Drug release from layered double hydroxides and from their polylactic acid (PLA) nanocomposites

Applied Clay Science 71 (2013) 1–7

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Research paper

Drug release from layered double hydroxides and from their polylactic acid (PLA) nanocomposites María S. San Román, María J. Holgado, Beatriz Salinas, Vicente Rives ⁎ GIR-QUESCAT, Departamento de Química Inorgánica, Universidad de Salamanca, 37008 Salamanca, Spain

a r t i c l e

i n f o

Article history: Received 21 July 2012 Received in revised form 19 October 2012 Accepted 26 October 2012 Available online 7 December 2012 Keywords: Drug release Polylactic acid Nanocomposites Layered double hydroxides

a b s t r a c t Layered double hydroxides (LDH) with intercalated diclofenac, chloramphenicol and ketoprofen have been supported and dispersed in semicrystalline polylactic acid (PLA). The solids have been characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and Fourier-Transform infrared spectroscopy (FT-IR). Drug release from the systems prepared was followed by UV–vis spectroscopy. Release is almost complete after 24 h for ketoprofen from the drug-LDH systems, but lower values were measured for the other drugs (60 and 80%, respectively, for diclofenac and chloramphenicol); however, when supported on PLA the release was much slower and could be related to the degradation of the polymer. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Inorganic materials combined with biopolymers have been widely investigated in recent years to be used in the development of implants, to modulate the activation of tissue repair and the regeneration processes, base of the healing process. Another important field of investigation of this kind of biomaterials is for obtaining a precise and tuneable control of anionic molecules release (Ambrogi et al., 2009; Bordes et al., 2009; Lee and Chen, 2006; Vainionpaa et al., 1989; Vallet-Regí et al., 1997; Vieille et al., 2004; Vivero-Escoto and Huang, 2011). The possibility of local diffusion of a drug through biocompatible and reabsorbable matrixes offers the advantage of a drastic reduction of the administered dose, if compared to the usual oral administration (Ambrogi et al., 2003; Ribeiro et al., 2009), which in turn can be related to a decrease in side effects. A wide range of additives, both organic and inorganic molecules, can be immobilized inside the biomaterial to incorporate directly this “active” molecules. Many different polymers are currently being used in this field. Some of them are stable and are used for permanent applications, e.g., poly(methylmetacrylate), PMMA, or polyethylene (PE). In addition, biodegradable polymers are also used for temporary applications (Chakrabortia et al., 2011). Kulkarni et al. introduced some 60 years ago the concept of bioabsorbable material (Kulkarni et al., 2005). Many papers have dealt with biodegradable polymers, such as those derived from lactic

⁎ Corresponding author at: Dpto. de Química Inorgánica, Universidad de Salamanca, 37008-Salamanca, Spain. Tel.: +34 923 29 44 89; fax: +34 923 29 45 74. E-mail address: [email protected] (V. Rives). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2012.10.014

acid, e.g., polylactic acid (PLA), and polyglycolic acid (PGA), which have demonstrated their interest and applicability in biomaterials (Ambrogi et al., 2008; Dagnon et al., 2009; Hsu et al., 1999; Quintanar-Guerrero et al., 1998; Sturesson et al., 1999). The most useful property of these compounds is that they are compatible with the tissues, and are degraded at a given time after implanted, leading to non-toxic subproducts which can be released or metabolized by the organism. Biodegradable polymers belong to this group, although some ceramic materials are also reabsorbable. Bioabsorbable materials need to fulfil some specific characteristics to be used as implants in human organisms, for instance, the materials and their subproducts should not be mutagenic, carcinogenic, antigenic, toxic, and obviously should be antiseptic, sterilizable, and compatible with the tissue where they are implanted; they should be also easily processed and able to be conformed in different shapes. Most of the current research in the area of polymers for biomedical applications addresses mainly to develop synthetic polymers, such as polylactic acid, and natural ones, such as collagen or dextran. The first biodegradable polymers being developed and most widely used are those prepared from polyglycolic (PGA) and polylactic (PLA) acids, which are used in the medical industry, for instance, in biodegradable suture. Numerous devices based on PGA or PLA, as well as other materials, have been developed since. Polylactic acid (PLA) is a thermoplastic, amorphous (or semicrystalline) polymer, very widely used for hosting drugs for controlled delivery, biodegradable sutures and different implants for fixing fractures and for cardiovascular devices (Lasprilla et al., 2012). As lactic acid is a common intermediate in carbohydrate metabolism in human organisms, the use of this hydroxyacid is considered an optimum situation from a toxicological point of view. Degradation of PLA takes place via hydrolysis and can be accelerated in vivo by enzymes, leading

