Biomacromolecules 2002, 3, 214-218
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NMR Imaging of High-Amylose Starch Tablets. 1. Swelling and Water Uptake Wilms E. Baille,† Ce´ dric Malveau,† Xiao Xia Zhu,*,† and Robert H. Marchessault‡ De´ partement de chimie, Universite´ de Montre´ al, C.P. 6128, Succ. Centre-ville, Montre´ al, Que´ bec, Canada H3C 3J7, and Department of Chemistry, McGill University Pulp and Paper Center, 3420 University Street, Montre´ al, Que´ bec, Canada H3A 2A7 Received October 12, 2001
Pharmaceutical tablets made of modified high-amylose starch have a hydrophilic polymer matrix into which water can penetrate with time to form a hydrogel. Nuclear magnetic resonance imaging was used to study the water penetration and the swelling of the matrix of these tablets. The tablets immersed in water were imaged at different time intervals on a 300 MHz NMR spectrometer. Radial images show clearly the swelling of the tablets and the water concentration profile. The rate constants for water diffusion and the tablet swelling were extracted from the experimental data. The water diffusion process was found to follow case II kinetics at 25 °C. NMR imaging also provided spin density profiles of the water penetrating inside the tablets. Introduction Biomacromolecules such as polysaccharides are widely used in the pharmaceutical industry as excipients for the formulation of controlled release devices.1 These controlled release systems based on hydrophilic excipients offer many advantages over conventional solid dosage forms, including improved bioavailability, better pharmacological effectiveness of the drug, and lower cost.2 To achieve maximum pharmacological effectiveness, the mechanism of drug release and the factors that affect the release process (water penetration, swelling of the tablet, and interaction between the dissolution medium and the release system) must be understood. Many techniques have been used in the past to study these systems, including optical methods,3-8 Rutherford backscattering spectrometry,9 electron spin resonance,10 and nuclear magnetic resonance (NMR) imaging.11-17 NMR imaging has been primarily used in medical diagnostics, but recently it has been used to visualize solvent penetration in cross-linked polymers and pharmaceutical excipients.18 It is noninvasive and nondestructive and can be easily adapted to monitor the water uptake and tablet swelling. Amylose and amylopectin are the major carbohydrate polymers in starch. High-amylose starch is a hybrid, which is commercially available and usually contains about 70% amylose with a molecular weight of about 500 000. While amylose is a linear polysaccharide, amylopectin is branched with multiple short chains arranged in a racemose fashion. Amylopectin has a molecular weight of 50-100 million. The tablets used in this study were made according to a patented process wherein the high-amylose starch granules were gelatinized in 4% aqueous NaOH and then cross-linked with * To whom correspondence should be addressed. E-mail: julian.zhu@ umontreal.ca. † Universite ´ de Montre´al. ‡ McGill University Pulp and Paper Center.
epichlorohydrin in the highly swollen state.19 The washed and spray-dried product has the trade name Contramid and is a commercial excipient from Labopharm Inc. (Laval, QC, Canada). Contrary to other cross-linked polymer matrixes, an increase in the degree of cross-linking leads to an increase of water uptake in the tablets and the drug release rate goes through a maximum.20-22 The process of solvent penetration in these tablets leads to a water gradient, which influences the profile of drug release. The solvent transport phenomenon in excipients of this kind also involves a diffusion process governed by the concentration gradient and a relaxation process due to the response of polymer chains to the swelling stress. Two extreme cases could be observed, namely, Fickian diffusion, where the relaxation is faster than the diffusion, and the case II process, where the diffusion is faster than the relaxation.23 In this paper, we present NMR images of the water uptake and the swelling of Contramid tablets at two different temperatures, 25 and 37 °C. Experimental Section Tablet Preparation. The modified high-amylose starch used was Contramid, a product synthesized according to Mateescu et al.19 The tablets were prepared from a spraydried powder of Contramid using a single-station tablet press (Stokes model-S4). The “as-received” tablets have the following dimensions: 8.64 mm diameter and 2.72 mm thickness corresponding to 200 mg weight at room humidity. The tablets were kindly provided by Labopharm Inc., Laval, QC. Magnetic Resonance Imaging Experiments. The 1H NMR imaging experiments were carried out on a Bruker DSX300 NMR spectrometer operating at a frequency of 300 MHz (7 T) and equipped with a microimaging probe allowing a sample size of 20 mm in diameter. The system
10.1021/bm015621e CCC: $22.00 © 2002 American Chemical Society Published on Web 12/01/2001
High-Amylose Starch Tablets
Figure 1. Schematic diagram of the tablet arrangement for the water uptake experiment.
