Modeling of drug release from delivery systems based on hydroxypropyl methylcellulose (HPMC)

The objective of this article is to review the spectrum of mathematical models that have been developed to describe drug release from hydroxypropyl methylcellulose (HPMC)-based pharmaceutical devices. The major advantages of these models are: (i) the elucidation of the underlying mass transport mechanisms; and (ii) the possibility to predict the effect of the device design parameters (e.g., shape, size and composition of HPMC-based matrix tablets) on the resulting drug release rate, thus facilitating the development of new pharmaceutical products. Simple empirical or semi-empirical models such as the classical Higuchi equation and the so-called power law, as well as more complex mechanistic theories that consider diffusion, swelling and dissolution processes simultaneously are presented, and their advantages and limitations are discussed. Various examples of practical applications to experimental drug release data are given. The choice of the appropriate mathematical model when developing new pharmaceutical products or elucidating drug release mechanisms strongly depends on the desired or required predictive ability and accuracy of the model. In many cases, the use of a simple empirical or semi-empirical model is fully sufficient. However, when reliable, detailed information are required, more complex, mechanistic theories must be applied. The present article is a comprehensive review of the current state of the art of mathematical modeling drug release from HPMC-based delivery systems and discusses the crucial points of the most important theories.

Mathematical modeling of bioerodible, polymeric drug delivery systems

The aim of this article is to give an introduction into mathematical modeling approaches of bioerodible controlled drug delivery systems and to present the most important erosion theories reported in the literature. First, important parameters such as degradation and erosion are defined and physicochemical methods for their investigation are briefly presented. Then, phenomenological empirical models as well as models based on diffusion and chemical reaction theory are discussed. Due to the significant chemical and physicochemical differences among individual bioerodible polymers used for controlled drug delivery systems, various mathematical models have been developed to describe the chemical reactions and physical mass transport processes involved in erosion-controlled drug release. Various examples of practical applications of these models to experimental drug release data are given. For those involved in the design and development of biodegradable drug delivery systems this will help to choose the appropriate mathematical model for a specific drug release problem. Important selection criteria such as the desired predictive power and precision, but also the effort required to apply a model to a particular system will be discussed. Furthermore, before models can be used for drug release predictions certain parameters such as drug dissolution or polymer degradation rate constants, have to be known. The number of parameters to be determined significantly differs between the models. The practical benefit of carefully choosing the right model is that effects of composition and device geometry on the drug release kinetics can be predicted which can reduce laborious formulation studies to a minimum.

PLGA-based microparticles: elucidation of mechanisms and a new, simple mathematical model quantifying drug release

The two major aims of this study were: (i) to elucidate the underlying release mechanisms from drug-loaded, erodible microparticles based on poly(lactic-co-glycolic acid) (PLGA) showing biphasic drug release behavior: an initial 'burst' effect, followed by a zero order release phase; and (ii) to develop a new, simple mathematical model that allows the quantitative description of the observed in vitro drug release patterns from this type of delivery system. PLGA-based microparticles offer various advantages, such as the possibility to control the resulting drug release rate accurately over prolonged periods of time, easiness of administration (e.g., by stereotaxic injection), good biocompatibility and complete erosion (avoiding the removal of empty remnants). Consequently, the practical importance of these advanced drug delivery systems is remarkably increasing. However, only little knowledge is yet available concerning the processes controlling the release rate of the drug out of these devices. Various chemical and physical phenomena are involved, rendering the identification of the crucial mechanisms and the mathematical description of the resulting drug release kinetics difficult. In the present study, different physicochemical characterization methods (e.g., DSC, SEM, SEC, particle size analysis) were used to monitor the changes occurring within anticancer drug-loaded PLGA microparticles upon exposure to phosphate buffer pH 7.4. Based on these experimental findings, the most important underlying drug release rate controlling mechanisms were identified and a new mathematical model was developed that allows the quantitative description of the resulting release patterns.

