS-15, FMSC-18 and FMS-20, the CEL (in the pores of FMS-15 and FMSC-18) was in a non-crystalline state, while in the pores of FMS-20 it was still in the original crystalline state. It was concluded that FMS15 was responsible for the non-crystalline state of CEL due to the fact that nanopores of mesoporous materials can retain a drug in the noncrystalline state by space confinement [40–42]. Furthermore, more drug loading by FMS-15 was produced by the large pore size. When the same amount of CEL was loaded into FMS-15 and FMS-20, all drugs were completely loaded into the pores of FMS-15, while some of the drug remained on the surface of FMS-20, leading to the disappearance of the crystalline form for FMSC-15 and the presence of the crystalline form for FMSC-20. The larger the pore size, the greater the degree of drug loading. It was also concluded that CEL molecules can be effectively prevented from growing into a crystalline form when they were confined to the nanopores of FMS-15. 3.5. In vitro drug dissolution Dissolution tests were carried out in order to study the application of FMS-15 with a larger pore size (about 16.0 nm) and a 3D face-centered cubic mesoporous structure as a drug carrier and to investigate the effect of different pore sizes of FMS samples on the dissolution behavior of CEL. The in vitro drug release of CEL from FMSC-15, FMSC-18 and FMSC-20 can be mainly divided into two stages: burst release (0–10 min) and prolonged release (10–60 min). The burst release phenomenon is present in many drug release processes which use mesoporous materials as drug carriers [28,39]. The reason for the initial burst release was mainly
Fig. 7. XRD patterns of FMS-15, FMS-18, FMS-20, FMSC-15, FMSC-18, FMSC-20 and pure CEL.
Fig. 9. Dissolution profiles of FMSC-15, FMSC-18, FMSC-20 and pure CEL.
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attributed to the presence of drug in the outer pores of mesoporous materials. As shown in Fig. 9, all the release rates of CEL from FMSC samples were much faster than that of CEL during the initial stage. The dissolution rate of CEL within 10 min was only about 13% due to its poor dissolution as a BCS class II drug. However, the cumulative release of CEL from FMSC15, FMSC-18 and FMSC-20 was 63%, 46% and 58%, respectively. The most likely reason for the improvement in dissolution rate may be that the spatial confinement within the nanosized pores of FMS reduced the degree of CEL crystallization or maintained CEL in a non-crystalline state, since it is well known that if a drug is in a non-crystalline state this effectively increases their dissolution rate [43,44]. Furthermore, the burst release for FMSC-20 was more significant than that of FMSC-15 and FMSC18 within the 5 min. This may be due to the fact that the amount of drug remaining in the external pores with FMS-20 was greater than that of FMS-15 and FMS-18, which was caused by the smaller pore size of FMS-20 compared with that of FMS-15 and FMS-18. This was also confirmed by the results of XRD and DSC analysis. The excess drug present in the external pores of FMS-20 resulted in the presence of the crystalline form for CEL, which cannot be covered up by FMS-20 (Figs. 7 and 8). Additionally, the cumulative release amounts within 60 min followed the sequence: FMSC-15 N FMSC-18 N FMSC-20. This was consistent with their pore size: FMS-15 (16.0 nm) N FMS-18 (6.9 nm) N FMS-20 (3.7 nm). So, it was concluded that the release behavior of CEL was mainly influenced by the pore size of the FMS samples. The cumulative release amounts of CEL increased on increasing the pore size (3.6 nm– 16.0 nm). This could be explained as follows: One reason may be that drugs in the larger pores encountered less diffusion resistance due to the spacious pore channels and moved more easily into the dissolution medium. For example, the pore size of FMS-15 (16.0 nm) was much larger than that of FMS-18 (6.9 nm) and FMS-20 (3.7 nm). The cumulative release amounts of FMSC-15 was the fastest of all, which could be attributed to the fact that the CEL molecules in the pores of FMS-15 with relatively pore size of 16.0 nm had more opportunity to escape from pores and enter the dissolution medium. Another reason may be that the diffusion-controlled process could be used to scribe the drug release process. When the dissolution medium penetrated into the pore channels of the carriers, the drug molecules would dissolve in the dissolution medium and escape from the pores along the pore channels filled with the solvent. The pore channels with larger pore volume would be filled with the more dissolution medium, which would dissolve more drug molecules. So the drug molecules that stayed in the larger pores would have more chance to escape from the carriers. Therefore, it was predicted that FMS-15 with a face-centered cubic pore structure could significantly increase the release rate of poorly water-soluble drugs by taking advantage of its large pores and open pore structure. Many forms of mesoporous silica with a large pore size have been used to load proteins [45]. However, to the best of our knowledge, there is almost no information about the use of 3D face-centered cubic mesoporous silica with a large pore size (up to over 10 nm) as a carrier for loading poorly water-soluble drugs, which was the main reason for using FMS-15 as a carrier for the poorly water-soluble drug CEL in our study. FMS-15 has the following advantages as a carrier for poorly water-soluble drugs: Firstly, FMS-15 with a large pore size (about 16.0 nm) could significantly improve the drug release rate due to the fact that the diffusion resistance of drugs in the larger pore sizes was reduced. Secondly, FMS-15 with an open pore structure (face-centered pore structure) and threedimensional interconnected pore structure (cubic pore structure) could facilitate drugs transfer and reduce the diffusion resistance of drugs. Thirdly, FMS-15 with a nanopore structure could effectively reduce drug crystallinity or inhibit formation of the crystalline form. Highly ordered crystals could not be formed due to the spatial confinement within nanosized pores of FMS-15. As is well known, changing the drug crystal form is an effective way of improving the dissolution rate of poorly water-soluble drugs [46,47]. It was concluded that FMS-15 with a large pore size (about 16.0 nm) and face-centered cubic pore structure was especially suitable as a carrier for poorly water-soluble drug CEL.
