Drug release modeled by dissolution, diffusion, and immobilization

This article presents a novel drug release model that combines drug dissolution, diffusion, and immobilization caused by adsorption of the drug to the tablet constituents. Drug dissolution is described by the well-known Noyes-Whitney equation and drug adsorption by a Langmuir-Freundlich adsorption isotherm, and these two processes are included as source and sink terms in the diffusion equation. The model is applicable to tablets that disintegrate into a number of approximately spherical fragments. In order to simplify the analysis it is assumed that liquid absorption, matrix swelling, and tablet disintegration are much faster than drug dissolution and subsequent drug release. The resulting model is shown to yield release characteristics in good agreement with those observed experimentally.

Theoretical investigation of drug release from planar matrix systems: effects of a finite dissolution rate

Drug release from planar matrix systems has been investigated with special emphasis on the influence of a finite dissolution rate on the drug release profile. A mathematical model of the drug dissolution and release processes was formulated in terms of two coupled nonlinear partial differential equations (PDEs). These were solved numerically by using well-established FORTRAN routines. An approximate analytical solution, valid during the early stages of the release process, was derived. The analytical solution was compared to the numerical one and to drug release models existing in the literature. From this comparison, it was established that the analytical approximation provided a good description of the major part of the release profile, irrespective of the dissolution rate. Existing literature models, based on instantaneous dissolution, were found to agree with the numerical solution only when drug dissolution proceeded very rapidly in comparison with the diffusion process. Consequently, the new analytical short-time approximation of the drug release complements the formulas existing in the literature, since it provides a superior description of the release of slowly dissolving drugs.

Modelling of drug release from coated granular pellets

A mathematical model of drug release from coated pellets with a granular core has been developed. The model includes a dynamic description of all three main processes contributing to drug release from such a system, i.e. liquid inflow, drug dissolution, and liquid efflux caused by diffusion across the coating. The cumulative fraction of released drug has been shown to be determined by three rate constants, one for each process mentioned above, together with two dimensionless parameters. These parameters are related to the porosity of the pellet core and the solubility of the drug in the dissolution medium. The model has been validated by comparison with experimentally determined release profiles for pellets consisting of a granular core of microcrystalline cellulose containing dispersed salicylic acid, coated by a thin layer of ethyl cellulose.

Finite element analysis of the release of slowly dissolving drugs from cylindrical matrix systems

Drug release from matrix systems of cylindrical shape is analyzed in detail by using the finite element method. The model used combines the Noyes-Whitney and diffusion equations, and thus takes the effects of a finite dissolution rate into account. The model is valid for all drug solubilities and dissolution rates, and allows accurate predictions of the drug release to be made. Anisotropic drug transport that may result from the manufacturing process is properly accounted for. Model calculations show that a finite dissolution rate may affect the release profile significantly, producing an initial delay. The equivalence between anisotropic release and isotropic release from a matrix with different dimensions is demonstrated. Comparisons are made with the predictions of a recently proposed pseudo-steady state (PSS) analysis of drug release from cylindrical matrices [Y. Zhou, J. S. Chu, T. Zhou, X. Y. Wu, Modeling of dispersed-drug release from two-dimensional matrix tablets, Biomaterials 26 (2005) 945-952]. This comparison reveals that important discrepancies exist between the numerical and analytical results, which are attributed to the simplifying assumption made in the PSS analysis that the region containing solid drug remains cylindrical in shape throughout the release process. The proposed model is shown to describe experimental release data well.

Compression behaviour and tablet-forming ability of spray-dried amorphous composite particles

The aim of this study was to investigate the compression behaviour and tablet-forming ability of spray-dried amorphous two- and three-component composite particles. Particles of lactose alone, two-component particles of lactose and PVP, and three-component particles of lactose, PVP and a small amount of polysorbate 80 were prepared by spray-drying. Two qualities of PVP with different molecular weights were used for the preparation of both types of particles. The particles were characterised with respect to permeametry surface area, moisture content, particle and bulk density, glass transition and crystallisation temperature, and heat of crystallisation. The tablet tensile strength of the different particles formed at a series of applied pressures was determined and compression parameters were derived from the Heckel and Kawakita equations. The presence of PVP gave particles that were less prone to deform permanently during compression while the presence of surfactant gave particles that were less able to form tablets. In conclusion, the compression behaviour and tablet-forming ability of spray-dried amorphous lactose can be modulated by the addition of stabilising polymers or surfactants to the spray feed solution.