How porosity and size affect the drug release mechanisms from PLGA-based microparticles

Porous, poly(lactic-co-glycolic acid) (PLGA)-based microparticles were prepared using a water-in-oil-in-water (W/O/W) solvent extraction/evaporation technique. Lidocaine was used as a model drug and different-sized particle fractions were obtained by sieving. The physicochemical properties of the devices and changes thereof upon exposure to phosphate buffer pH 7.4 were studied using optical and scanning electron microscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC), gravimetric analysis and in vitro drug release measurements. In contrast to non-porous microparticles of identical composition, the relative drug release rate was found to decrease with increasing system size. SEC, DSC and gravimetric analysis showed that the degradation rate of the polymer increased with increasing microparticle dimension, indicating that autocatalytic effects play an important role even in small and highly porous PLGA-based microparticles. However, these effects were much less pronounced than in non-porous devices. Importantly, they were overcompensated by the effects of the increasing diffusion pathway lengths with increasing system dimension. Thus, high initial microparticle porosities do not only lead to increased drug mobilities, but can also fundamentally alter the underlying mass transport mechanisms.

HPMC-matrices for controlled drug delivery: a new model combining diffusion, swelling, and dissolution mechanisms and predicting the release kinetics

PURPOSE

The purpose of this study was to investigate the drug release mechanisms from hydroxypropyl methylcellulose (HPMC)-matrices, and to develop a new model for quantitative predictions of controlled drug delivery.

METHODS

The dissolved mass of pure HPMC-matrices and the drug release rate from propranolol HCl-loaded HPMC-matrices were determined experimentally. Based on Fick's second law of diffusion for cylinders, the transport of water and drug were modeled considering (i) both radial and axial diffusion, (ii) concentration-dependent drug diffusivities, (iii) matrix swelling and (iv) HPMC dissolution.

RESULTS

Good agreement between theory and experiment (dissolved mass and drug release studies) was obtained, proving the validity of the presented model. The water and drug diffusivities are strongly dependent on the matrix swelling ratio. Diffusion, swelling and dissolution are the governing mechanisms involved in the overall drug release process.

CONCLUSIONS

The practical benefit of the presented model is to identify the required shape and dimensions of drug-loaded HPMC-matrices in order to achieve desired release profiles, thus facilitating the development of new controlled drug delivery products. This will be demonstrated in a future study.

Effect of the size of biodegradable microparticles on drug release: experiment and theory

The aim of this study was to investigate the effect of the size of biodegradable microparticles (monolithic dispersions) on the release rate of an incorporated drug in a quantitative way. 5-Fluorouracil-loaded, poly(lactic-co-glycolic acid)-based microparticles were prepared with a solid-in-oil-in-water solvent extraction technique. In vitro drug release from different-sized particle fractions was measured in phosphate buffer pH 7.4. Differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and size exclusion chromatography (SEC) were used to monitor the degradation behavior of the polymer and morphological changes of the microparticles upon exposure to the release medium. Based on these experimental results, an appropriate mathematical theory was identified and used to get further insight into the underlying physical and chemical processes, which are involved in the control of drug release. Interestingly, the relative as well as the absolute release rate of the drug increased with increasing microparticle radius, despite of the increasing diffusion pathways. SEC, DSC and SEM analysis revealed that the degradation behavior of the matrix forming polymer was not significantly affected by the size of the devices and that autocatalytic effects do not seem to play a major role. Importantly, the initial drug loading significantly increased with increasing radius of the drug delivery system. Thus, large microparticles became more porous during drug release than small microparticles, leading to higher apparent diffusivities and drug transport rates. This effect overcompensated the effect of the increasing diffusion pathways with increasing microparticle radius, resulting in increased drug release rates with increasing device dimension. The applied mathematical model, considering drug diffusion with non-constant diffusivities (to account for polymer degradation) was able to quantitatively describe the observed drug release patterns. Importantly, an exponential relationship could be established between the diffusion coefficient and the initial loading of the drug. Based on this dependency, it was possible to predict the resulting drug release kinetics for arbitrary microparticle sizes in a quantitative way.

Hydrophilic matrices for controlled drug delivery: an improved mathematical model to predict the resulting drug release kinetics (the "sequential layer" model)

PURPOSE

The aims of this study were (i) to elucidate the transport mechanisms involved in drug release from hydrophilic matrices; and (ii) to develop an improved mathematical model allowing quantitative predictions of the resulting release kinetics.

METHODS

Our previously presented model has been substantially modified, by adding: (i) inhomogeneous swelling; (ii) poorly water-soluble drugs; and (iii) high initial drug loadings. The validity of the improved model has been tested experimentally using hydroxypropyl methylcellulose (HPMC)-matrices, containing either a poorly or a freely water-soluble drug (theophylline or chlorpheniramine maleate) at various initial loadings in phosphate buffer pH 7.4 and 0.1 N HCl, respectively.