Furthermore, the dissolution rate of CEL could be controlled by regulating the pore size of 3D face-centered cubic mesoporous silica. The optimization of mesopore size in order to enhance the dissolution properties for the poorly water-soluble drugs in this study should be based on uniform drug loading without pore blocking. However, uniform drug loading for all samples (FMS-15, FMS-18 and FMS-20) was not to be obtained by the same drug loading method. This may be because the pore channels of FMS-18 and FMS-20 with a smaller pore size be much easier to be blocked, compared with that of FMS-15. In the case of FMS-15, the pore diameter after the loading of CEL was changed from 16.0 to12.3 nm (Fig. 6). However, pore diameter of FMS-18 and FMS-20 (6.9 nm and 3.8 nm, respectively) after the loading of CEL (6.3 nm and 3.9 nm, respectively) was almost unchanged (Fig. 6). Moreover, isotherm type and hysteresis loop shape of FMSC15 were both different from that of FMS-15. The type of isotherms and hysteresis loop for FMSC-18 and FMSC-20 remained almost unchanged compared with that of FMC-18 and FMS-20. The above results probably indicated that CEL may not be uniformly introduced into the mesopore of FMSC-18 and FMSC-20, compared with that of FMSC-15. This may cause nonuniform distribution of drugs in the pore channels or the blocked pore windows. Especially, the burst release for FMSC-20 was more significant than that of FMSC-15 and FMSC-18 within the 5 min. Most drugs may accumulate at the pore window and part of drugs may be loaded into the pore channels due to irregular pore filling for FMSC-20. So drugs at the pore window diffused rapidly to the dissolution medium, which resulted in significant burst release of FMSC-20. Moreover, irregular pore filling for FMSC-20 hampered the drug release, resulting in 74.4% of cumulative release. So pore filling may also have a direct relationship with the drug release. In comparison, the cumulative release percentage of FMSC-15 within 60 min was higher than that of FMSC-18 and FMSC-20. This may be attributed to the more regular pore filling of FMS-15 than that of FMSC-18 and FMSC-20. The regular pore filling could facilitate the well-diffusion pathway of the dissolution medium. In conclusion, uniform drug loading and pore size both play an important role in enhancing the dissolution properties for the poorly water-soluble drugs. 4. Conclusion The mesoporous silica (FMS-15) with a large pore size (16.0 nm) and faced-centered cubic structure was successfully synthesized and then the rapid release of CEL (a poorly water-soluble drug) from FMS15 was carried out in our present study. This was the first report of the use of 3D face-centered cubic mesoporous silica with large pore sizes (up to over 10 nm) as a drug carrier. Characterization using SEM, TEM, BET, FT-IR, TGA, XRD and DSC demonstrated the successful incorporation of CEL into the FMS samples (FMS-15, FMS-18 and FMS-20). A great reduction in crystallinity, partial or total crystalline-amorphous transformation was observed for CEL after it was loaded into FMS samples. The marked crystallinity loss or disappearance was probably due to spatial confinement of nanopores. Moreover, we further investigated the effect of pore size on the release of CEL from FMS samples. The results obtained showed that the cumulative release amounts within 60 min of CEL from FMS samples increased on increasing the pore size (3.7–16.0 nm) of the FMS samples. This may be due to the fact that less diffusion resistance was encountered for drugs in the larger pores and more dissolution medium penetrated into pore channels would dissolve more drug molecules. In addition, the results of in vitro dissolution tests also showed that CEL released from FMS samples all exhibited a significant improvement of dissolution rate. The following factors appear to be responsible for the dissolution improvement: the reduction in crystalline form and formation of the amorphous form, nanopore size and the interconnected cubic pore channels of the FMS samples, Moreover, uniform drug loading and pore size both play an important role in enhancing the dissolution properties for the poorly watersoluble drugs. It was worth noting that FMSC-15 had the fastest