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to release of the monomer (lactic acid). These monomers are incorporated into physiological processes at the cell level, where degradation proceeds, leading to the metabolic route, which can undergo chain disruption in the human body and degrades into nontoxic byproducts, lactic acid, carbon dioxide and water which are subsequently eliminated through the Krebs cycle and in the urine. Many drugs are insoluble in aqueous media or biological fluids and thus their effect is diminished. One of the methods currently used to enhance their solubility is to host them in the interlayer space of layered compounds, such as clays (if they are positively charged) or hydrotalcites (if they are negatively charged), a family of layered materials which can be described as layered double hydroxides (LDH). The main advantage of these LDHs is that they increase the solubility of the drug and control its release, while supporting them on a polymeric excipient decreases the mobility of ionic drugs and reduces their aggregation, increasing the effect of the drug on a long time term (Ha and Xanthos, 2011). Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds or anionic clays, are two dimensional materials. The best known of these materials is natural hydrotalcite, with the brucite structure, where for each set of eight Mg2+ cations, two are substituted by Al 3+, and the positive charge in excess is balanced by carbonate anions hosted together with water molecules in the interlayer (Cavani et al., 1991; Miyata, 1975, 1983). The normalized formula for all LDHs can be written as [M1−xM´x(OH)2](An−)x/n mH2O, where M and M´ usually stand for divalent and trivalent metal cations, respectively (Rives, 2001). Applications of these materials have been studied (Arco et al., 2008; Begu et al., 2009; Gasser, 2009; Holgado et al., 2001; Li et al., 2009) and reviewed (Duan and Evans, 2006; Rives et al., 2010; Wypych and Satyanarayana, 2004). In this work we have prepared hydrotalcites containing Zn(II) and Al(III) in the brucite-like layers by the coprecipitation method, and we have intercalated nonsteroidal antiinflamatory drugs (NSAIDs), such as ketoprofen and diclofenal, which are scarcely soluble in water, and a water soluble antibiotic, chloramphenicol succinate in its sodium salt form. These hybrids have been characterized and they have been dispersed in a biodegradable polymer (PLA), leading to nanocomposites (LDH-drug P), to compare the drug release from the layered host and from the polymer-supported host. We have used PLA for a double purpose: it acts covering the hybrids here synthesized and secondly as a biomaterial support for regeneration of injured human tissues. Our aim has been to study a slow release of the drugs, which are not orally administrated, but implanted in the body, for instance for soldering bones or tissue regeneration, aiming to release the drug for periods as long as several months. 2. Materials and methods All drugs, diclofenac sodium salt, chloramphenicol succinate sodium salt, and ketoprofen, were supplied by Sigma-Aldrich (Spain). Ketoprofen and diclofenac are very soluble in methanol; aqueous solubility is low, but increases with increasing pH (Ha and Xanthos, 2011; Kincl et al., 2004). Poly(L-lactide) was supplied by Aldrich (Spain). Zinc nitrate hexahydrate and aluminum nitrate-9 hydrate were supplied by Panreac (Spain). Sodium hydroxide pellets were from Panreac (Spain). All chemical were used without further purification. 2.1. Preparation of hydrotalcite-intercalated drugs A salts solution was prepared by adding 150 mL of decarbonated water to Zn(NO3)2 6H2O (0.125 mol) and Al(NO3)3 9H2O (0.0625 mol). The solution thus formed was slowly added onto a basic solution of the drug prepared by dissolving 0.125 mol of the sodium salt of chloramphenicol succinate, diclofenac or ketoprofen in 150 mL of solvent (decarbonated water for chloramphenicol succinate sodium salt and ethanol for ketoprofen and diclofenac), also containing the amount of

a 1 M NaOH solution required to yield pH = 9. The whole process was carried out at room temperature. The required amount of 1 M NaOH was further added during addition of the salt solution in order to maintain the pH at a value close to 9. Once the addition was completed the suspension was vigorously stirred under nitrogen atmosphere at room temperature for 24 h. The suspension was then centrifuged and the solid was washed several times with decarbonated water. The solid was filtered and dried in a desiccator at room temperature. The samples were named as ZnAl-Chlo, ZnAl-Dicl and ZnAl-Ket (LDH-drug) for those containing intercalated chloramphenicol, diclofenac or ketoprofen, respectively.