RESULTS

By overcoming the assumption of homogeneous swelling we show that the agreement between theory and experiment could be significantly improved. Among others, the model could describe quantitatively even the very complex effect on the resulting relative release rates (first slowing down, then accelerating drug release) observed when increasing the initial loading of poorly water-soluble drugs.

CONCLUSIONS

The practical benefit of this work is an improved design model that can be used to predict accurately the required composition and dimensions of drug-loaded hydrophilic matrices in order to achieve desired release profiles, thus facilitating the development of new pharmaceutical products.

pH-independent release of a weakly basic drug from water-insoluble and -soluble matrix tablets

Weakly basic drugs or salts thereof demonstrate pH-dependent solubility. The resulting release from conventional matrix tablets decreases with increasing pH-milieu of the gastrointestinal tract. The aim of this study was to overcome this problem and to achieve pH-independent drug release. Two different polymers were used as matrix formers, the water-insoluble and almost unswellable ethylcellulose (EC), and the water-soluble and highly swellable hydroxypropyl methylcellulose (HPMC). Two different approaches to solve the problem of pH-dependent release of weakly basic drugs are demonstrated in this paper. The first one is based on the addition of hydroxypropyl methylcellulose acetate succinate (HPMCAS, an enteric polymer), the second one on the addition of organic acids such as fumaric, succinic or adipic acid to the drug-polymer system. The first approach failed to achieve pH-independent drug release, whereas the addition of organic acids to both matrix formers was found to maintain low pH values within the tablets during drug release in phosphate buffer (pH 6.8 or 7.4). Thus, the micro-environmental conditions for the dissolution and diffusion of the weakly basic drug were almost kept constant. The release of verapamil hydrochloride from tablets composed of ethylcellulose or HPMC and organic acids was found to be pH-independent.

A new model describing the swelling and drug release kinetics from hydroxypropyl methylcellulose tablets

A novel mathematical model for the water transport into and drug release from hydroxypropyl methylcellulose (HPMC) tablets is presented. Fick's second law of diffusion is used to describe the mass transfer processes in the three-component system drug/polymer/water. Numerical solutions of the respective set of partial differential equations are provided, considering axial and radial diffusion within cylindrical tablets. It is shown that the diffusion coefficients strongly depend on the water concentration (parameters quantifying this dependence have been determined). Swelling of the device is considered using moving boundary conditions, whereas dissolution processes are neglected. Experiments proved the applicability of the theory. The practical benefit of the new model is to calculate the required shape and dimensions of HPMC tablets to achieve a desired release profile.

Diffusion-controlled drug delivery systems: calculation of the required composition to achieve desired release profiles

The aim of this study was to investigate the effect of the composition of diffusion-controlled release devices (type and amount of plasticizer, type of polymer) on the drug diffusivity and the resulting release kinetics in a quantitative way. Diltiazem hydrochloride and theophylline were investigated in ethyl cellulose (EC) and Eudragit((R)) RS 100, plasticized with various amounts of acetyltributyl citrate (ATBC), acetyltriethyl citrate (ATEC), dibutyl phthalate (DBP), dibutyl sebacate (DBS), diethyl phthalate (DEP), and tributyl citrate (TBC). Thin drug-containing films (monolithic solutions) were used to determine the diffusion coefficients experimentally. The effect of the type and amount of plasticizer on the drug diffusivity was found to be significant, whereas the chain length of the polymer only played a minor rule in the investigated systems. Interestingly, a quantitative relationship between the diffusion coefficient of the drug and the plasticizer level could be established. Based on these results, the release kinetics of diffusion-controlled drug delivery systems could be predicted. In this study, the release patterns from microparticles were calculated and the significant effect of the composition of the device on the resulting release rate was simulated. The latter could be effectively modified by varying the type and amount of plasticizer. Independent experiments verified the theoretical predictions. The practical benefit of the presented method is to calculate the required composition of diffusion-controlled drug delivery systems (monolithic solutions) to achieve desired release profiles.

