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J. Phys. Chem. C 2008, 112, 20222–20226
Drug-Matrix Interactions in Nanostructured Materials Containing Acetyl Salicylic Acid Using an Enteric Polymer As a Coating M. Gonza´lez,† A. Galano,‡ J. Rieumont,§ T. Lo´pez,*,|,# D. Dupeyron,∇ and Leon Albaran‡,⊥ Engineering and Chemical Research Center, C.P. 10600, HaVana City, Cuba, Department of Chemistry, Autonomous Metropolitan UniVersitysIztapalapa, San Rafael Atlixco 186, Col. Vicentina, Iztapalapa C.P. 09340, Mexico D. F., Mexico, Faculty of Chemistry, Department of Physical Chemistry, UniVersity of HaVana, C.P. 10600 HaVana City, Cuba, Health Department, Autonomous Metropolitan UniVersitysXochimilco, Calz. Del Hueso 1100, Col. Villa Quietud, Delegacio´n Coyoaca´n, 04960 Me´xico D.F., Mexico, National Institute of Neurology and Nerosurgery, MVS Nanotechnology Laboratory, AV. Insurgentes Sur 3877, Col. La Fama, 14269 Me´xico D.F., Mexico, Department of Chemical and Biochemical Engineering, Tulane UniVersity, New Orleans, Louisiana 70118, and Institute of Materials and Reagents, C.P. 10600, HaVana City, Cuba ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: September 10, 2008
Infrared spectra of nanostructured materials previously obtained and studied as controlled release devices were recorded in order to reveal drug-matrix interactions. These nanostructured materials contain acetyl salicylic acid (ASA) as the drug and the enteric copolymer poly(methacrylic acid methyl methacrylate) as the matrix. Thus, carboxylic interactions are expected to be operating. However the complex nature of the carboxylic acid carbonyl signals in the IR region resulted weak evidence about these interactions. However, using the density functional theory formalism, a stable complex drug--copolymer through the carboxyl/ASA-carboxyl/ copolymer groups can be suggested as the main type of interaction in this system. It was demonstrated that this complex is as stable as the complex formed between ASA molecules. Raman spectra of nanostructured materials afforded additional information through the signals corresponding to the complex theoretically predicted. Introduction The design and application of systems of controlled dosage of medications and systems of targeting the bioactivity of certain drugs are, at the moment, one of the aspects of more relevance in the development in new medication procedures. The uses of polymeric materials as supports of drugs to regulate and to dose their release in specific applications are a perspective that has acquired great interest.1 Due to their great versatility, polymers are the most used materials in microencapsulation. Biocompatible or biodegradable polymers such as alginates, quitosane, lactic copolymers, or poly(hydroxy alcanoate)s have been extensively used2 for this purpose. Other materials such as random acrylic copolymers with an interesting solubility dependence on the pH, the socalled enteric polymers, have been used to avoid any contact of drug with the enzymes and the low gastrointestinal track pH. Copolymers with solubility dependent on the pH, such as the commercially well known Eudragit,3-5 offer wide possibilities to control the release of the encapsulated material to the intestine. This behavior is because a hydrophobic and a hydrophilic monomer form part of the copolymer. Thus a turning point in solubility takes place as the pH is increased due to the carboxylic ionization rendering conformations that occupy more volume enhancing the solubility. At low pH, the material is able to close †
Engineering and Chemical Research Center. Autonomous Metropolitan UniversitysIztapalapa. University of Havana. | Autonomous Metropolitan UniversitysXochimilco. # Tulane University. ∇ Institute of Materials and Reagents. ⊥ National Institute of Neurology and Nerosurgery. ‡ §
Figure 1. Structural formula of the used polymer.
