NMR imaging of high-amylose starch tablets. 1. Swelling and water uptake

Using NMR chemical shift imaging to monitor swelling and molecular transport in drug-loaded tablets of hydrophobically modified poly(acrylic acid): methodology and effects of polymer (in)solubility

A new technique has been developed using NMR chemical shift imaging (CSI) to monitor water penetration and molecular transport in initially dry polymer tablets that also contain small low-molecular weight compounds to be released from the tablets. Concentration profiles of components contained in the swelling tablets could be extracted via the intensities and chemical shift changes of peaks corresponding to protons of the components. The studied tablets contained hydrophobically modified poly(acrylic acid) (HMPAA) as the polymer component and griseofulvin and ethanol as hydrophobic and hydrophilic, respectively, low-molecular weight model compounds. The water solubility of HMPAA could be altered by titration with NaOH. In the pure acid form, HMPAA tablets only underwent a finite swelling until the maximum water content of the polymer-rich phase, as confirmed by independent phase studies, had been reached. By contrast, after partial neutralization with NaOH, the polyacid became fully miscible with water. The solubility of the polymer affected the water penetration, the polymer release, and the releases of both ethanol and griseofulvin. The detailed NMR CSI concentration profiles obtained highlighted the clear differences in the disintegration/dissolution/release behavior for the two types of tablet and provided insights into their molecular origin. The study illustrates the potential of the NMR CSI technique to give information of importance for the development of pharmaceutical tablets and, more broadly, for the general understanding of any operation that involves the immersion and ultimate disintegration of a dry polymer matrix in a solvent.

NMR imaging of chitosan and carboxymethyl starch tablets: swelling and hydration of the polyelectrolyte complex

The hydration and swelling properties of the tablets made of chitosan, carboxymethyl starch, and a polyelectrolyte complex of these two polysaccharides have been studied by NMR imaging. We studied the effect of pH and ionic strength on the swelling of the tablets and on the diffusion of fluid into the tablets in water and simulated physiological fluids. The pH value of the fluids exerts a more significant effect than their ionic strengths on the swelling of the tablets. The tablets are compared also with those made of cross-linked high amylose starch. The formation of complex helps to keep the integrity of the tablets in various media and render a slow and restricted swelling similar to that of the tablets of the cross-linked high amylase starch, which is significantly lower than the swelling of chitosan and of carboxymethyl starch. The capacities to modulate the release rate of drugs in different media are discussed by comparing the matrices and evaluating the preparation process of the complex. A sustained release of less soluble drugs such as aspirin in gastrointestinal fluids can be provided by the complex, due to the ionic interaction and hydrogen bonding between the drug and the biopolymer complex.

Magnetic resonance imaging studies on gadonanotube-reinforced biodegradable polymer nanocomposites

We report about the in vitro cytotoxicity and MRI studies of Gd(3+)ions-doped ultra-short single-walled carbon nanotube (gadonanotubes), gadonanotubes- reinforced poly(lactic-co-glycolic acid) (PLGA) polymer nanocomposites and in vivo small animal MRI studies using the gadonanotubes. These studies were performed to explore the suitability of gadonanotubes-reinforced PLGA polymer nanocomposite as a model scaffold for noninvasive magnetic resonance imaging (MRI) to evaluate nanotube release during the degradation process of the scaffold and their biodistribution upon release from the polymer matrix in vivo. The gadonanotubes at 1-100 ppm and the gadonanotubes/PLGA nanocomposites (2 wt % gadonanotubes) did not show any cytotoxicity in vitro as demonstrated using the LIVE/DEAD viability assay. For the first time, r(2) relaxivity measurements were obtained for the superparamagnetic gadonanotubes. In vitro 7T MRI of the superparamagnetic gadonanotubes ([Gd] = 0.15 mM) suspended in a biocompatible 1% Pluronic F127 solution, gave a r(2) value of 578 mM(-1) s(-1). Upon subcutaneous injection of the gadonanotubes suspension into the dorsal region of rats, the high r(2) value translated into excellent and prolonged negative contrast enhancement of in vivo T(2)weighted proton MRI images. The in vitro characterization of the nanocomposite discs and their degradation process by MRI, showed strong influence of the gadonanotube on water proton relaxations. These results indicate that the gadonanotubes/PLGA nanocomposites are suitable for further in vivo studies to track by MRI the biodegradation release and biodistribution of gadonanotubes.

NMR 1D-imaging of water infiltration into mesoporous matrices

It is shown that coupling nuclear magnetic resonance (NMR) 1D-imaging with the measure of NMR relaxation times and self-diffusion coefficients can be a very powerful approach to investigate fluid infiltration into porous media. Such an experimental design was used to study the very slow seeping of pure water into hydrophobic materials. We consider here three model samples of nuclear waste conditioning matrices which consist in a dispersion of NaNO(3) (highly soluble) and/or BaSO(4) (poorly soluble) salt grains embedded in a bitumen matrix. Beyond studying the moisture progression according to the sample depth, we analyze the water NMR relaxation times and self-diffusion coefficients along its 1D-concentration profile to obtain spatially resolved information on the solution properties and on the porous structure at different scales. It is also shown that, when the relaxation or self-diffusion properties are multimodal, the 1D-profile of each water population is recovered. Three main levels of information were disclosed along the depth-profiles. They concern (i) the water uptake kinetics, (ii) the salinity and the molecular dynamics of the infiltrated solutions and (iii) the microstructure of the water-filled porosities: open networks coexisting with closed pores. All these findings were fully validated and enriched by NMR cryoporometry experiments and by performing environmental scanning electronic microscopy observations. Surprisingly, results clearly show that insoluble salts enhance the water progression and thereby increase the capability of the material to uptake water.