Floating matrix tablets based on low density foam powder: effects of formulation and processing parameters on drug release

The aim of this study was to develop and physicochemically characterize single unit, floating controlled drug delivery systems consisting of (i). polypropylene foam powder, (ii). matrix-forming polymer(s), (iii). drug, and (iv). filler (optional). The highly porous foam powder provided low density and, thus, excellent in vitro floating behavior of the tablets. All foam powder-containing tablets remained floating for at least 8 h in 0.1 N HCl at 37 degrees C. Different types of matrix-forming polymers were studied: hydroxypropyl methylcellulose (HPMC), polyacrylates, sodium alginate, corn starch, carrageenan, gum guar and gum arabic. The tablets eroded upon contact with the release medium, and the relative importance of drug diffusion, polymer swelling and tablet erosion for the resulting release patterns varied significantly with the type of matrix former. The release rate could effectively be modified by varying the "matrix-forming polymer/foam powder" ratio, the initial drug loading, the tablet geometry (radius and height), the type of matrix-forming polymer, the use of polymer blends and the addition of water-soluble or water-insoluble fillers (such as lactose or microcrystalline cellulose). The floating behavior of the low density drug delivery systems could successfully be combined with accurate control of the drug release patterns.

Effects of the type of release medium on drug release from PLGA-based microparticles: Experiment and theory

The major objectives of the present study were: (i) to prepare 5-fluorouracil (5-FU)-loaded, poly(lactic-co-glycolic acid) (PLGA)-based microparticles, which can be used for the treatment of brain tumors, (ii) to study the effects of the type of release medium on the resulting drug release kinetics, and (iii) to get further insight into the underlying drug release mechanisms. Spherical microparticles were prepared by a solvent extraction method and characterized using different techniques, including size exclusion chromatography (SEC), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and particle size analysis before and upon exposure to various release media. Interestingly, very different drug release patterns (including mono-, bi- and tri-phasic ones) were observed, depending on the pH, osmolarity and temperature of the release medium. An adequate mathematical theory was used to quantitatively describe the experimentally measured 5-FU release patterns. The model considers the limited solubility of the drug, polymer degradation as well as drug diffusion and allowed to determine system and release medium specific parameters, such as the diffusion coefficient of the drug. In particular, the pH and temperature of the release medium were found to be of major importance for the resulting release patterns. Based on the obtained knowledge the selection of an appropriate release medium for in vitro tests simulating in vivo conditions can be facilitated, and "stress tests" can be developed allowing to get rapid feedback on the release characteristics of a specific batch.

Calculation of the required size and shape of hydroxypropyl methylcellulose matrices to achieve desired drug release profiles

The aim of this study was to develop methods for the design of hydroxypropyl methylcellulose (HPMC) tablets with specified drug profiles. This was achieved by the use of a mathematical model developed to predict the release kinetics of water-soluble drugs from HPMC matrices. The required model parameters were determined experimentally for propranolol HCl and chlorpheniramine maleate in 0. 1 N HCl and phosphate buffer pH 7.4, respectively. Then, the effects of the dimensions and aspect ratio (radius/height) of the tablets on the drug release rate were evaluated. Independent experiments were conducted to verify the theoretical predictions. Acceptable agreement between theory and experiment was found, irrespective of the type of release medium and drug. However, statistical analysis revealed a structure in the resulting residuals. Drug release rates are overestimated at the beginning and underestimated at the end of the process. Possible explanations and modifications of the model are thoroughly discussed. Both, theoretical and experimental data showed that a broad spectrum of drug release patterns can be achieved by varying the size and shape of the tablet. The effect of the initial matrix radius on release was found to be more pronounced than the effect of the initial thickness. The practical benefit of the proposed method is to predict the required size and shape of new controlled drug delivery systems to achieve desired release profiles, thus significantly facilitating the development of new pharmaceutical products.

Blends of enteric and GIT-insoluble polymers used for film coating: physicochemical characterization and drug release patterns