2.2. Preparation of polymer supported hydrotalcite Nanocomposites were prepared by dispersing 4 mg of LDH-drug in 1 g of polylactic acid from Aldrich, previously melted at 180 °C, for 2 h. Mixing was carried at room temperature. The samples were named as LDH-drug P. Previous studies (San Román et al., 2012) have shown that the pristine drugs and the intercalated drugs are thermally stable up to at least 300 °C, and thus they are not decomposed during mixing with the polymer.

2.3. Sample characterization Element chemical analysis was carried out in Servicio General de Análisis Químico Aplicado (Universidad de Salamanca, Spain) by atomic absorption in a Mark 2 ELL-240 apparatus after previous dissolution of the samples in nitric acid. Carbon, nitrogen and hydrogen contents were determined through a LECO CHNS-932 equipment in Centro de Microanálisis Elemental (Universidad Complutense de Madrid, Spain). Powder X-ray diffraction (PXRD) patterns were recorded on a Siemens D-500 apparatus equipped with a Daco-MP microprocessor and Diffrac-AT software, using graphite-filtered CuKα radiation (λ = 1.54 Å). The instrument was set at a current of 30 mA, operating voltage of 40 kV (power 1200 W), and scanning speed of 2°/min (step size 0.05° and step time 1.5 s) in the 2–70° range (2θ scale). Identification of the crystalline phases was made by comparison with the JCPDS files (JCPDS, 1977) and literature data. The Fourier-Transform infrared spectra (FT-IR) of the samples were recorded on a Perkin-Elmer 1600 spectrometer, in the 4000 to 400 cm−1 range, with a nominal resolution of 4 cm−1 and averaging 100 scans to improve the signal-to-noise ratio. The samples were pressed in KBr (Merck) pellets (sample/KBr mass ratio ca. 1:300). Thermogravimetric analysis (TGA) was carried out in a TG-7 instrument from Perkin-Elmer. The analyses were carried out under oxygen (L´Air Liquid, Spain) gas flow (ca. 30 mL min−1) at a heating rate of 10 °C min−1.

2.4. Drug release Drug release from the nanohybrid and from the composite was studied by UV–vis spectroscopy; the spectra were recorded in a Perkin-Elmer Lambda 35 instrument. The wavelengths selected were 276, 276, and 260 nm, for chloramphenicol, diclofenac or ketoprofen, respectively. A regression plot was previously obtained for each drug to determine the molar extinction coefficient in the studied concentration range. For the release studies, these LDHdrug-Polymer (LDH-drug P) nanohybrids (4 mg of LDH-drug in 1 g of PLA) were added to 50 mL of saline solution (NaCl, 0.154 mol/L) at 37 °C, simulating the conditions of the living organisms; 5 mL of liquid was withdrawn at preselected times to analyze the amount of drug released, and 5 mL of physiological serum was added to maintain a constant volume of the solution.

