Modelling of drug release from coated granular pellets

Effect of talc and vitamin E TPGS on manufacturability, stability and release properties of trilaurin-based formulations for hot-melt coating

This study was focused on one particular case of hot-melt coating with trilaurin - a solid medium-chain monoacid triglyceride. The challenge of using trilaurin as coating agent in melting-based processes is linked to its relatively low melting profile: 15.6 °C (Tm,α), 35.1 °C ( [Formula: see text] ) and 45.7 °C (Tm,β). From a process perspective, the only possibility to generate products coated with formulations composed of trilaurin is by setting thermal operational conditions above Tm,α. From a material perspective, this processing possibility depends principally on trilaurin crystallisation which was investigated via a set of analytical techniques including turbidimetry, calorimetry, hot-melt goniometry, and polarised light microscopy. A highly soluble drug model substrate (sodium chloride crystals) was coated with three selected trilaurin-based formulations: (i) trilaurin, (ii) trilaurin plus talc, and (iii) trilaurin plus vitamin E TPGS and talc. Coated salt crystals were then analysed to investigate processing performance, coating quality, stability and release properties under digestion effect. The results show that firstly, talc addition promotes nucleation and crystal growth and, as a consequence, it facilitates the manufacture of trilaurin-based formulations. Secondly, the formulation of a solid triglyceride and a hydrophilic surfactant could potentially cause release instability, but formula (iii) was found to be stabilised by a mechanism whereby trilaurin crystallization enhanced in the presence of talc immobilised vitamin E TPGS in its crystal lattice. Thirdly, talc addition did not significantly influence trilaurin digestion which endows products with an immediate release in lipolytic conditions instead of an extended liberation in pure water. Nor did the addition of one or two additives alter the extent of trilaurin digestion under the conditions studied. These important findings relate to product manufacturability, stability, and release properties. A good understanding of material properties (e.g. crystallisation, polymorphism, digestibility) is essential for melt-processing, lipid coating stabilising and modulation of release profile of solid lipid-coated product, as demonstrated in this case study with trilaurin.

Quantification of porogen effect on the drug release from single- and multi-layered ethylcellulose coated pellets containing single or combined drugs

The aim of this work was to develop a mathematical model to estimate the drug release from a conventional single-compartment reservoir pellet and extend its applicability to multi-compartment reservoir pellets. Conventional pellets were prepared by layering the drug onto starter-core then applying various ethylcellulose/HPC coatings for drug release control. Multi-layered pellets comprised a first drug layer of propranolol HCl (D₁) followed by a first controlled release coating (C₁) and consecutively a second drug layer of carbamazepine or caffeine (D₂) and then a second controlled-release coating (C₂). Drug release from single- and multi-compartment pellets generally increased with an increase of the water-soluble HPC in the coatings. The response described a sigmoidal curve, which agreed with a cumulative normal distribution function. The developed mathematical model facilitated quantification of the drug release of pellets as a function of the porogen content and the coating level. Additionally, the model was applied successfully in multi-compartment pellets to calculate theses effects on the release of drugs with a broad range of aqueous solubility.

Theoretical and Experimental Studies of the Controlled Release of Tetracycline Incorporated into Bioactive Glasses

Several authors have studied the release profile of drugs incorporated in different devices. However, to the best of our knowledge, although many studies have been done on the release of tetracycline, in these release devices, no study has investigated if the released compound is actually the tetracycline, or, instead, a degraded product. This approach is exploited here. In this work, we analyse the influence of two drying methods on the tetracycline delivery behaviour of synthesised glasses using the sol-gel process. We compare the drying methods results using both theoretical models and practical essays, and analyse the chemical characteristic of the released product in order to verify if it remains tetracycline. Samples were freeze-dried or dried in an oven at 37°C and characterised by several methods such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TG), differential thermogravimetric analysis (DTG), differential thermal analyses (DTA) and gas adsorption analysis (BET). The released concentration of tetracycline hydrochloride was studied as a function of time, and it was measured by ultraviolet spectrophotometry in the tetracycline wavelength. The drug delivery profiles were reasonably consistent with a diffusion model analysis. In addition, we observed higher release rates for the freeze-dried compared to those dried in an oven at 37°C. This higher release can be attributed to larger pore size for the freeze-dried sample systems with tetracycline, which promoted more water penetration, improving the drug diffusion. The analysis of the solution obtained in the release tests using high-performance liquid chromatography- mass spectrometry (HPLC-MS) confirmed that tetracycline was being released.