THE OBJECTIVES OF THIS STUDY WERE: (i). to use blends of gastrointestinal tract (GIT)-insoluble and enteric polymers (ethyl cellulose and Eudragit L) as coating materials for multiparticulate controlled release dosage forms; (ii). to investigate the effects of the polymer blend ratio and coating level on the resulting drug release patterns; and (iii). to explain the observed phenomena based on the physicochemical properties of the systems. Propranolol HCl-loaded pellets were coated in a fluidized bed coater with organic polymer solutions; thin, drug-containing and drug-free, polymeric films were prepared using a casting knife. In vitro drug release, water uptake and dry weight loss studies were performed in 0.1 M HCl and phosphate buffer pH 7.4, respectively. The apparent drug diffusion coefficients within the polymeric systems were determined using different experimental and theoretical techniques (side-by-side diffusion cells, in vitro drug release from thin films; exact and approximate solutions of Fick's second law of diffusion). A broad range of drug release patterns from coated pellets could be achieved by varying the GIT-insoluble:enteric polymer blend ratio. With increasing relative amounts of Eudragit L, the release rates in both media significantly increased. The increase at low pH could be attributed to an increase in water uptake, as observed with thin films. Interestingly, only partial Eudragit L leaching occurred in phosphate buffer pH 7.4 even at high enteric polymer contents, indicating that the GIT-insoluble polymer effectively hindered the dissolution of the entrapped Eudragit L. At high pH, both polymer leaching and polymer swelling contributed to the control of drug release. The determined apparent drug diffusion coefficients take the two effects adequately into account.

Floating microparticles based on low density foam powder

The aim of this study was to develop a novel multiparticulate gastroretentive drug delivery system and to demonstrate its performance in vitro. Floating microparticles consisting of (i) polypropylene foam powder; (ii) verapamil HCl as model drug; and (iii) Eudragit RS, ethylcellulose (EC) or polymethyl methacrylate (PMMA) as polymers were prepared with an O/W solvent evaporation method. The effect of various formulation and processing parameters on the internal and external particle morphology, drug loading, in vitro floating behavior, in vitro drug release kinetics, particle size distribution and physical state of the incorporated drug was studied. The microparticles were irregular in shape and highly porous. The drug encapsulation efficiency was high and almost independent of the theoretical loading. Encapsulation efficiencies close to 100% could be achieved by varying either the ratio 'amount of ingredients: volume of the organic phase' or the relative amount of polymer. In all cases, good in vitro floating behavior was observed. The release rate increased with increasing drug loading and with decreasing polymer amounts. The type of polymer significantly affected the drug release rate, which increased in the following rank order: PMMA<EC<Eudragit RS. A broad spectrum of release patterns could be obtained with the investigated formulations. In contrast, the effect of the volume of the aqueous phase on drug release was not very pronounced. The size of the microparticles was almost independent of the drug loading, but strongly depended on the amount of polymer. The drug was partly dissolved and partly in the amorphous form distributed throughout the system.

Pharmaceutical Applications of Shellac: Moisture-Protective and Taste-Masking Coatings and Extended-Release Matrix Tablets

Shellac is a natural polymer, which is used as enteric coating material in pharmaceutical applications. The major objective of the present study was to investigate the potential of shellac for other purposes, namely to provide moisture-protective and taste-masking coatings as well as extended-release matrix tablets. The efficiency of shellac to achieve moisture protection and taste masking was compared with that of hydroxypropyl methylcellulose (HPMC), which is most frequently used for these purposes. Shellac-coated tablets showed lower water uptake rates than HPMC-coated systems at the same coating level. The stability of acetylsalicylic acid was higher in tablets coated with shellac compared with HPMC-coated systems, irrespective of the storage humidity. Therefore, lower shellac coating levels were required to achieve the same degree of drug protection. Shellac coatings effectively masked the unpleasant taste of acetaminophen tablets. Compared to HPMC, again lower coating levels were required to achieve similar effects. The resulting drug release in simulated gastric fluid was not significantly altered by the thin shellac coatings, which rapidly ruptured due to the swelling of the coated tablet core. In addition, shellac was found to be a suitable matrix former for extended-release tablets. The latter could be prepared by direct compression or via wet granulation using ethanolic or ammoniated aqueous shellac binder solutions. The resulting drug-release patterns could effectively be altered by varying different formulation and processing parameters.

Understanding and predicting drug delivery from hydrophilic matrix tablets using the "sequential layer" model

PURPOSE

The objectives of this work were (i) to study and understand the physicochemical phenomena which are involved in the swelling and drug release from hydrophilic matrix tablets using the "sequential layer" model, and (ii) to predict the effect of the initial radius height and size of the tablets on the resulting drug release profiles.