NMR imaging study of cross-linked high-amylose starch tablets — The effect of drug loading

NMR imaging techniques were used to study the effect of drug loading in cross-linked high-amylose starch tablets. The tablets contained acetaminophen with loading levels from 10 to 40 wt%. The absolute amount of the drug released increased with a larger amount of drug loading, but the percentages of drug released had only minor differences for the different tablets, probably due to the rapid formation of a gel layer for all the tablets, which slowed down drug release significantly. The release of drugs from the tablets in all cases is dominated by a diffusion mechanism before the disappearance of the dry core of the tablets. Radial and axial swelling and water uptake were found to increase with the amount of drug loading. The diffusion rates of water were comparable at the initial stage for all the tablets with different loadings, but became faster later for the tablets with higher amounts of drug loading as water diffusion may be facilitated by the hydrophilicity of the drug.

Synchrotron X-ray microtomographic study of tablet swelling

Tablet swelling behaviour was investigated by following the movements of embedded glass microsphere tracers, using X-ray microtomography (XmicroT) with intense illumination from a synchrotron. Specimens were prepared using combinations of hydroxypropyl-methyl-cellulose (HPMC) and microcrystalline cellulose (MCC) or pre-gelatinised starch (PGS), three materials commonly used as excipients for compacted tablets. The results revealed significant differences in swelling behaviour due to excipient type and compaction conditions. In particular, a sudden change was observed from gel-forming behaviour of formulations containing PGS or high HPMC content, to more rapid expansion and disintegration for formulations above 70% MCC. Although some radial expansion was observable with the higher PGS formulations and during later stages of swelling, axial expansion (i.e. the reverse of the compaction process) appeared to dominate in most cases. This was most pronounced for the 10/90 HPMC/MCC specimens, which rapidly increased in thickness, while the diameter remained almost unchanged. The expansion appeared to be initiated by hydration and may be due to the relaxation of residual compaction stress. This occurred within 'expansion zones', which initially appeared as thin bands close to the compacted (upper and lower) faces, but gradually advanced towards the centre and spread around the sides of the tablets. These zones exhibited lower X-ray absorbance, probably because they contained significant amounts of bubbles, which were formed by air released from the swelling excipients. Although, in most cases, these bubbles were too small to be resolved (<60 microm), larger bubbles (diameter up to 1mm) were clearly evident in the rapidly swelling 10/90 HPMC/MCC specimens. It is suggested that the presence of these bubbles may affect subsequent water ingress, by increasing the tortuosity and occluding part of the gel, which may affect the apparent diffusion kinetics (i.e. Fickian or Case II). These observations also suggested that axial expansion, initiated by water ingress, may be an important mechanism during tablet swelling.

Diffusion of molecular probes and the effects of their interactions with polymer matrices as studied by pulsed-field gradient NMR spectroscopy

Pulsed-field gradient NMR spectroscopy was used to study the interactions between small molecular probes and polymers bearing interacting groups. The self-diffusion coefficients of ethylene glycol and its oligomers and their methyl ester derivatives in poly(vinyl alcohol) gels were measured to study the effect of hydrogen bonding. The self-diffusion coefficients of small molecular probes containing hydroxyl, amine, and carboxylic acid groups were determined in several polymer matrices including poly(vinyl alcohol), poly(allylamine), and poly(acrylic acid) bearing lateral hydroxyl, amine, and carboxylic acid groups, respectively. The ionic interactions between the functional groups of the diffusants and of the polymers exhibited a marked effect on the diffusion of the molecular probes. For example, the reduced self-diffusion coefficients measured for a diffusant with a carboxylic acid group in a poly(allylamine) matrix were shown to be lower even though the molecular masses of the diffusants are similar.Key words: pulsed-field gradient NMR spectroscopy, self-diffusion, intermolecular interactions.

Study of hydration of cross-linked high amylose starch by solid state 13C NMR spectroscopy

Starch is subjected to chemical treatments such as cross-linking or hydroxypropylation to meet the material requirements for food uses or controlled release in the pharmaceutical industries. In this work, two types of cross-linking formulations have been employed for the preparation of high amylose starch for use as an excipient for sustained drug release. The structural differences and chain dynamics of the modified starches in the dry and hydrated states have been compared by the use of variable contact time cross polarization-magic angle spinning solid state (13)C NMR spectroscopy.