its structure and to encapsulate drugs. Therefore, release of the drug should occur in the intestine not in the stomach. The aim of this work is to reveal the kind of interactions between acetyl salicylic acid (ASA) and the Eudragit L-100 (Figure 1), a copolymer of methacrylic acid and methyl methacrylate supplied by the Rohn Pharm, which behaves as an enteric polymer. For this purpose, IR spectra of nanostructured materials previously obtained6 with these materials were recorded, and theoretical calculations were performed indicating the possibility that strong drug-matrix interactions are operating through a carboxylic-carboxylic complex. The knowledge of such interactions should allow us to understand the release profiles of these nanodevices. Experimental Methods FTIR Spectroscopy. Infrared spectra were collected with a FT-IR-BRUKER Vector 33. The sample (∼2 mg) and KBr (∼150 mg) were ground together in an agate mortar with an agate pestle until the sample was well dispersed and the mixture had the consistency of fine flour. Then, a translucent disk was prepared, according to Ryczkowski7 for making the sample sufficiently transparent for transmission measurements. FTIR
10.1021/jp806572g CCC: $40.75 2008 American Chemical Society Published on Web 12/04/2008
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J. Phys. Chem. C, Vol. 112, No. 51, 2008 20223
Figure 2. Scanning electron micrographs of ASA NPs (sample ASA1).
TABLE 1: Selected Geometrical Parameters Relevant to the Modeled Interactions ASA dimer d(C1-C2) d(C2-O3) d(C2-O4) d(O4-H5) d(O3-O4′) d(H5-O3′) ∠(C1-C2-O3) ∠(C1-C2-O4) ∠(C2-O4-H5) ∠(O3-C2-O4)
complex I
expt9
ref 10
this work
this work
1.487 1.239 1.289 0.989 2.649 1.665 119.5 118.0 111.5 122.5
1.487 1.237 1.322 1.006 2.643 1.637 122.8 113.9 110.5 123.3
1.488 1.237 1.321 1.007 2.631 1.625 123.5 113.7 110.2 122.8
1.491 1.237 1.319 1.016 2.637 1.634 120.6 116.2 110.3 123.1
spectra were obtained in the wavenumber region between 500 and 4000 cm -1. Raman Spectroscopy. The samples were characterized in a commercial Micro Raman system (Labram model) equipped with a 100 mW Ar laser emitting at 632.8 nm, coupled with an Olympus Bx40 microscope with 50× objective. All measurements were carried out at room temperature with no special sample preparation. Morphology and Distribution of Particles Sizes. Morphology of the nanostructured materials was analyzed by scanning
electron microscopy (SEM JEOL-JSM -6060LV). Samples were recovered by sputtering with a layer of gold of approximately 20 nm using an EMS 550 sputter coater. Computational Details. The theoretical calculations were carried out with the Gaussian 038 package of programs within the density functional theory (DFT) formalism. Geometry optimizations have been performed without any symmetry constraint, using the B3LYP hybrid functional in conjunction with the 6-31G(d,p) basis sets. Frequency calculations were made for all the studied systems at the same level of theory and the minima character of the modeled structure was identified by the number of imaginary frequencies (NIMAG ) 0). The energies of all the stationary points were improved by singlepoint calculations at B3LYP/6-311++G(d,p) level of theory. Thermodynamic corrections at 298.15 K were included in the calculation of the relative energies. Results and Discussion Characteristics of the Nanostructured Materials Used. The technique for nanostructured material preparation and the controlled release features of them were previously reported by our laboratory.6 A micrograph is shown in Figure 2. These nanostructured materials are formed by the enteric matrix of the copolymer Eudragit L-100, as well as drug loading (ASA). Two nanostructured materials samples were used for this study, both with a copolymer/drug weight ratio of 10. Sample ASA-1 was obtained with a lower amount of surfactant and stirring rate than sample ASA-2, giving diameters around 700 nm (ASA-1) and 400 nm (ASA-2) as shown in Figure 2. Encapsulation efficiency for these samples was 86% and 92 % respectively. Infrared Study. The IR spectra of the pristine polymer and nanostructured materials are shown in Figure 3a. Those spectra present the signals corresponding to the structure such as the CdO vibrations around 1740 cm-1 and the wide absorption band of the associated OH groups between 2500-3900 cm-1. Furthermore, the CHx vibrations appear in the range of 2900-3000 cm-1. Those spectra are similar, but the carbonyl signal for the polymer is broader and a pronounced shoulder is observed. This fact is related to the presence of the carboxylic and the ester groups in the copolymer as well as the possible interchain and intrachain interactions, turning this signal very complex. However for ASA-1 and ASA-2 samples, the carboxyl
Figure 3. FT-IR spectra of the sample (a) in the zone 4000-500 cm-1 and (b) in the zone of the CdO groups.