was also equipped with three orthogonal field gradient coils permitting a maximum gradient of 100 G/cm. A standard slice-selective spin-echo imaging sequence was used to acquire images of the tablet placed inside a 15 mm o.d. NMR tube containing a 1:1 mixture of H2O and D2O (Figure 1). A slice of 500 µm thickness was selected perpendicular to the main magnetic field (axial axis) using a sinc-shaped pulse and a gradient strength of 12 G/cm. Four scans were accumulated to obtain 256 × 256 pixel images, leading to an in-plane resolution of 78 and 59 µm for the tablets at 25 and 37 °C, respectively. An echo time (TE) of 4 ms and a repetition time (TR) of 2 s were fixed leading to an acquisition time of about 35 min for each image. Highamylose starch tablets were placed on a support, which allowed three-dimensional water penetration. Results and Discussion To quantify the tablet swelling and the water penetration, the solvent diffusion along the radial direction of the tablet was recorded at 25 °C over 20 h (Figure 2). The dark blue circle area, which represents the dry diameter of the tablet in the tube, decreases as a function of time, while the extreme limits of the tablets, represented by the red circle on the images, show the extent of swelling of the tablet. The high intensity of water shown by the red circle on the images is a result of attenuation by relaxation of the water molecules
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at the tablet interface. The pulse sequence, in particular the short recovery time, provides a distinction of the water signals inside and outside the tablet. The longitudinal relaxation time (T1) of the free water outside is about 5 s, whereas the maximum T1 of water inside the tablet (i.e., at equilibrium) is about 800 ms. The recovery time, however, was fixed at 2 s, so that the magnetization of free water does not return to equilibrium, contrary to the magnetization of water inside the tablet. This results in a higher signal intensity for water inside the tablet although its spin density is lower. This allows us to follow more easily the swelling of the tablet with time. Of course, T1 of water inside the tablet depends on water concentration (i.e., T1 decreases with decreasing concentration). But this should not have affected the signal intensity of the water at the different locations because the recovery time is sufficiently long for all the spins inside the tablet to relax. Figure 2 also shows clearly that diffusion is isotropic in the radial plane and that the equilibrium is not reached even after about 20 h. NMR imaging experiments were also carried out at 37 °C (Figure 3). Obviously, the penetration process at 37 °C is faster than at 25 °C, and the images of the tablet exhibit greater extent of swelling. Water advances rapidly to the center of the sample, as shown by the red area of the images. The same explanation, given above for the high intensity of water inside the tablets at 25 °C, applies also at 37 °C. In addition, the increase in the area of the red part with time may also be due to a higher water content because of a greater extent of swelling of the tablet at 37 °C. The water penetration observed is also isotropic in the radial plane, and the equilibrium is reached after about 39 h. Figure 4 illustrates the decrease of the “dry” diameter (defined as the tablet diameter where the water concentration is equal to 1/6 of the concentration at the interface with the solvent) as a result of water penetration. We can see that for short immersion times (less than 600 min) no significant difference is observed in the decrease of the dry diameter at the two temperatures. This corresponds to the period where the swelling effect is more significant and the water is mostly
Figure 2. Water penetration in the high-amylose starch tablet at 25 °C as a function of time. Each image is the sum of 4 accumulations with a repetition time of 2 s, yielding an experimental time of 35 min. The spatial resolution in plane is 78 µm, with a slice thickness of 500 µm.