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3. Results 3.1. Element chemical analyses Element chemical analyses data for metals, carbon, nitrogen and hydrogen are included in Table 1. There is an acceptable agreement, within experimental error, between the Zn/Al ratio in the solids and in the starting solutions, 95% for ZnAl-Chlo, the value decreasing to 86 and 84%, respectively, for samples ZnAl-Dicl and ZnAl-Ket. The lack of full agreement between these ratios is rather common in the literature and has been ascribed to a preferential precipitation of one or another cation as a hydroxide during the stirring process (Holgado et al., 1996; San Román et al., 2012). As it will be shown below, the sample containing chloramphenicol also contains a small amount of intercalated nitrate. 3.2. Powder X-ray diffraction Powder X-ray diffraction diagrams have been recorded to check intercalation of the drug anionic molecules in the interlayer space of the LDHs and also to analyze the dispersion degree of the nanohybrids in the polymer. As an example, the diagram for PLA, the ZnAl-Ket sample and the LDH dispersed in PLA are included in Fig. 1; similar results were obtained for the other samples. The PXRD patterns for the LDH-drug samples show formation of, at least, one well crystallized hydrotalcite-like phase, with diffraction lines due to basal planes (00l), confirming the layered structure of these solids. The diffraction lines due to the basal planes are symmetric and sharp and are recorded in the low diffraction angle range, while non-basal reflections are less intense, and somewhat asymmetric, being recorded at larger angle values. Samples ZnAl-Ket and ZnAl-Dicl are very crystalline, with interlayer spacing of 21.97 and 23.35 Å, respectively (San Román et al., 2012), confirming the intercalation of the organic anion in the interlayer space of the inorganic host. However, sample ZnAl-Chlo contains three crystalline phases: in addition to the diffraction maxima due to a layered material, weak diffraction lines due to chloramphenicol succinate are recorded at 2θ values of 13.22 and 16.01°, which positions are coincident with those recorded for the sodic salt of this anion. Probably it is adsorbed on the external surface of the crystallites and is not removed even after several washing cycles. The phase corresponding to the hydrotalcite-like solid with intercalated chloramphenicol shows basal diffractions at 11.65 and 5.96 Å, but a second set of maxima due to a layered material are recorded at 7.47 and 3.95 Å, due to a solid with intercalated nitrate (San Román et al., 2012); the presence of nitrate is further confirmed from the FT-IR spectra of this solid (San Román et al., 2012). Tammaro et al. (2007) have reported, however, a basal spacing of 24.7 Å, but using a Mg,Al matrix, while we have used a Zn,Al one. A detailed study of the lattice parameters of these solids has been reported elsewhere (San Román et al., 2012), and suggests different orientations of the drug anionic molecules within the interlayer space of the host. So, chloramphenicol is intercalated with its molecules parallel to the layers, in a similar way as that previously reported for a Mg,Al hydrotalcite solid with intercalated lactate (San Román et al.,

Fig. 1. X-ray diffraction patterns of polylactic acid and of the ketoprofen-containing samples.

2008). While in the samples with ketoprofen and diclofenac the interlayer anionic molecules are forming a sort of bilayer perpendicular to the layers, but slightly tilted or slightly superimposed (Arco et al., 2009; Jaubertie et al., 2006; San Román et al., 2008). The PXRD patterns of PLA and the LDH-drug P samples show the diffraction maxima of the aluminum sample holder (38.44 and 44.8°), which can be used as a sort of internal reference for a precise determination of the positions of the diffraction maxima. PLA is a very highly crystalline solid (despite the information from the supplier describes it as a semicrystalline solid); the maximum at 16.6° is due to planes (200) and/or (110) of the orthorhombic structure (Chiang and Wu, 2010). The PXRD patterns of the ZnAl-DiclP and ZnAl-KetP nanocomposites show a diffraction maximum due to planes (003) characteristic of the hydrotalcite-like phase at 3.69°, corresponding to a spacing of 24.11 Å; however, it was recorded at 3.98 and 3.83°, respectively, for the original hydrotalcites containing ketoprofen and diclofenac, respectively. This behavior (i.e., an increase in the spacing of the layers) has been previously reported by other authors (Ha and Xanthos, 2011), and is probably due to the fact that after the hot melting processing a small amount of the polymer might have migrated into the interlayer; also Bugatti et al. (2010) have reported this sort of behavior when using polycaprolactone as a support. The maximum due to diffraction by planes (003) is not recorded in the nanocomposite containing chloramphenicol. Even upon repeating the experiment preparing a new sample with a larger dose of LDH the maximum was not either recorded. Tammaro et al. (2007) reported this finding and claimed that the layered material might have been exfoliated during preparation of the composite. Wang et al. (2010) have supported a Zn,Al LDH containing dodecylsulphate on PLA and also confirm the absence of the diffraction maxima due to the hydrotalcite phase; these authors, however, explain their results on

Table 1 Results from element chemical analysis and formulae of the sample prepared.a Sample

ZnAl-Chlo ZnAl-Dicl ZnAl-Ket a b c

Znb

23.19 16.56 20.21

Alb

5.02 3.96 4.95

Cb

15.29 24.24 32.13

All values have been rounded to two figures. Mass percentage. Molar ratio.

