Clay/(PEG-CMC) biocomposites as a novel delivery system for ibuprofen

2.1 Materials

PEG (Mw=10,000 g mol⁻¹), sodium CMC (M_W=700,000; Degree of substitution=0.9) were purchased from Aldrich Chemical Company (St. Louis, MO, USA). Ib (purity 99%) was used as supplied by Sigma-Aldrich Co. (St. Louis, MO, USA). The chemical structures of the drug and polymers used are illustrated in Figure 1.

Figure 1:

Structural formula of (A) ibuprofen (2-(4-isobutylphenyl) propionic acid), (B) sodium salt carboxymethylcellulose (CMC), and (C) poly(ethylene glycol) (PEG).

The clay used is an Mt-rich bentonite (Mostaganem, Algeria) provided by the national non-ferrous mining products and valuable substances ENOF (Algiers, Algeria), and analyzed by its central laboratory. This native clay contains SiO₂ (58.5%), Al₂O₃ (19.5%), Fe₂O₃ (2.8%), MnO (0.06%), CaO (2.5%) MgO (3.4%), Na₂O (2.1%), K₂O (2.2%), H₂O (7.1%), as determined by chemical analysis. The cation exchange capacity was determined to be about 76 meq/100 g by conductometric titration. It was treated according to a standard procedure [24].

In a first step, the raw clay was purified with several washings in de-ionized water. The submicron fraction of the clay was separated by gravity sedimentation. In a second step, we treated the purified clay in an HCl (Sigma-Aldrich, St. Louis, MO, USA) acid solution (1 N) in order to activate it. The third stage of the change was to standardize the existing cations in the interlayer space of Mt, and the saturation of the clay with NaCl (Sigma-Aldrich, St. Louis, MO, USA) solution (0.1 N). The dispersion was reacted three times with NaCl solution, centrifuged, and washed with de-ionized water until free of chloride as tested by AgNO₃ (Sigma-Aldrich, St. Louis, MO, USA) solution. The clay recovered at the end of this step was sodium Mt.

K₂HPO₄ and NaOH (Sigma-Aldrich, St. Louis, MO, USA) used for the preparation of phosphate buffer solution, pH 7.4, were analytical grade and used as received.

2.2 Drug loading

Some 4 g of Mt was dispersed in 100 ml of distilled water with vigorous stirring for 2 h at room temperature. The pH was adjusted to 13 with (0.1 m) NaOH. Then, 0.6 g of Ib was added and ultrasonically stirred at 70°C. After 2 h, the mixture was centrifuged at 3500 rpm for 30 min, and supernatant was filtrated to remove the floating small Mt particles. The Ib concentration in the supernatant was determined with an UV spectrophotometer at λ_max=264 nm after dilution. The incorporation efficiency of MtIb was 13%.

2.3 Preparation of MtIb/polymer composites

CMC, PEG and a CMC/PEG blend in a 50/50 weight ratio were dissolved in de-ionized water and stirred for 2 h to obtain homogeneous solutions. Some 0.4 g of MtIb hybrid was added to 0.1 g of each biopolymer or polymer blend solution and stirred for 3 h into an ultrasonic bath at 60°C to obtain homogeneous suspension. The mixture was then casted onto plates. After evaporation of most of the solvent at room temperature, the samples were placed in a vacuum at 50°C. For the drug release study, the films were milled and then compressed to obtain 6 mm diameter tablets. The obtained samples were named MtIb/PEG, MtIb/CMC and MtIb/PEG-CMC.

2.4 Characterization

The amount of drug intercalated into Mt was calculated from the difference between the concentration of the solutions before and after intercalation, by UV spectroscopy and confirmed by TGA study.

XRD patterns were recorded at 4°/min on a Phillips PW1710 diffractometer by using CuKα radiation at a generator voltage of 50 kV and a generator current of 180 mA. All samples were examined in powder form.

FTIR spectra were recorded at room temperature with a Perkin Elmer FTIR Spectrum Two, using 32 scans and a resolution of 2 cm⁻¹, from 4000 cm⁻¹ to 400 cm⁻¹. The dry powdered samples were mixed with KBr (7% w/w).

TGA was performed on a TAQ 500 instrument under nitrogen atmosphere from room temperature to 600°C at a heating rate of 10°C/min.

The DSC study was carried out using a Perkin Elmer, DSC Diamond. Samples with weights between 5 mg and 10 mg were sealed into an aluminum pan. An empty aluminum sealed pan was used as reference material. The temperature was raised from 25°C to 180°C at a heating rate of 20°C/min under nitrogen atmosphere.