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Figure 3. Water penetration in the high-amylose starch tablet at 37 °C as a function of time. Each image is the sum of 4 accumulations with a repetition time of 2 s, yielding an experimental time of 35 min. The spatial resolution in plane is 59 µm, with a slice thickness of 500 µm.
Figure 4. Variation of the diameter of the remaining dry tablet at 25 and 37 °C.
Figure 5. Swelling of high-amylose starch tablet as a function of time at 25 and 37 °C.
consumed for the swelling of the tablet, especially at the surface. Afterward, the dry diameter decreases more rapidly at 37 °C than at 25 °C. At 37 °C, the beginning of this rapid decrease in dry diameter also corresponds to the end of the rapid swelling of the tablet. This behavior is in keeping with the fact that this modified starch also has its gelatinization temperature at ca. 37 °C. The dry diameter variation was fitted by a linear equation (with a rate of 5.4 × 10-6 cm/s) at 25 °C and by an exponential equation (with a rate constant of 3.5 × 10-3 min-1) at 37 °C. The dimensional changes of the tablet at 25 and 37 °C shown in Figure 5 were directly extracted from the images in Figures 2 and 3. The size of the tablet includes the outer (wet, as shown in red) and inner (dry, as shown in blue) layers of the tablet. Figure 5 illustrates a rapid swelling of the tablets for the two temperatures at short immersion times (up to 600 min). In both cases, a plateau is reached after about 18 h. The isotherms in Figure 5 can be fitted to the following equation:
Table 1. Initial Tablet Diameter (d0), Maximum Swelling Diameter (dmax), and Rate Constants of the Swelling (ks) of Contramid Tablets at Two Different Temperatures
d ) dmax - c exp(-kst)
(1)
where d represents the diameter of the swollen tablet, dmax is the maximum diameter of the swollen tablet at equilibrium,
temperature (°C)
d0 (mm)
dmax (mm)
ks (×10-3 min-1)
25 37
8.64 8.64
10.48 11.57
3.52 6.42
c is a constant, t is the immersion time, and ks is the rate constant of the swelling. The resulting fits show a good agreement with the experimental data. The values of dmax and ks obtained from the fitting procedure are listed in Table 1. The values of dmax and ks at 37 °C are higher than those at 25 °C. The swelling of the tablet is more extensive at 37 °C, corresponding to an increase of 34% of tablet diameter, while the increase is only 21% at 25 °C. The results obtained for ks values indicate that the swelling rate is faster at 37 °C than at 25 °C. An activation energy of swelling of 38.5 kJ/ mol can be roughly estimated from ks values at the two temperatures with the Arrhenius equation. It seems that the value obtained is realistic. It could be in part a result of the breakup of hydrogen bonds, which involves an activation energy in the range of 20-40 kJ/mol. The swelling process is unlike a reaction since the activation energy of reactions occurring in hours is in the range of 50-100 kJ/mol.
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High-Amylose Starch Tablets
Figure 6. Advance of solvent front expressed as r, the distance of the solvent front from the edge of the tablet toward the center of the tablet, at 25 and 37 °C as a function of immersion time.