Nb

2.67 2.02 0.14

Hb

3.42 3.63 4.24

Zn/Alc

Formula

Solution

Solid

2 2 2

1.9 1.72 1.68

[Zn0.65 Al0.35 (OH)2] (C11H11Cl2N2O5)0.16(NO3)0.19 0.13 H2O [Zn0.63 Al0.37 (OH)2] (C14H11NCl2O2)0.36 0.28 H2O [Zn0.63 Al0.37 (OH)2] (C16H13O3)0.34 0.07 H2O

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the basis of a good dispersion of the hydrotalcite in the polymer, as checked by TEM, and concluding that most of LDH layers are disorderly oriented, providing a direct evidence of crystal layer exfoliation. This result is supported by the absence of a distinct peak below 11.65 Å for ZnAl-ChloP in its PXRD diagrams. Some authors (Tammaro et al., 2007, 2010) have also indicated that a partial intercalation of the polymer chain in the hydrotalcite galleries can take place. In such cases, the reflexion was very broad, probably due to a distribution or unequal distance with a maximum centered around 3.69° (2θ). 3.3. Thermogravimetric analysis All samples were submitted to thermogravimetric analysis to determine the degradation temperature and to determine the interlayer anion content. As an example, Fig. 2 includes the plots for ZnAl-Ket, the polymer-supported ZnAl-KetP sample and the polymer alone (PLA). Curve 2.a corresponds to the thermal decomposition of a hydrotalcitelike phase with progressive mass losses as the temperature is linearly raised. The first mass loss is associated to removal of physisorbed water from the external surface of the crystallites, and the second mass loss to the removal of hydroxyl groups from the layers, together with water molecules from the interlayer, both mass losses amounting ca. 20% of the initial mass sample. The last mass loss is associated to combustion of the intercalated organic drug and extends up to 600 °C, forming a residue composed of ZnO and ZnAl2O4 (Tammaro et al., 2010). From the mass losses recorded and element chemical analysis data, the molecular formulae of the solids have been determined; they are included in Table 1. Curve 2.b corresponds to the thermogravimetric curve of the polymer, with a single mass loss centered around 340 °C; it is totally decomposed to CO2 and H2O. When the nanohybrid is loaded on the PLA polymer (Fig. 2.c) the polymer degradates ca. 15 °C below. The Td midpoint of the composites is in the range where the intercalation compounds lose the constitutional water (Fig. 2.a) leading to formation of the oxides before decomposition of the organic guests. Hence, the anticipation of the Td midpoint, relative to that for pure PLA, could be ascribed to the action of the constitutional water on the ester bonds giving rise to a base-catalyzed decomposition of the polymer, in a similar way as previously observed with PCL (Polycaprolactone) (Chrissafis et al., 2007; Costantino et al., 2009). The thermogravimetric curves for all three nancomposites and pure PLA are included in Fig. 3. In all cases, the average degradation temperature of the nanocomposites is lower than that for the pure

Fig. 2. Thermogravimetric analysis curves of polylactic acid and of the ketoprofencontaining samples.

Fig. 3. Thermogravimetric analysis curves of polylactic acid and of the three nanocomposites studied.

polymer, the decrease being of 45 °C for sample ZnAl-DiclP. Mass loss is lower than 100% because of the formation of the (ZnO+ ZnAl2O4) residue, and it is larger for sample ZnAl-DiclP because the drug loading is somewhat larger than in the other cases. 3.4. FT-IR Spectroscopy The FT-IR spectra of all samples prepared have been recorded; those corresponding to the ketoprofen series are included in Fig. 4, where only the 2000–400 cm −1 range is shown, as it is where the most significant bands are recorded. Outside this range, PLA shows bands at 3002 and 2955 cm −1 corresponding to the antisymmetric and symmetric valence vibrations of the C\H group in methyl units, respectively. An associated band due to the \C_O bonds in the polymer is recorded at 1757 cm−1. These results are similar to those previously

Fig. 4. FT-IR spectra of polylactic acid and of the ketoprofen-containing samples.