Relationships between surface coverage ratio and powder mechanics of binary adhesive mixtures for dry powder inhalers

The aim of this paper was to study relationships between the content of fine particles and the powder mechanics of binary adhesive mixtures and link these relationships to the blend state. Mixtures with increasing amounts of fine particles (increasing surface coverage ratios (SCR)) were prepared using Lactopress SD as carrier and micro particles of lactose as fines (2.7 µm). Indicators of unsettled bulk density, compressibility and flowability were derived and the blend state was visually examined by imaging. The powder properties studied showed relationships to the SCR characterised by stages. At low SCR, the fine particles predominantly gathered in cavities of the carriers, giving increased bulk density and unchanged or improved flow. Thereafter, increased SCR gave a deposition of particles at the enveloped carrier surface with a gradually more irregular adhesion layer leading to a reduced bulk density and a step-wise reduced flowability. The mechanics of the mixtures at a certain stage were dependent on the structure and the dynamics of the adhesion layer and transitions between the stages were controlled by the evolution of the adhesion layer. It is advisable to use techniques based on different types of flow in order to comprehensively study the mechanics of adhesive mixtures.

Linking carrier morphology to the powder mechanics of adhesive mixtures for dry powder inhalers via a blend-state model

The aim of this study was to investigate how the carrier morphology affects the expression of blend states in adhesive mixtures as a function of surface coverage ratio (SCR) and to identify where transitions between the different states occur. Adhesive mixtures of five lactose carriers with varying contents of lactose fines, corresponding to blends with different SCR ranging from 0 to 6, were produced by low-shear mixing. The powder mechanics of the mixtures were characterized by bulk density, compressibility and permeability. The appearance of the carriers and blends was studied by scanning electron microscopy, light microscopy and atomic force microscopy. The size and morphology of the carriers had a crucial impact on the evolution of the blend state, and affected the powder mechanical properties of the mixtures. It was found that smaller carriers with little or no surface irregularities were more sensitive to additions of fines resulting in self-agglomeration of fines at relatively low SCR values. On the contrary, carriers with irregular surface structures and larger sizes were able to reach higher SCR values before self-agglomeration of fines occurred. This could be attributed to an increased deagglomeration efficiency of irregular and larger carriers and to fines predominantly adhering to open pores.

Drug release from reservoir pellets compacted with some excipients of different physical properties

The aim of the present study was to investigate the influence of the size and the porosity of excipient microcrystalline cellulose (MCC) particles on the densification and the deformation during compaction and the consequent effect on the drug release from reservoir pellets. Drug pellets consisting of salicylic acid and microcrystalline cellulose were prepared by extrusion-spheronisation and spray-coated with ethyl cellulose (ethanol solution). Excipient pellets of different size and porosity were prepared by extrusion-spheronisation or direct spheronisation. Five binary mixtures of reservoir pellets and excipient particles were prepared in the proportion 1:7 and lubricated. After compaction the reservoir pellets were retrieved and analysed to determine the intragranular porosity, surface area, shape and drug release. The reservoir pellets were shown to undergo extensive deformation and densification during compaction, resulting in a preserved or even prolonged drug release time. The mode of deformation of the reservoir pellets seems to be critical for the compression-induced change in drug release. Formation of large indents has a negative effect on the release time, while the use of small particles or small deformable agglomerates has a protective effect. We also hypothesize that the coating structure changes during compaction and the final structure of the coating is the net effect of two parallel processes, one reducing and one prolonging the drug transport time across the coating.