2.5 In vitro drug release experiments

The in vitro drug release behavior was carried out by placing the drug loaded samples of constant size and surface area in simulated intestinal fluid (phosphate buffer solution pH 7.4), in a water bath with stirring at a speed of 100 rpm at 37±0.5°C. Aliquots (5 ml) were withdrawn at periodic intervals and were immediately replaced by the same volume of fresh medium. The aliquots, after suitable dilution, were analyzed using the UV-visible Perkin Elmer lambda 20 spectrophotometer. The UV-visible absorbances of Ib solutions were measured at λ_max=264 nm. The release amount was calculated by a previously established calibration curve. The test was carried out in triplicate using the basket method. For comparative purposes, commercial 400 mg Ib tablets from Nadpharmadic Production (Algeria), containing corn starch, pregelatinized starch, sodium starch glycolate, stearic acid and aquapolish as excipients, were also tested.

3.1 XRD study

XRD analysis was first used to evaluate the intercalation of the drug into the interlayer space of the Mt galleries. The XRD patterns of Ib, Mt and MtIb are shown in Figure 2A. The (001) reflection for pure Mt was observed at 7.06° corresponding to a d-spacing of 12.51 Å as calculated from Bragg’s law. This peak shifted to lower angles at 5.56° upon adding Ib, suggesting that the d-spacing of Mt increased to 15.88 Å. The increased basal spacing after the introduction of Ib within Mt is evidence of the intercalation of Ib molecules into Mt galleries and confirmed the successful preparation of the intercalated MtIb hybrid. Pure Ib exhibited typical reflection at 2θ 6.16°, 12.21°, 16.61°, 19.00 and 22.30°. The disappearance of these peaks in the MtIb diffractogram revealed that the Ib incorporated in the clay sample was in the amorphous state [25]. This observation indicates that the clay platelets inhibit the crystallization of Ib as a consequence of the presence of good interactions between the drug and the clay.

Figure 2:

X-ray diffraction patterns of (A) ibuprofen (Ib), montmorillonite (Mt) and MtIb hybrid, (B) MtIb, poly(ethylene glycol) (PEG), carboxymethylcellulose (CMC), and MtIb/PEG, MtIb/CMC, MtIb/PEG-CMC composites.

A second XRD study was carried out to investigate the possibility of the intercalation of the polymer macromolecules into the drug loaded Mt galleries. In Figure 2B, the Mt peak for the (001) plane is shifted toward lower angles in MtIb /PEG than in the MtIb pattern, suggesting that the intercalation of the PEG macromolecules further increased the interlayer space in the clay layers. The same phenomenon was observed for the MtIb/PEG-CMC diffractogram. The displacements to lower angles are reported in Table 1.

The CMC biopolymer displayed a common broad diffraction peak at 2θ=22.5° illustrating the amorphous nature of CMC. In the case of the MtIb/CMC formulation, there was no peak observed in the diffraction pattern corresponding to the layered Mt clay. This phenomenon implied that Mt layers in the MtIb/CMC composites were exfoliated as a result of the presence of strong interactions between the clay and the carbohydrate polymer.

3.2 FTIR study

FTIR spectroscopy was first used to confirm the insertion of the Ib molecules within the Mt layers. FTIR spectra of Mt and MtIb are illustrated in Figure 3A. In the FTIR Mt spectrum, the broad band centered near 3446 cm⁻¹ is due to the -OH stretching mode of interlayer water. The bands at 3621 cm⁻¹ and 3696 cm⁻¹ are due to the -OH stretching mode of Al-OH and Si-OH of the Mt structure [26]. The absorption peak in the region of 1641 cm⁻¹ is attributed to the -OH bending mode of absorbed water. The Si-O and Al-O bonds are, respectively, observed at 1037 cm⁻¹ and 620 cm⁻¹, and the Mg-O bond is assigned to the band at 530 cm⁻¹. In the Ib spectrum, a strong absorption band at 1706 cm⁻¹ assigned to the acid carbonyl groups was observed. The bands between 3090 cm⁻¹ and 3000 cm⁻¹ were related to the Ib C-H aromatic ring. The bands between 3000 cm⁻¹ and 2800 cm⁻¹ were the alkyl stretching vibrations of Ib. All characteristic bands belonging to Mt and Ib appeared in the spectrum of the MtIb hybrid, especially the bands in the 3040–2877 cm⁻¹ region corresponding to -C-H stretching of Ib, indicating the good insertion of the drug molecules in the Mt galleries. Furthermore, the displacement of the carbonyl band of Ib from 1706 cm⁻¹ to lower temperatures revealed that Ib interacted strongly with the Mt layer. The appearance of new bands at 1617 cm⁻¹ (asymmetric stretching) and 1550 cm⁻¹ (symmetric stretching) indicated the presence of carboxylate ions arising from electrostatic interactions between the drug and Mt. Indeed, the negatively charged sites at the clay surface are capable of extracting the carboxylic proton of Ib (Figure 4). The presence of these interactions may improve the solubility and/or modify the profile of drug release [27]. The negative charge on the clay surface results from the substitution of silica cations (Si⁴⁺) by aluminum cations (Al³⁺) in the clay sheet structure. The FTIR spectra of PEG, CMC, PEG-CMC and the corresponding biocomposites MtIb/CMC, MtIb/PEG and MtIb/PEG-CMC are presented in Figure 3B. To better visualize the interactions developed in the different systems, the FTIR spectra are displayed in the 4000–1250 cm⁻¹ region. In the MtIb/CMC, MtIb/PEG and MtIb/PEG-CMC composites spectra, the bands at 3696 cm⁻¹ and 3621 cm⁻¹ correspond to the stretching vibration of OH groups situated on the surface of the clay. In the CMC spectrum, a broad band with a double maximum at 1677 cm⁻¹ and 1598 cm⁻¹ corresponding to carboxylic and carboxylates (-COO⁻) groups, respectively, was observed. In the MtIb/CMC spectrum, this band becomes thinner and more symmetric with a maximum at 1598 cm⁻¹, suggesting that all carboxylic groups have been converted to carboxylate groups after their interaction with the negatively charged clay surface (Figure 4). The MtIb/PEG-CMC spectrum showed a broad band at about 3423 cm⁻¹ corresponding to the O-H stretching groups of both PEG and CMC. The bands between 3000 cm⁻¹ and 2800 cm⁻¹ are the -CH alkyl stretching vibration of CMC, PEG and Ib [28]. The band of carboxylate groups which has been observed in the MtIb/CMC spectrum appeared at the same position in the MtIb/PEG-CMC spectrum, suggesting the same developed interaction between the CMC and the clay surface in the blend biocomposite.