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reported by other authors (Hoidy et al., 2010) and are slightly shifted with respect to the values reported by Rincon Lasprilla et al. (2011). These statements are similar those described by other authors (Jahno et al., 2007, 2010; Nikolic et al., 2010). The spectrum of hydrotalcite supported on the polymer shows the bands associated to the polymer, but no band due to the inorganic component is recorded, probably because of its low loading, may be below than the detection level of the technique. The spectrum of the LDH shows its characteristic, expected, bands due to normal vibration modes corresponding to layer hydroxyl groups, water molecules, M\O and M\O\M´ stretching vibrations. Also the bands due to the aromatic ring are recorded in all cases; the band at 3030 cm−1 due to the _C\H group is slightly shifted due to the presence of the substituents; for instance, in sample ZnAl-Chlo, the presence of the nitro group shifts this band up to 3070 cm−1, in agreement with the position of this band as recorded in the spectrum of nitrobenzene (Clarkson and Smith, 2003). The spectrum of this same sample shows a broad band at 3430 cm−1due to the hydroxyl groups of the brucite-like layers and of chloramphenicol succinate. The secondary amide is responsible for the intense band at 1680 cm−1 (San Román et al., 2012). The band due to the C\Cl group is recorded in all cases in the 730–720 cm−1 range (Begu et al., 2009), but is obscured in the spectra of our samples by the bands due to the M\O vibrations. Finally, the presence of cointercalated nitrate is concluded from a sharp band at 1367 cm−1 due to mode v3. Concerning the spectrum of sample ZnAl-Ket, the intense bands at 1533 (overlapped to the band due to the bending mode of water molecules) and 1393 cm−1 are due to the antisymmetric and symmetric stretching modes of the carboxylate group (Nakamoto, 1997). The band associated to the presence of amino groups in sample ZnAl-Dicl is overlapped to the band due to the hydroxyl groups. The band at 1380 cm−1 is due to the stretching mode of the C\N bonds of the amino group, while the bands due to the C_O bond are recorded in the 1650–1450 cm−1 range (Ha and Xanthos, 2011). 3.5. Drugs release Calibration curves for all three drugs, relating the absorbance of the solution with the concentration of the drug, were built from measurements at 276, 276, and 260 nm for chloramphenicol succinate, diclofenac and ketoprofen, respectively, in solutions of physiological serum in the expected concentration range, obtaining straight lines in all cases with regression coefficients larger than 0.9; only a slight deviation from linearity was observed for high concentrations of ketoprofen (Tammaro et al., 2007). Drug release from all three drug-intercalated hydrotalcites along 24 h in physiological serum at 37 °C is shown in Fig. 5. A similar behavior is observed in all cases, namely, a fast initial release, (the so-called “burst” effect (Costantino et al., 2008), almost linearly up to 1 h for diclofenac, 2 h for chloramphenicol or 5.5 h for ketoprofen. This fast release can be explained assuming that it corresponds to organic anions that weakly adsorbed on the external surface of the layered crystallites. Actually, the amount released from the external surface of the crystallites is expected to be very low and hardly modifies the formula calculated. In this so-called “burst” effect the intercalated anionic molecules are released, but only those close to the borders of the crystallites would be released; afterwards the drug molecules deeper within the layers would be released. After that the release is slower, as it corresponds to the anion exchange with anions in the solution and proceeds through diffusion within the interlayer space (Costantino et al., 2008). As relatively large organic anions are exchanged by smaller anions (i.e., chloride, carbonate), a decrease in the interlayer spacing is expected (Costantino et al., 2008). No further release is observed after 8.5 h for sample ZnAl-Ket, and the release plot shows a plateau; the maximum amount of drug released is 1.99 mg, when starting from 4 mg of sample. The amount of ketoprofen

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Fig. 5. Release profiles of drugs from the nanohybrid compounds.

in this sample was 49.98%; consequently, the maximum amount released, as determined by UV spectroscopy, coincides with the maximum amount expected. No clear plateau is observed in the plot for sample ZnAl-Chlo, and after ca. 4 h the drug is continuously and slowly released, but the maximum amount able to be released is not reached even after 24 h. As this sample contained 40.48% drug, the maximum amount to be released, from initial 4 mg of sample, would be 1.61 mg; as the maximum amount experimentally released was 1.06, this means that 66% of the initial amount of drug has been released. Sample ZnAl-Dicl shows a similar behavior, the release being very slow after 2 h, and amounting 60% of the initial amount of drug (2.16 mg, releasing 1.30 mg). Release of the drug from the encapsulated samples is shown in Fig. 6. This study has been carried out along 3 months (12 weeks) in physiological serum at pH 5.5; the polymer is not completely degraded in this time. The slow degradation of the polymer under these experimental conditions permits a slow release of the hydrotalcite and thereof the deintercalation of the organic drug. Such a release is faster during the first weeks of the experiment, probably because it takes place from the hydrotalcite particles supported on the surface of the polymer; after this first step, the release is slower, but significant differences can be observed from one sample to another. The drug is not completely released from sample ZnAl-KetP even after 12 weeks, while for sample ZnAl-ChloP the total amount of drug released is slightly larger than that released from the hydrotalcite during 24 h, the total amount released after 12 weeks being 70% of

Fig. 6. Release profiles of drugs from the composites prepared with PLA.