From the results in Figures 4 and 5, more information on the diffusion phenomenon can be extracted. Figure 6 shows the advancing of the solvent front (r) toward the center of the tablet at 25 and 37 °C as a function of immersion time. The dashed line is fitted to the Frisch equation: r ) DtR + c
(2)
where r represents the distance between the extreme limit of the tablet and the dry part (where water concentration is lower than 1/6 of maximum concentration), t is the immersion time, and c is a constant. D is a constant proportional to the diffusion rate and represents the diffusion coefficient or velocity for the Fickian or case II process, respectively. The parameter R is representative of the diffusion kinetics
and equals 0.5 for Fickian diffusion and 1 for a case II (nonFickian) process.24,25 Intermediate values of R between 0.5 and 1 indicate anomalous diffusion. These values of R are applied in the case of diffusion in a flat sheet and must be corrected for other sample geometry and when the material swells on ingress of a solvent.26,27 The results in Figure 6 show clearly the difference in the diffusion processes at the two temperatures. In fact, the advancing of the diffusion front is directly proportional to the immersion time of the tablet at 25 °C, and the parameter R is found to be equal to 0.92, which is characteristic of case II diffusion for cylindrical geometry.27 A velocity of 0.8 × 10-5 cm/s is obtained from the slope. Figure 6 also shows clearly that the diffusion process of water at 37 °C does not follow case II kinetics. However, the Frisch equation is rather empirical when the material swells and the effects of the diffusion from the top and bottom of the tablet must be taken into account. Figure 5 shows that after an immersion time of about 400 min the swelling becomes small and negligible for the two temperatures. Moreover, transversal images (not shown here) have demonstrated that the diffusion fronts in the axial direction (i.e., from the upper and lower parts of the tablets) were only reached after 900 and 350 min at 25 and 37 °C, respectively, and thus the effects of diffusion from the top and bottom of the tablet could be ignored until these times. Equation 2 could be applied between 400 and 900 min for the tablet at 25 °C, but not at 37 °C since the contribution from diffusion from the top and the bottom of the tablet could not be neglected after an immersion time of 350 min. We
Figure 7. Spin density profiles of water (water gradient) extracted from images obtained at 25 °C (A-C) and 37 °C (D-F) at short immersion times.
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can expect that diffusion at 37 °C follows not the case II process but more probably anomalous diffusion. The gelatinization of Contramid occurs at about 37 °C, which probably influences the kinetics of water diffusion. The transition from case II to Fickian diffusion (via anomalous diffusion) with increased temperature was observed by Weisenberg et al. in the case of a PMMA-methanol system.28 These results are coherent since the change of swelling with temperature was expected. The concentration gradient of water can be derived from Figures 2 and 3 as shown by the profiles in Figure 7 for the tablets at 25 and 37 °C. These figures provide a realistic view of the water diffusion process. Clearly visible are the advancing water concentration gradients in the tablet. At very short times, these concentration profiles exhibit at 25 °C a rather abrupt decrease going from the swollen region to the core, which is characteristic of case II diffusion. Water concentration profiles have nearly the same shape at 37 °C as at 25 °C. The diffusion process follows probably a quasicase II kinetics at 37 °C. These results are in agreement with the data obtained from the advance of the solvent front in Figure 6. Conclusion Clearly, NMR imaging is a useful tool to study the slow release behavior of pharmaceutical tablets in an aqueous environment. The diffusion of water into the tablet and the swelling of the tablet upon absorption of water can be visualized with NMR imaging. More quantitative data can be obtained from the images, in particular the kinetics of the diffusion and swelling processes, than by optical imaging.20 