Nanocarrier Hydrodynamics and Binding in Targeted Drug Delivery: Challenges in Numerical Modeling and Experimental Validation

This review discusses current progress and future challenges in the numerical modeling of targeted drug delivery using functionalized nanocarriers (NC). Antibody coated nanocarriers of various size and shapes, also called functionalized nanocarriers, are designed to be injected in the vasculature, whereby they undergo translational and rotational motion governed by hydrodynamic interaction with blood particulates as well as adhesive interactions mediated by the surface antibody binding to target antigens/receptors on cell surfaces. We review current multiscale modeling approaches rooted in computational fluid dynamics and nonequilibrium statistical mechanics to accurately resolve fluid, thermal, as well as adhesive interactions governing nanocarrier motion and their binding to endothelial cells lining the vasculature. We also outline current challenges and unresolved issues surrounding the modeling methods. Experimental approaches in pharmacology and bioengineering are discussed briefly from the perspective of model validation.

Predictability of drug release from cochlear implants

A simplified mathematical theory is presented allowing for in silico simulation of the effects of key parameters of miniaturized implants (size and composition) on the resulting drug release kinetics. Such devices offer a great potential, especially for local drug treatments, e.g. of the inner ear. However, the preparation and characterization of these systems is highly challenging, due to the small system dimensions. The presented mathematical theory is based on Fick's second law of diffusion. Importantly, theoretical predictions do not require the knowledge of many system-specific parameters: Only the "apparent" diffusion coefficient of the drug within the implant matrix is needed. This parameter can be easily determined via drug release measurements from thin, macroscopic films. The validity of the theoretical model predictions was evaluated by comparison with experimental results obtained with a cochlear implant. The latter consisted of miniaturized electrodes, which were embedded in a silicone matrix loaded with various amounts of dexamethasone. Importantly, independent experimental results confirmed the theoretical predictions. Thus, the presented simplified theory can help to significantly speed up the optimization of this type of controlled drug delivery systems, especially if long release periods are targeted (e.g., several months or years). Straightforward experiments with thin, macroscopic films and computer simulations can allow for rapid identification of optimal system design.

In-line prediction of drug release profiles for pH-sensitive coated pellets

A new method for the prediction of the drug release profiles during a running pellet coating process from in-line near infrared (NIR) measurements has been developed. The NIR spectra were acquired during a manufacturing process through an immersion probe. These spectra reflect the coating thickness that is inherently connected with the drug release. Pellets sampled at nine process time points from thirteen designed laboratory-scale coating batches were subjected to the dissolution testing. In the case of the pH-sensitive Acryl-EZE coating the drug release kinetics for the acidic medium has a sigmoid form with a pronounced induction period that tends to grow along with the coating thickness. In this work the autocatalytic model adopted from the chemical kinetics has been successfully applied to describe the drug release. A generalized interpretation of the kinetic constants in terms of the process and product parameters has been suggested. A combination of the kinetic model with the multivariate Partial Least Squares (PLS) regression enabled prediction of the release profiles from the process NIR data. The method can be used to monitor the final pellet quality in the course of a coating process.

Sperm chemotaxis as revealed with live and synthetic eggs

Fertilization is one of the least understood fundamental biological processes. How sperm search for and find an egg remains enigmatic. Sperm attraction to egg-derived chemical cues may be significant evolutionarily for maintaining species barriers and important ecologically for increasing gamete encounters. New tools are needed, however, to resolve the functional consequences of these dissolved signal molecules. Freshly spawned eggs from red abalone (Haliotis rufescens) naturally release l-tryptophan, which stimulates chemotactic responses by conspecific sperm. Here, microspheres were manufactured to the approximate size and the same shape as female gametes and formulated to emit controlled doses of chemoattractant, imitating natural l-tryptophan release rates. When experimentally tested for effectiveness, male gametes did not distinguish between chemically impregnated mimics and live eggs, demonstrating that l-tryptophan alone is both necessary and sufficient to promote chemotaxis, and confirming the identity of a native sperm attractant. The techniques that we describe can be used to create synthetic eggs for most animal and plant species, including humans. Egg mimics increase the capacity for experimental manipulation and enable realistic studies of sperm behavior even in the absence of female gametes.

Development and in vitro evaluation of sustained release multiparticulate tablet of freely water soluble drug

Blends of aqueous dispersion of a hydrophobic and hydrophilic polymer, namely Surelease®: hydroxypropyl methylcellulose (Surelease®: HPMC E15) were used as coating materials to control the drug release from coated pellets of the highly water soluble drug metoprolol succinate. Varying the polymer blends, ranges of drug release patterns were obtained at pH 6.8. The present study dealt with diffusion of drug through plasticized Surelease®/ hydroxypropyl methylcellulose (HPMC E15) films prepared by coating of drug and polymers onto non-pareil seeds using the solution layering technique. The release of metoprolol succinate from coated pellets was decreased with increased coating load of polymer. The optimized formulation was obtained by 3² full factorial design. The release profile revealed that the optimized formulation follows zero order release kinetics. The stability data showed no interaction for storage at 25ºC and 60% relative humidity.