20224 J. Phys. Chem. C, Vol. 112, No. 51, 2008
Gonza´lez et al.
Figure 4. Geometries of ASA-EL100 complexes fully optimized at B3LYP/6-31G(d,p) level of theory.
Figure 5. Geometry of ASA dimer fully optimized at B3LYP/631G(d,p) level of theory.
TABLE 2: Enthalpies and Gibbs Free Energies of Reaction (kcal/mol) for the Complex Formation ASA-dimer I II
∆H1atm
∆G1atm
∆G1M
Sol ∆G1M
-13.73 -14.12 -9.31
-2.03 -2.55 2.56
-3.92 -4.44 0.67
-6.46 -6.98 -1.87
signal is more symmetric and sharp and no such shoulder appears. Thus, the ASA inside nanostructured materials seems to modify to a certain extent this picture, disrupting the interactions of the polymer and forming new ones. However a net shift in frequency is not clearly observed, due to the complex nature of the carboxyl signal, as is usually seen in other more simple systems (Figure 3b). These facts lead us to carry out theoretical calculations in order to get a deeper insight into the features of drug-matrix interactions for this system. Theoretical Modeling. ASA has a carboxylic acid group, which can interact with the functional groups of the polymer. Two different interactions have been considered: the first one between the acid group in ASA and the acid group in EL100 (I) and the second one involving the acid group in ASA and the acid and carboxylic fragment from the ester groups in the polymer (II). The fully optimized geometries of the resulting complexes are shown in Figure 4. According to the O · · · H distances involved, the interactions in complex I are stronger than those in complex II. Since there are previously reported experimental9 and theoretical10 data on the structure of the ASA dimer, which is formed through interactions similar to those in complex I, the geometrical features of this dimer have been used to test the accuracy of the geometries obtained in the present work. Selected geometrical parameters of the ASA dimer, relevant to the modeled interactions, are reported in Table 1, compared with the equivalent parameters in complex I. The used numbering is shown in Figure 5. Taking into account that experimental results were obtained from the crystal and the computational modeling corresponds to vacuum, the agreement between experimental
Figure 6. Computed IR spectra of ASA, ASA dimer, EL100, and complexes I and II.
and calculated data is good. There is also an excellent agreement with the results from this work and those calculated by Boczar et al.9 at B3LYP/6-31++G(d,p) level of theory. Based on this analysis, it seems reasonable to assume that the accuracy of the present calculations is high enough to provide reliable data. The energy releases associated with the interactions between ASA and Eudragit L 100, in terms of enthalpies and Gibbs free energies are reported in Table 2. In addition, the equivalent data for the ASA dimer formation has also been included, for comparison purposes. The two first columns in this table correspond to values in vacuum using standard state of 1 atm, as calculated by the Gaussian program. However, since the actual process takes place in solution, the Gibbs free energies of reaction have been corrected to include the solvent cage effects. For that purpose, the reference state has been changed from 1 atm to 1 M, and the solvent cage effects have been included according to the corrections proposed by Okuno,11 taking into account the free volume theory.12 These corrections are in good agreement with those independently obtained by Ardura et al.13 and have been successfully used by other authors.14,15 The expression used to correct Gibbs free energy is Gas (2n-2) ∆GSol ] - (n - 1)} 1M = ∆G1M - RT{ln[n10
(1)
where n represents the total of reactant moles. According to expression 1, the cage effects in solution cause ∆GSol 1M to decrease by 2.54 kcal/mol for bimolecular reactions at 298 K with respect Gas. This lowering is expected since the cage effects of to ∆G1M
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TABLE 3: Selected Vibrational Frequencies (cm-1) for ASA, EL100, Complex I and Complex II Calculateda
Description
Experimental
ASA ν(O-H) ν(CdO)ester ν(CdO)acid
3628 (m)3000 (vs) dimer 1797 (s)1790 (w) dimer 1744 (s)1694 (m) dimer
1754,9,19 1693,19 16919
EL100 ν(O-H) ν(CdO)ester ν(CdO)ester + ν(CdO)acid ν(CdO)acid + ν(CdO)ester δν(H-O-C)
3613 1723 1737 1761 1290
(m) (s) (m) (m) (m)
2999 2995 2764 1794 1736 1684
(vs) (vs) (vs) (m) (m), 1723 (m) (s)
3318 3255 1779 1739 1689 1667
(vs) (m) (m) (m) (m) 1727 (vw) (w)
ester 1730, 172416 acid 1705, 171016
Complex I νS(O-H) νS(O-H) νA(O-H) ν(CdO)ester-ASA ν(CdO)ester-EL100 ν(CdO)acid-ASA + ν(CdO)acid-EL100 Complex II νS(O-H) νA(O-H) ν(CdO)ester-ASA ν(CdO)ester-EL100 + ν(CdO)acid-EL100 ν(CdO)acid-ASA + ν(CdO)ester-EL100 + ν(CdO)acid-EL100 ν(CdO)acid-ASA + ν(CdO)ester-EL100 a
Intensities: vs ) very strong; s ) strong; m ) medium; w ) weak; vw ) very weak.