The water penetration at 25 °C is a case II diffusion process, as confirmed by water concentration profiles. The extent of swelling of the tablets depends on the temperature and reaches a maximum after about 10 h. The NMR imaging technique also shows a steep advancing solvent front which should be responsible for the slow release characteristics of this microporous drug delivery system.29,30 The observed difference in swelling between 25 and 37 °C, as seen by NMR imaging, is likely caused by the gelatinization temperature of the excipient Contramid at about 37 °C. In both cases, the tablet, whose dry bulk density was about 0.7 g/mL, is converted into a microporous hydrogel by the swelling process. A recent rheological study of the swollen tablet shows a spongelike behavior.31 Thus, the pressed powder particles in the dry tablet are fused by the swelling into a coarse network. The latter results from pseudo-cross-linking of amylose chains as water penetrates the tablet and converts the mostly noncrystalline Contramid into a highly crystalline state dominated by the double helix “B” crystal polymorph
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of amylose.22,30 As a result of this mechanism, the Contramid tablet self-assembles in a microporous hydrogel with limited swelling.30 Acknowledgment. The authors acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada for their financial support. R.H.M. is grateful for the financial support from Labopham Inc. and a strategic grant from NSERC. References and Notes (1) Fyfe, C. A.; Blazek-welsh, A. I. J. Controlled Release 2000, 68, 313. (2) Harding, S.; Baumann, H.; Gren, T.; Seo, A. J. Controlled Release 2000, 66, 81. (3) Gao, P.; Meury, R. H. J. Pharm. Sci. 1996, 85, 725. (4) Cutts, L. S.; Hibberd, S.; Adler, J.; Melia, C. D. J. Controlled Release 1996, 42, 115. (5) Colombo, P.; Bettini, R.; Massino, G.; Catellani, P. L.; Santi, P.; Peppas, N. A. J. Pharm. Sci. 1995, 85, 991. (6) Thomas, N. L.; Windle, A. H. Polymer 1978, 19, 255. (7) Wan, L. S. C.; Prased, K. P. P. Drug DeV. Ind. Pharm. 1990, 16, 921. (8) Lee, P. I. Pharm. Res. 1993, 10, 980. (9) Mills, P. J.; Palmstrøm, C. J.; Kramer, E. J. J. Mater. Sci. 1986, 21, 1479. (10) Li, D.; Zhu, S.; Hamielec, A. E. Polymer 1993, 34, 1383. (11) Weisenberg, L. A.; Koenig, J. L. Macromolecules 1990, 23, 2445. (12) Grinsted, R. A.; Clark, L.; Koenig, J. L. Macromolecules 1992, 25, 1235. (13) Grinsted, R. A.; Koenig, J. L. Macromolecules 1992, 25, 1229. (14) Rajabi-Siahboomi, A. R.; Bowtell, R. W.; Mansfield, P.; Henderson, A.; Davies, M. C.; Melia, C. D. J. Pharm. Sci. 1995, 84, 1072. (15) Rothwell, W. P.; Holecek, D. K.; Kershaw, J. A. J. Polym. Sci., Polym. Lett. Ed. 1984, 22, 241. (16) Rajabi-Siahboomi, A. R.; Bowtell, R. W.; Mansfield, P.; Henderson, A.; Davies, M. C.; Melia, C. D. Pharm. Res. 1996, 13, 376. (17) Ghi, P. Y.; Hill, D. J. T.; Whittaker, A. K. Biomacromolecules 2001, 2, 504. (18) Koenig, J. L. Spectroscopy of polymers, 2nd ed.; Elsevier: New York, 1999. (19) Mateescu, M. A.; Lenaerts, V.; Dumoulin, V. U. S. Patent 5,618,650. (20) Moussa, I. S.; Cartilier, L. H. J. Controlled Release 1996, 42, 47. (21) Moussa, I. S.; Lenaerts, V.; Cartilier, L. H. J. Controlled Release 1998, 52, 63. (22) Lenaerts, V.; Moussa, I. S.; Dumoulin, L.; Mebsout, F.; Chouinard, F.; Szabo, P.; Mateescu, M. A.; Cartilier, L. H.; Marchessault, R. J. Controlled Release 1998, 53, 225. (23) Hyde, T. M.; Gladden, L. F.; Mackley, M. R.; Gao, P. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1795. (24) Crank, J.; Park, G. S. In Diffusion in Polymers, 1st ed.; Academic Press: New York, 1968. (25) Crank, J. In The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, 1975. (26) Ritger, P. L.; Peppas, N. A. J. Controlled Release 1987, 5, 23. (27) Ritger, P. L.; Peppas, N. A. J. Controlled Release 1987, 5, 37. (28) Weisenberg, L. A.; Koenig, J. L. Appl. Spectrosc. 1989, 43, 1117. (29) Le Bail, P.; Morin, F. G.; Marchessault, R. H. Int. J. Biol. Macromol. 1993, 26, 193. (30) Shiftan, D.; Ravenelle, F.; Mateescu, M. A.; Marchessault, R. H. Starch/Sta¨ rke 2000, 52, 186. (31) Ravenelle, F.; Marchessault, R. H.; Le´gare´, A.; Buschmann, M. D. Carbohydr. Polym. 2001, 47, 259.
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