METHODS

Tablets were prepared by direct compression, using hydroxypropyl methylcellulose (HPMC) grades with different average molecular weights as matrix-forming polymers. The in vitro release of chlorpheniramine maleate, propranolol HCl, acetaminophen, theophylline and diclofenac sodium was studied in phosphate buffer (pH 7.4) and 0.1 M HCl, respectively. The initial drug loading varied from 1 to 70%, while the radius and height of the tablets varied from 1 to 8 mm.

RESULTS

The "sequential layer" model considers water and drug diffusion with non-constant diffusivities and moving boundary conditions, non-homogeneous polymer swelling, drug dissolution, and polymer dissolution. We showed that this model was able to predict the resulting drug release kinetics accurately in all cases.

CONCLUSIONS

The "sequential layer" model can be used to elucidate the swelling and drug release behavior from hydrophilic matrix tablets and to simulate the effect of the device geometry on the drug release patterns. Hence, it can facilitate the development of new pharmaceutical products.

Polymer blends used for the aqueous coating of solid dosage forms: importance of the type of plasticizer

The aim of this study was to investigate the importance of the type of plasticizer in polymer blends used for the coating of solid dosage forms, comparing a lipophilic and a hydrophilic plasticizer (dibutyl sebacate (DBS) and triethyl citrate (TEC)). In vitro drug release from propranolol hydrochloride (propranolol HCl)-loaded pellets coated with blends of ethyl cellulose (EC) and Eudragit L (100:0, 75:25, 50:50, 25:75 and 0:100 w/w) was investigated at low as well as at high pH. To better understand the underlying mass transport mechanisms, the physicochemical properties of the film coatings (e.g. mechanical resistance, water uptake and dry weight loss behavior) were determined. Interestingly, drug release strongly depended on the type of plasticizer. Importantly, not only the slope but also the shape of the release curves was affected, indicating that the chemical nature of the plasticizer plays a major role for the underlying drug release mechanisms. Diffusion through the intact polymer coatings and/or through water-filled cracks was found to be dominating for the control of drug release. The relative importance of these pathways strongly depended on the polymer blend ratio and type of plasticizer. In contrast to DBS, TEC rapidly leached out of the coatings, resulting in decreasing mechanical resistances of the films and, thus, facilitated crack formation. In addition, the hydrophilicity of the plasticizer significantly affected the water uptake behavior of the film coatings and, hence, changes in the coatings' toughness and drug permeability. Also the relative affinity of the plasticizer to the different polymers was found to be of significance. In contrast to TEC, DBS has a higher affinity to EC than to Eudragit L, resulting in potential redistributions of this plasticizer within the polymeric systems and changes in the release profiles during storage. Importantly, these effects could be avoided with appropriate curing conditions and preparation techniques for the coating dispersions.

How to adjust desired drug release patterns from ethylcellulose-coated dosage forms

The aim of this study was to provide an easy and efficient tool to adjust desired drug release kinetics from (aqueous) ethylcellulose-coated solid dosage forms and to better understand the underlying mass transport mechanisms. Pure ethylcellulose films are poorly permeable for many substances and can result in very low release rates for certain drugs from coated dosage forms, if the film coatings are completely formed and remain intact upon exposure to the release media. To increase the permeability of the polymeric membranes, different amounts of a water-soluble poly(vinyl alcohol)-poly(ethylene glycol) graft copolymer (PVA-PEG graft copolymer) were added to an aqueous ethylcellulose dispersion (Aquacoat ECD). Importantly, the presence of only a low percentage of this hydrophilic copolymer significantly increased the resulting water uptake rate and extent, dry weight loss and drug permeability of the films. In contrast to hydroxypropyl methylcellulose (HPMC), the PVA-PEG graft copolymer does not cause flocculation of the colloidal coating dispersion (leading to potentially variable release rates). Interestingly, the transport of water as well as of the model drug theophylline through the polymeric networks was primarily controlled by pure diffusion. The penetration kinetics could be quantitatively described by Fick's law of diffusion, irrespective of the type of release medium and PVA-PEG graft copolymer content. Most important from a practical point of view, a broad spectrum of pH-independent drug release rates can easily be obtained from drug-loaded pellets by simply varying the PVA-PEG graft copolymer content. An appropriate curing step after coating is required, but interestingly the investigated curing conditions (differing in time and relative humidity) resulted in very similar drug release patterns, indicating that stable film structures are likely to be achieved.