Figure 7. Raman spectrum of nanostructured materials.
the solvent reduce the entropy loss associated with any addition reaction or transition state formation in reactions with molecularity equal or larger than two. Therefore, if the translational degrees of freedom in solution are treated as in the gas phase, the cost associated with their loss when two or more molecules form a complex system in solution is overestimated, and consequently these processes are kinetically over-penalized in solution leading to rate constants that are artificially underestimated. As the values from Table 2 show, the complex formation in solution is an exothermic and exergonic process in all the Sol large enough modeled cases with the values of ∆H and ∆G1M to overcome any inaccuracy inherent to the performed calculations. In addition, the energy evolution associated with the interactions ASA-EL100 (I) is slightly larger than that of ASA-ASA. Between the two studied ASA-EL100 complexes, complex I was found to be about 5 kcal/mol lower in free energy than complex II, which is in line with the corresponding geometrical features described above. Analyzing these results together, it seems reasonable to assume that in the vicinity of
the polymer ASA would form complexes with EL100, mainly through structure I. The formation of such complexes would compete with the dimeric association of crystalline ASA, since both are energetically equivalent. Accordingly it should be expected that ASA-ASA and ASA-EL100 complexes occur in similar proportion. However, the encapsulation of nanostructured materials is carried out using the ratio polymer/drug equal to 10, indicating that actually in the nanostructured materials the main interaction should be through the complex I. A similar polymer-active principle association has been previously described for Eudragit encapsulation of ketoprofene.16 The infrared (IR) spectra of ASA, as a free molecule and in dimeric form, EL100, and complexes I and II have also been computed and are shown in Figure 6 in the range 1450-1950 cm-1. At this point, it is important to mention that the aim of the theoretical calculations on vibrational spectra of large systems, at least in this work, is not to reproduce the experimental data to a high wavenumber accuracy but to reflect the major general features and trends and luckily to help in the assignment of the most prominent bands. In general, the calculated wavenumbers are expected to diverge to a certain extent from the experimental ones due to the anharmonicity effects, the approximate treatment of electron correlation effects, and the truncation of the basis set. The frequency values associated with the IR bands most relevant to the interaction ASA-polymer are reported in Table 3 and were calculated using the scaling factor 0.9627, recommended by Merrick et al.17 The assignment of the normal modes was performed by direct inspection using the Gaussview18 program to visualize them. A general agreement was obtained between calculated and experimental results. However, the calculated bands are slightly shifted, which is not unusual for systems relatively large that make imperative the use of modest levels of theory9,15 In the case of ASA, the calculated-experimental correspondence is better for the dimeric form, which is an expected result since crystalline aspirin is actually in dimer or