A new method for characterizing the release of drugs from tablets in low liquid surroundings

The purpose of this article is to introduce a method capable of determining early drug dissolution in small amounts of liquid. The method is based on the measurement of the alternating ionic current through a cell containing the dissolution medium and the substance to be dissolved. Both the initial and more prolonged absorption of liquid into tablets can also be determined by using the same technique. The method has been tested on two tablet formulations containing agglomerated micronized cellulose and NaCl as a model drug. Release of NaCl was delayed from both formulations; the extent of the delay was strongly formulation-dependent only when the surrounding liquid was in short supply. This finding shows that new drug dissolution phenomena may be encountered in small liquid volumes; these phenomena would not have been seen with the large volume methods normally used in in vitro dissolution tests. Hence, for formulations intended for sublingual, buccal, or rectal administration, i.e., in areas where liquid is scarce, in vitro dissolution tests should be performed in small volumes of dissolution medium.

Theoretical analysis of the release of slowly dissolving drugs from spherical matrix systems

Drug release from spherical matrix systems has been investigated theoretically, with numerical as well as analytical methods. The model used combines the Noyes-Whitney and diffusion equations, and thus takes the effects of a finite dissolution rate into account. The release profile has been determined numerically, by using well-established FORTRAN routines. An approximate analytical formula for the amount of released drug has been derived, which is valid during the early stages of the release process. This analytical short-time approximation was compared to the numerical result, and to drug release models existing in the literature. From this comparison it was concluded that the analytical approximation provided a good description of the major part of the release profile, irrespective of the dissolution rate. Existing literature models, based on instantaneous dissolution, provided a good description of the release only when drug dissolution proceeded very rapidly in comparison with the diffusion process. Consequently, the new analytical short-time approximation complements the formulas existing in the literature, since it provides a superior description of the release of slowly dissolving drugs.

Electrostatic Swelling Transitions in Surface-Bound Microgels

Herein, electrostatic swelling transitions of poly(ethyl acrylate-co-methacrylic acid) microgels covalently bound to silica surfaces are investigated. Confined at a solid surface, microgel swelling is anisotropically hindered and the structure is flattened to an extent dictated by pH and microgel composition. Microgel deformation under applied load is also shown to depend on microgel charge density, with the highest deformation observed at intermediate charge densities. Two modes of microgel deformation under load were observed, one elastic and one viscoelastic, related to polymer strand deformation and displacement of trapped water, respectively. Results on polymer strand dynamics reveal that the microgels are highly dynamic, as the number of strand-tip interaction points increases 4-fold during a 10 s contact time. Furthermore, finite element modeling captures these effects qualitatively and shows that stress propagation in the microgel network decays locally at the rim of contact with a solid interface or close to the tip probe. Taken together, the results demonstrate a delicate interplay between the surface and microgel which determines the structure and nanomechanical properties of the latter and needs to be controlled in applications of systems such as pH-responsive surface coatings in biomaterials.

Conductivity Percolation in Loosely Compacted Microcrystalline Cellulose: An in Situ Study by Dielectric Spectroscopy during Densification

The present study aims at contributing to a complete understanding of the water-induced ionic charge transport in cellulose. The behavior of this transport in loosely compacted microcrystalline cellulose (MCC) powder was investigated as a function of density utilizing a new type of measurement setup, allowing for dielectric spectroscopy measurement in situ during compaction. The ionic conductivity in MCC was found to increase with increasing density until a leveling-out was observed for densities above approximately 0.7 g/cm3. Further, it was shown that the ionic conductivity vs density followed a percolation type behavior signifying the percolation of conductive paths in a 3D conducting network. The density percolation threshold was found to be between approximately 0.2 and 0.4 g/cm3, depending strongly on the cellulose moisture content. The observed percolation behavior was attributed to the forming of interparticulate bonds in the MCC and the percolation threshold dependence on moisture was linked to the moisture dependence of particle rearrangement and plastic deformation in MCC during compaction. The obtained results add to the understanding of the density-dependent water-induced ionic transport in cellulose showing that, at given moisture content, the two major parameters determining the magnitude of the conductivity are the connectedness of the interparticluate bonds and the connectedness of pores with a diameter in the 5-20 nm size range. At densities between approximately 0.7 and 1.2 g/cm3 both the bond and the pore networks have percolated, facilitating charge transport through the MCC compact.