Drugs acting as plasticizers in polymeric systems: A quantitative treatment

The objective of the present study was to investigate and quantify the effects of ibuprofen, chlorpheniramine maleate and metoprolol tartrate on the thermal, mechanical and diffusional properties of polyacrylate-based films. Thin drug-containing films were prepared from organic Eudragit RS solutions and physicochemically characterized with respect to their glass transition temperature, mechanical properties and drug release kinetics in phosphate buffer pH 7.4. The apparent diffusion coefficient of the drug within the polymeric systems was determined by fitting an adequate solution of Fick's second law of diffusion to the experimentally determined release profiles. Importantly, the glass transition temperature of the films significantly decreased with increasing initial drug content, whereas the film flexibility and drug release rate increased. This clearly indicates that the three drugs act as efficient plasticizers for Eudragit RS. Interestingly, the mathematical analysis revealed that drug release was primarily controlled by diffusion. An increase in the initial drug content resulted in increased drug diffusivities and, thus, accelerated (absolute and relative) drug release rates. Importantly, quantitative relationships could be established between the drug diffusivity and the initial drug content. Based on this knowledge, the effects of the films' composition and thickness on the resulting drug release kinetics (also from coated solid dosage forms) can be predicted in a quantitative way.

Paclitaxel-loaded microparticles and implants for the treatment of brain cancer: preparation and physicochemical characterization

The aim of this study was to prepare different types of paclitaxel-loaded, PLGA-based microparticles and lipidic implants, which can directly be injected into the brain tissue. Releasing the drug in a time-controlled manner over several weeks, these systems are intended to optimize the treatment of brain tumors. The latter is particularly difficult because of the blood-brain barrier (BBB), hindering most drugs to reach the target tissue upon systemic administration. Especially paclitaxel (being effective for the treatment of ovarian, breast, lung and other cancers) is not able to cross the BBB to a notable extent since it is a substrate of the efflux transporter P-glycoprotein. Both, biodegradable microparticles as well as small, cylindrical, glycerol tripalmitate-based implants (which can be injected using standard needles) were prepared with different paclitaxel loadings. The effects of several formulation and processing parameters on the resulting drug release kinetics were investigated in phosphate buffer pH 7.4 as well as in a diethylnicotinamide (DENA)/phosphate buffer mixture. Using DSC, SEM, SEC and optical microscopy deeper insight into the underlying drug release mechanisms could be gained. The presence of DENA in the release medium significantly increased the solubility of paclitaxel, accelerated PLGA degradation, increased the mobility of the polymer and drug molecules and fundamentally altered the geometry of the systems, resulting in increased paclitaxel release rates.

Bimodal drug release achieved with multi-layer matrix tablets: transport mechanisms and device design

The aim of this study was to develop new multi-layer matrix tablets to achieve bimodal drug release profiles (fast release/slow release/fast release). Hydroxypropyl methylcellulose acetate succinate (HPMCAS, type MF) was chosen as a matrix former, because it is water-insoluble at low, and water-soluble at high pH values. Studies focused on the elucidation of the drug release mechanisms from HPMCAS-MF:drug tablets. In 0.1 N HCl the resulting release kinetics can be described using Fick's second law of diffusion, taking into account axial and radial mass transfer in cylindrical geometry. As the diffusion coefficients are found to be constant and the boundary conditions to be stationary, these systems are purely drug diffusion-controlled. In contrast, the dominating mass transport phenomena in phosphate buffer pH 7.4 are more complex. Due to polymer dissolution the resulting matrix structure is time-variant, leading to increasing drug diffusion coefficients and decreasing tablet dimensions, and thus moving boundary conditions. Drug release is affected by water imbibition, drug diffusion and polymer dissolution and is faster compared to 0.1 N HCl. With knowledge of these underlying release mechanisms, multi-layer matrix tablets were developed to achieve bimodal drug release. HPMCAS-MF:drug mixtures were used as tablet cores. As expected, changing the release medium from 0.1 N HCl to phosphate buffer pH 7. 4 after 2 h, lead to a significant increase in drug release. The abruptness of this rate change could be enhanced by adding two drug-free HPMCAS-MF barrier layers (one on each side) to the system. The addition of a fourth, drug-containing and fast disintegrating initial dose layer yielded the desired bimodal drug release patterns. The process and formulation parameters affecting the resulting release rates were investigated using theophylline and acetaminophen as model drugs.