20226 J. Phys. Chem. C, Vol. 112, No. 51, 2008 even larger associations. Comparing the spectrum of EL100 with those of complexes I and II, it can be observed that due to the ASA-polymer interactions, the vibration of the carboxylic acid carbonyl is shifted to lower wavenumbers. In addition, the magnitude of this shift is larger for complex I, where the interaction is stronger. The other band that is also left-shifted due to the ASA-EL100 association is the O-H stretching, which appears at lower frequencies in the complexes, and again the stronger the interaction in the complex the more significant the shift, compared with free EL100. In this context, the Raman spectra of nanostructured materials exhibit in the carbonyl region (Figure 7) two signals at 1651 and 1656 cm-1 for the two samples of nanostructured materials that were not observed in the FTIR spectra. Those experimental signals at lower wavenumber confirm theoretical data shown in Table 3, which pointed out the presence of similar signals due to the interaction ASA-copolymer in the complex type I. Conclusions Weak evidence of drug-matrix interaction for the system aspirin poly(methacrylic acid methyl methacrylate) was obtained by recording IR spectra of nanostructured materials formed with those materials and comparing them with the IR of the copolymer. However, detailed theoretical calculations allowed us to conclude that actually these drug-matrix interactions are operatingintherealsystembecause(a)thecomplexASA-copolymer is as stable as complex ASA-ASA through the carboxyl-carboxyl interactions, (b) the carboxylic acid carbonyl signal of this complex is shifted to lower wavenumbers, as well as the O-H stretching, and (c) item a was experimentally confirmed by Raman spectra of nanostructured materials. The signals at 1651-1656 cm-1 should correspond to this type of complex. Those facts all together indicate that the ASA-enteric copolymer interactions help to encapsulate the drug at low pH but also to release the drug because both drug and matrix are ionized at higher pH, contributing the repulsion between the ionized species to the release. Acknowledgment. The authors thank Engineering and Chemical Research Center of La Havana, UAM and CONACyTMe´xico for financial support.
Gonza´lez et al. References and Notes (1) Microencapsulation: Methods and industrial applications; Benita, S., Ed.; Marcel Dekker Inc.: New York, 1996. (2) Deasey, P. Microencapsulation and Related Drug Process; Marcel Dekker: New York, 1996. (3) Mooustafine, R. I. T.; Kabanova, V.; Kemenova, V. A.; Mooter, G.V. J. Controlled Release 2005, 103, 191. (4) Dupeyro´n, D.; Gonza´lez, M.; Sa´ez, V.; Ramo´n, J.; Rieumont, J. IEEE Proc. Nanobiotechnol. 2005, 152, 165. (5) Vachon, M. G.; Nairn, J. G. J. Microencapsulation 1995, 12, 287. (6) Gonza´lez, M.; Rieumont, J.; Dupeyron, D.; Perdomo, I.; Fernandez, E.; Castan˜o, V.; Abdo´n, L. ReV. AdV. Mater. Sci. 2007, 17, 71. (7) Ryczkowski., J. Catal. Today 2001, 68, 268. (8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P; . Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W. ; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (9) Kim, Y.; Machida, K.; Taga, T.; Osaki, K. Chem. Pharm. Bull. 1985, 33, 2641. (10) Boczar, M.; Wojcik, M. J.; Szczeponek, K.; Jamroz, D.; Zieba, A.; Kawalek, B. Chem. Phys. 2003, 286, 63. (11) Okuno, Y. Chem.sEur. J. 1997, 3, 210. (12) Benson, S. W. The Foundations of Chemical Kinetics: R.E. Krieger, Malabar, FL, 1982. (13) Ardura, D.; Lopez, R.; Sordo, T. L. J. Phys. Chem. B 2005, 109, 23618. (14) Alvarez-Idaboy, J. R.; Reyes, L.; Cruz, J. Org. Lett. 2006, 8, 1763. (15) Galano, A. J. Phys.Chem. A 2007, 111, 1677. (16) Eerikainen, H.; Peltonen, L.; Raula, J.; Hirvonen, J.; Kauppinen, E. I. Pharm. Sci. Technol. 2004, 5, 68. http://www.aapspharmscitech.org/ default/issueView.asp?vol)05&issue)04 (accessed May 10, 2008). (17) Merrick, J. P.; Moran, D.; Radom, L. J. Phys. Chem. A 2007, 111, 11683. (18) GaussView 2.0; Gaussian, Inc., Pittsburgh, PA. (19) Paasch, S.; Salzer, R. Anal. Bioanal. Chem. 2004, 380, 5.
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