Polymer mobilization and drug release during tablet swelling. A 1H NMR and NMR microimaging study

The objective of this study was to investigate the swelling characteristics of a hydroxypropyl methylcellulose (HPMC) matrix incorporating the hydrophilic drug antipyrine. We have used this matrix to introduce a novel analytical method, which allows us to obtain within one experimental setup information about the molecular processes of the polymer carrier and its impact on drug release. Nuclear magnetic resonance (NMR) imaging revealed in situ the swelling behavior of tablets when exposed to water. By using deuterated water, the spatial distribution and molecular dynamics of HPMC and their kinetics during swelling could be observed selectively. In parallel, NMR spectroscopy provided the concentration of the drug released into the aqueous phase. We find that both swelling and release are diffusion controlled. The ability of monitoring those two processes using the same experimental setup enables mapping their interconnection, which points on the importance and potential of this analytical technique for further application in other drug delivery forms.

A Study of Water Diffusion into a High-Amylose Starch Blend: The Effect of Moisture Content and Temperature

The effect of moisture content and temperature on water diffusion into a modified high amylose (< or = 90%) maize thermoplastic starch blend was investigated. Gravimetric and magnetic resonance imaging (MRI) studies were conducted to elucidate the diffusion mechanism and diffusion coefficients for this system. The diffusion coefficient data demonstrated that the rate of water diffusion into this blend was significantly dependent upon temperature and moisture content. Water diffusion was faster at higher temperatures and generally for samples stored at higher relative humidity environments. It was revealed from the gravimetric data that water diffusion into this starch blend was Fickian; however, further analysis of the MRI images found that the water diffusion mechanism was exponentially dependent on the concentration. This result was determined by comparing experimental water concentration profiles to a theoretical model calculated using the implicit Crank-Nicolson finite difference method.

Imaging of High-Amylose Starch Tablets. 3. Initial Diffusion and Temperature Effects

The penetration of water into cross-linked high amylose starch tablets was studied at different temperatures by nuclear magnetic resonance (NMR) imaging, which follows the changes occurring at the surface and inside the starch tablets during swelling. It was found that the swelling was anisotropic, whereas water diffusion was almost isotropic. The water proton image profiles at the initial stage of water penetration were used to calculate the initial diffusion coefficient. The swelling and water concentration gradients in this controlled release system show significant temperature dependence. Diffusion behavior changed from Fickian to Case II diffusion with increasing temperature. The observed phenomena are attributed to the gelatinization of starch and the pseudo-cross-linking effect of double helix formation.

Restricted diffusion and release of aroma molecules from sol-gel-made porous silica particles

The aim of the current study is to predict the release kinetics of organic molecules entrapped in sol-gel-made silica particles using both pulsed field gradient-nuclear magnetic resonance (PFG-NMR) techniques and model calculations to describe restricted diffusion. The macroscopic release profile of aroma molecules from sol-gel-made particles is measured directly by UV-VIS spectroscopy, while the release kinetics are calculated by the Crank equation. The microscopic restricted pore diffusion coefficient of the aroma molecules in the Crank equation is obtained in situ by pulsed field gradient (PFG) magic angle spinning (MAS) nuclear magnetic resonance (NMR). Furthermore, restricted pore diffusion coefficients obtained by model calculations are in agreement with those measured by PFG-MAS-NMR, indicating the potential of the latter for characterization and screening of encapsulation formulations. Measured and calculated release profiles agree within experimental error.

In Situ Investigation of Drug Diffusion in Hydrogels by the Refractive Index Method

This work describes a simple but novel analytical method for in situ monitoring of the diffusion process of drugs in hydrogels based on refractive index measurements. The diffusion process was monitored by recording the refraction of a laser beam passing through a triangular cell, which allows the determination of changes in the refractive index distribution from the deviated distance of the linear beam. Compared to conventional methods, this new method exhibits advantages such as more simplicity, lower cost, and speed. Further, the refractive index method permits the determination of the concentration distribution of solutes in the hydrogels at any time during the diffusion process under nondestructive circumstances. The precision was determined by successfully applying this new method to the diffusion of a typical antibiotic drug, cefazolin sodium, in agarose gels of various concentrations. By employing Fick's second law, the diffusion behavior was investigated and the diffusion coefficients of cefazolin sodium in agarose gels were therefore obtained. Amsden's physical model based on obstruction effect was applied to the simulation of the diffusion process of cefazolin sodium and turned out to fit the results quite well.

NMR imaging of high-amylose starch tablets. 2. Effect of tablet size

Carbohydrate polymers are widely used for pharmaceutical applications such as the controlled release of drugs. The swelling and water mobility in high-amylose starch tablets are important parameters to be determined for these applications. They have been studied at different time intervals by nuclear magnetic resonance imaging (NMRI) after the immersion of the samples in water. These tablets have a hydrophilic matrix, which swells anisotropically and forms a hydrogel in water. NMRI shows clearly the anisotropy of the water penetration and the swelling along the radial and axial dimensions of the tablets. Empirical relationships are established to describe the kinetics of water penetration and swelling of the tablets. Results show that water uptake and tablet swelling strongly depend on the size of the tablets. Gravimetric measurements of water uptake were also performed in comparison with the NMRI results.