Figure 3:

Fourier transform infrared (FTIR) spectra of (A) montmorillonite (Mt), ibuprofen (Ib) and MtIb hybrid, (B) MtIb, poly(ethylene glycol) (PEG), carboxymethylcellulose (CMC) and drug loaded composites.

Figure 4:

Illustration of clay/drug and clay/carboxymethylcellulose (CMC) interactions.

3.3 DSC study

The DSC thermogram of Ib showed a melting endotherm at 83°C due to its crystalline structure (Figure 5). DSC thermograms of Mt and MtIb revealed very similar broad endotherms beginning at about 100°C, with maxima occurring at 130°C, indicative of loss of adsorbed and intercalated water from the clay particles. The endothermic peak due to the melting of Ib crystals was not observed in the MtIb hybrid spectrum. This observation is an indication of the amorphous state of the drug [29], [30] as already observed by XRD analysis, and confirms the development of specific interactions between the clay and the drug which hinder crystal growth.

Figure 5:

Differential scanning calorimetry (DSC) thermograms of ibuprofen (Ib), montmorillonite (Mt) and MtIb hybrid recorded on heating.

The DSC thermogram for pristine PEG shows an endotherm at 70°C, which corresponds to its melting point. The melting endotherm has shifted slightly to lower temperatures at 68°C in the MtIb/PEG composite suggesting a less-ordered structure of PEG in the MtIb/PEG composite compared to its pure crystalline form Figure 6A. Furthermore, no peak corresponding to Ib was observed in the MtIb/PEG thermogram, confirming that the drug is still in its amorphous state in this composite. The PEG melting temperature decreased further after introduction of the CMC in the MtIb/PEG-CMC composite, which is an indication of the miscibility of PEG and CMC polymers [31]. Indeed, the decrease in the melting point in the blend composite is due to a decrease in PEG lamellar thickness as a result of polymer-polymer interactions.

Figure 6:

Differential scanning calorimetry (DSC) thermograms recorded (A) on heating (B) on cooling.

The DSC thermograms of pristine PEG and MtIb/PEG composites recorded upon cooling are illustrated in Figure 6B. The PEG thermogram showed an exothermic peak which corresponds to the crystallization temperature T_C=32°C. The crystallization temperature of PEG in the MtIb/PEG composite was higher by 7°C than that of the neat PEG, suggesting that the clay hybrid MtIb has acted as a nucleating agent and enhanced the crystallization rate of PEG. In this case, in the molten state, the segments of the PEG molecules can easily interact with the surface of Mt, developing crystallization nuclei. This may be due to the high surface area of the nano-sized particles that enhanced heterogeneous crystallization. Similar results have been reported for nanocomposites by several authors [20], [21].