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dissolution rate among all the FMSC samples, which was due to its special face-centered cubic pore structure, large pore size of 16.0 nm and uniform drug loading. We believe that our study will open up new opportunities for using 3D cubic mesoporous silica with large pores as delivery systems for poorly water-soluble drugs. Acknowledgment This work was supported by the National Basic Research Program of China (973 Program) (No. 2009CB930300) and the National Natural Science Foundation of China (No. 81273449). References [1] F. Chen, et al., Chitosan enclosed mesoporous silica nanoparticles as drug nanocarriers: sensitive response to the narrow pH range, Microporous Mesoporous Mater. 150 (2012) 83–89. [2] C.X. Lin, et al., Periodic mesoporous silica and organosilica with controlled morphologies as carriers for drug release, Microporous Mesoporous Mater. 117 (2009) 213–219. [3] T. Heikkila, et al., Mesoporous silica material TUD-1 as a drug delivery system, Int. J. Pharm. 331 (2007) 133–138. [4] Y. Zhu, et al., PEGylated hollow mesoporous silica nanoparticles as potential drug delivery vehicles, Microporous Mesoporous Mater. 141 (2011) 199–206. [5] X. Li, et al., Preparation of mesoporous calcium doped silica spheres with narrow size dispersion and their drug loading and degradation behavior, Microporous Mesoporous Mater. 102 (2007) 151–158. [6] M. Vialpando, et al., Risk assessment of premature drug release during wet granulation of ordered mesoporous silica loaded with poorly soluble compounds itraconazole, fenofibrate, naproxen, and ibuprofen, Eur. J. Pharm. Biopharm. 81 (2012) 190–198. [7] I. Slowing, et al., Mesoporous silica nanoparticles as controlled release drug delivery and gene transfection carriers, Adv. Drug Deliv. Rev. 60 (2008) 1278–1288. [8] M. Liu, et al., A novel liposome-encapsulated hemoglobin/silica nanoparticle as an oxygen carrier, Int. J. Pharm. 427 (2012) 354–357. [9] H. Peng, et al., A pH-responsive nano-carrier with mesoporous silica nanoparticles cores and poly(acrylic acid) shell-layers: fabrication, characterization and properties for controlled release of salidroside, Int. J. Pharm. 446 (2013) 153–159. [10] J. Liu, et al., Magnetic silica spheres with large nanopores for nucleic acid adsorption and cellular uptake, Biomaterials 33 (2012) 970–978. [11] S. Wang, Ordered mesoporous materials for drug delivery, Microporous Mesoporous Mater. 117 (2009) 1–9. [12] Y. Zhou, et al., 3D net-linked mesoporous silica monolith: new environmental adsorbent and catalyst, Catal. Today 166 (2011) 39–46. [13] Y. Hu, et al., Facile synthesis of 3D cubic mesoporous silica microspheres with a controllable pore size and their application for improved delivery of a water-insoluble drug, J. Colloid Interface Sci. 363 (2011) 410–417. [14] W. Na, et al., Large pore 3D cubic mesoporous silica HOM-5 for enzyme immobilization, Mater. Lett. 62 (2008) 3707–3709. [15] J. Huang, et al., Large-pore cubic Ia-3d mesoporous silicas: synthesis, modification and catalytic applications, J. Mol. Catal. A Chem. 271 (2007) 200–208. [16] Y. Hu, et al., 3D cubic mesoporous silica microsphere as a carrier for poorly soluble drug carvedilol, Microporous Mesoporous Mater. 147 (2012) 94–101. [17] S.K. Das, et al., Effects of surface acidity and pore size of mesoporous alumina on degree of loading and controlled release of ibuprofen, Microporous Mesoporous Mater. 118 (2009) 267–272. [18] F. Qu, et al., Controlled release of Captopril by regulating the pore size and morphology of ordered mesoporous silica, Microporous Mesoporous Mater. 92 (2006) 1–9. [19] Y.P. Wang, et al., Control of pore size in mesoporous silica templated by a multiarm hyperbranched copolyether in water and cosolvent, Microporous Mesoporous Mater. 120 (2009) 447–453. [20] M.J.K. Thomas, et al., Inclusion of poorly soluble drugs in highly ordered mesoporous silica nanoparticles, Int. J. Pharm. 387 (2010) 272–277.
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