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the initial loading, suggesting that the release will continue even after 3 months. These results indicate that the amount of drug released depends both on the precise nature of the intercalated anion and on the easiness of its diffusion through the polymer (Costantino et al., 2008). Concerning the ZnAl-DiclP sample, the release proceeds along the time studied; only after 7 weeks the increase in the release rate is slower and reaches 24% of the initial drug loading, a value markedly smaller than those measured for the other two systems studied, in a similar way as that observed from release from the unsupported hydrotalcites. 4. Conclusions Intercalation of the drugs studied in the interlayer space of the ZnAl hydrotalcite has given rise to an increase in the interlayer spacing of 11–22 Å. The spacing slightly further increases in two of the three cases studied upon supporting these hydrotalcite-drug systems into PLA, probably because an incipient intercalation of the polymer or exfoliation of the layers. Thermogravimetric analysis has shown that incorporation of the nanohybrid to PLA anticipates the degradation temperature of the composite by 15–45 °C. This effect could be ascribed to the action of the constitution water on the ester bonds giving rise to a base-catalyzed decomposition of the polymer. The amount of drug released after 24 h amounts 100% of the initial loading in the case of ketoprofen, but only 80 and 60% for chloramphenicol succinate and diclofenac, respectively. When the LDH is incorporated into the biodegradable PLA polymer the release of the drug is much slower and is not completed even after 3 months (the total time the study has been extended to) for any of the samples. Only 36% of ketoprofen is released from the hydrotalcitedrug-PLA system after 3 months, while the amounts released in the cases of diclofenac and chloramphenicol were 24 and 70%, respectively. Nevertheless, the net amounts of drug released might be therapeutically appropriated in all cases, as the amount of drug reaching the tissue will be of the same order as that by oral dosing. On the other hand, it should be mentioned that this release pathway might be also beneficial for the interaction between the drug and the tissue, as in the first stages after the implant the amount of drug required is usually rather large, while as the time elapsed is increased, the amount of drug needed will be lower and lower. Acknowledgments Authors thank financial support from ERDF and MICINN (grant MAT2009-08526). References Ambrogi, V., Fardella, G., Grandolini, G., Nocchetti, M., Perioli, L., 2003. Effect of hydrotalcite-like compounds on the aqueous solubility of some poorly watersoluble drugs. Journal of Pharmaceutical Sciences 92, 1407–1418. Ambrogi, V., Perioli, L., Ricci, M., Pulcini, L., Nocchetti, M., Giovagnoli, S., Rossi, C., 2008. Eudragit and hydrotalcite-like anionic clay composite system for diclofenac colonic delivery. Microporous and Mesoporous Materials 115, 405–415. Ambrogi, V., Perioli, L., Marmottini, F., Moretti, M., Lollini, E., Rossi, C., 2009. Chlorhexidine MCM-41 mucoadhesive tablets for topical use. Journal of Pharmaceutical Innovation 4, 156–164. Arco, M., Fernández, A., Martín, C., Sayalero, M.L., Rives, V., 2008. Solubility and release of fenamates intercalated in layered double hydroxides. Clay Minerals 43, 255–265. Arco, M., Fernández, A., Martín, C., Rives, V., 2009. Release studies of different NSAIDs encapsulated in Mg, Al, Fe-hydrotalcites. Applied Clay Science 42, 538–544. Begu, S., Pouessel, A.A., Polexe, R., Leitmanova, E., Lerner, D.A., Devoisselle, J.M., Tichit, D., 2009. New layered double hydroxides/phospholipid bilayer hybrid material with strong potential for sustained drug delivery system. Chemistry of Materials 21, 2679–2687. Bordes, P., Pollet, E., Avérous, L., 2009. Nano-biocomposites: biodegradable polyester/ nanoclay systems. Progress in Polymer Science 34, 125–155. Bugatti, V., Costantino, U., Gorrasi, G., Nocchetti, M., Tammaro, L., Vittoria, V., 2010. Nano-hybrids incorporation into poly(epsilon-caprolactone) for multifunctional applications: mechanical and barrier properties. European Polymer Journal 46, 418–427.

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