3.4 TGA

TGA was first used to prospect a possible delay in drug decomposition temperature because of drug intercalation in the clay interlayers. The thermal parameters used are T_onset and T_max. T_onset is defined as the temperature at which the weight loss just becomes detectable and T_max as the temperature corresponding to maximum rate degradation. Figure 7A shows the TGA thermograms of Ib, Mt and MtIb. As can be seen, two major weight loss patterns were observed in the temperature range of 80–130°C and 450–750°C for Mt. The first weight loss corresponds to evaporation of adsorbed water. The second weight loss is due to the loss of structural hydroxyl groups of Mt [32]. The thermal degradation of pristine Ib begins at 114°C with a maximum weight loss at 225°C, as determined from the derivative curve. The MtIb hybrid showed a weight loss in the temperature region from 393°C to 516°C, corresponding to the degradation of the drug in clay galleries. This delay in drug decomposition suggests a thermal stabilization effect resulting from drug intercalation of the drug molecules in the interlayer spaces of the clay.

Figure 7:

Thermogravimetric analysis (TGA) thermograms of (A) ibuprofen (Ib), montmorillonite (Mt) and MtIb hybrid, (B) poly(ethylene glycol) (PEG), carboxymethylcellulose (CMC) and drug loaded composites.

TGA analysis was also conducted on pure matrices PEG, CMC and PEG/CMC, and the MtIb loaded matrices MtIb/PEG, MtIb/CMC and MtIB/PEG-CMC. The results are presented in Figure 7B. PEG showed a single abrupt weight loss with a T_max located at 390°C corresponding to backbone chain scission, since PEG has a simple linear chain structure. The CMC thermogram revealed three weight loss regions, suggesting the coexistence of more than one degradation process. After moisture dehydration that starts just above room temperature, CMC showed a first chains degradation from 250°C to 315°C. A second stage of decomposition with a decreased rate is observed from 315°C to 515°C. The residual mass after this stage was about 36% indicating that up to 600°C, the total decomposition of the polymer did not take place.

MtIb/PEG and MtIb/CMC thermograms exhibited higher thermal stability than the corresponding polymers PEG and CMC separately. Furthermore, the initial decomposition of both MtIb/PEG and MtIb/CMC occurs at 267°C. In the case of the MtIb/PEG-CMC nanocomposite, the onset temperature shifts to the high values compared to MtIb/PEG and MtIb/CMC nanocomposites. This enhancement in the thermal stability can be attributed to the miscibility of the PEG-CMC blend, which is the result of the development of specific interactions between PEG and CMC as already observed in the DSC study. The increase in the onset temperatures can be associated to the break of the specific interactions that exist in the miscible blend.

3.5 In vitro drug release study

The sustained release profiles of Ib in a phosphate buffer solution at pH 7.4, from the different drug delivery systems compared to a commercial tablet are shown in Figure 8.

Figure 8:

Release profiles in simulated intestinal fluid (pH 7.4) at 37±0.5°C.

The MtIb hybrid exhibited a burst effect with 80% release of Ib within 1 h. This is probably due to the repulsive ionic interactions occurring between two negatively charged species. Indeed, in the buffer solution at pH 7.4, the drug Ib (pKa=5.2) will be in the conjugate base form (-COO⁻). By contrast, the negative charge on the clay surface increases in this medium [33]. Therefore, the repulsion forces between the clay surface and the drug molecules are strong enough to speed up the drug release.

In comparison, the commercial tablet showed a more continuous release since 80% of the drug was delivered in about 2.5 h. This is probably attributed to the presence of excipients in the tablet. The release rate was greatly decreased after the introduction of PEG, CMC and the PEG-CMC polymer blend in the system in comparison to both the MtIb hybrid and the commercial tablet. For the MtIb/PEG system, a sustained release of 80% was reached after 6.2 h. The reduced drug release rate may be attributed to the resulting intercalated structure in the MtIb/PEG composite as illustrated in the XRD study. Such an intercalated structure may increase the diffusion path length of the drug towards the dissolution medium. However, the problem of the burst effect was not completely suppressed by the introduction of PEG in the system.

In the MtIb/CMC system, the release period was further prolonged. However, the total cumulative percentage of the drug released was only about 50%. This can be explained by the decrease in the mobility of the drug molecules induced by the exfoliated structure of the MtIb/CMC composite. Furthermore, the presence of interactions between the Ib molecules and CMC macromolecular chains attached to the clay sheets, as illustrated in Figure 4, will prevent some drug molecules from being released.

The use of the MtIb/PEG-CMC system sustained the release of Ib more significantly, since 80% of the drug released was reached after 7 h. This can be explained by the presence of interactions between the carboxylic groups of the CMC and the PEG ether groups which will generate entanglement of the chains occupying a large space in the interfoliar space, thus delaying the release of the drug molecules. Furthermore, the phenomenon of burst effect has been totally avoided minimizing any initial toxicity associated with a high dose. An almost total release was obtained in 10 h.

3.6 Kinetics of drug release

To better understand the mechanism by which the drug release from the different carriers is governed, several kinetic models have been applied.