Ammonio Polymethacrylate-Coated Diltiazem: Drug Release from Single Pellets, Media Dependence, and Swelling Behavior

Drug release from single pellets was measured on an easily assembled flow-through system. Despite heterogeneity between pellets, the sum of the individual results resembled drug release from an ensemble. A typical pellet displayed a long lag followed by rapid release. Heterogeneity appeared to result from substrate properties rather than coating uniformity. Swelling behavior in acid and buffer was measured by dynamic image analysis and related to drug release. Drug release was sensitive to dissolution temperature but swelling was not. A description of the drug release process was proposed.

Assessment of Dynamic Image Analysis as a Surrogate Dissolution Test for a Coated Multiparticulate Product

Dynamic image analysis (DIA) was used to measure particle diameter (D50) of in-process samples removed during fluid bed coating. A single, rapid measurement gave D50 to within 4 mum. Samples removed at intervals of 2% weight gain were readily distinguishable by DIA and by their drug release profiles. Drug release was related to D50. DIA was assessed as a surrogate dissolution test with considerable potential. Current limitations of the approach were presented.

Performance of Multilayered Particles: Influence of a Thin Cushioning Layer

Nowadays, oral dosage forms with controlled release kinetics have known an increasing interest. The polymer coating of drug-loaded particles is one of the most common methods used for controlling drug delivery. Such multilayered particles could be either filled into capsules or compressed into tablets for their oral administration. However, many studies have noticed that coating films are damaged during the compression process, leading to significant changes in drug release profiles. The aims of this study were to investigate the effects of a thin cushioning layer [made of HydroxyPropylMethyl Cellulose (HPMC)] applied on coated theophylline particles upon particle characteristics, tablet properties, and then upon their dissolution performance. If no significant effect was shown with particles, this thin HPMC layer played an important role in the tablets. Tablet cohesiveness was decreased due to HPMC cushioning properties and moreover, the theophylline release rate was increased, as HPMC is a water-soluble polymer creating channels in polymer film for dissolution medium. Therefore, a cushioning layer helped to protect polymer coats from fracture during compression but could also affect drug release and so, both effects must be checked in such a drug delivery system.

Modeling vancomycin release kinetics from microporous calcium phosphate ceramics comparing static and dynamic immersion conditions

The release kinetics of vancomycin from calcium phosphate dihydrate (brushite) matrices and polymer/brushite composites were compared using different fluid replacement regimes, a regular replacement (static conditions) and a continuous flow technique (dynamic conditions). The use of a constantly refreshed flowing resulted in a faster drug release due to a constantly high diffusion gradient between drug loaded matrix and the eluting medium. Drug release was modeled using the Weibull, Peppas and Higuchi equations. The results showed that drug liberation was diffusion controlled for the ceramics matrices, whereas ceramics/polymer composites led to a mixed diffusion and degradation controlled release mechanism. The continuous flow technique was for these materials responsible for a faster release due to an accelerated polymer degradation rate compared with the regular fluid replacement technique.

Drug release properties of polymer coated ion-exchange resin complexes: experimental and theoretical evaluation

Although ion-exchange resins have been used widely as drug delivery systems, their exact release kinetics has not been reported yet. Usually only the rate-limiting step has been taken into account and the rest of the steps have been ignored as instantaneous processes. To investigate the exact release kinetics of polymer-coated drug/ion-exchange resin complexes for sustained drug delivery, the results of new mathematical modeling were compared with experimental results. Drug/resin complexes with a model drug, dextromethorphan, were prepared and used as cores for fluid-bed coating. An aqueous colloidal dispersion of poly(vinyl acetate) was applied for the coating. A comprehensive mathematical model was developed using a mechanistic approach by considering diffusion, swelling, and ion-exchange processes solved by numerical techniques. The rate-limiting factor of the uncoated resin particles was diffusion through the core matrix. Similarly, in the coated particles the rate-limiting factor was diffusion through the coating membrane. The mathematical model has captured the phenomena observed during experimental evaluations and the release dynamics from uncoated and coated (at different coat levels) particles were predicted accurately (maximum RMSE 2.4%). The mathematical model is a useful tool to theoretically evaluate the drug release properties from coated ion-exchange complexes thus can be used for design purposes.

Sustained Release Pellets Based on Poly(N-Isopropyl Acrylamide): Matrix and In Situ Photopolymerization-Coated Systems

The usefulness of poly(N-isopropyl acrylamide), PNIPA, for preparing sustained release matrix or photopolymerization-coated cellulosic pellets was evaluated. Theophylline pellets and granules were prepared using powdered cellulose (PC), poly(vinylpyrrolidone) (PVP), and PNIPA of Mw approximately 330 kDa, Mn approximately 93 kDa and low critical solubility temperature approximately 32 degrees C. The low consistency of wet mass, evaluated by torsion rheometry, due to hydrophilic character of PNIPA at room temperature, favored extrusion-spheronization. Theophylline (20%) pellets prepared with 15% PNIPA, 20% PVP and 45% PC, and granules obtained using 40% PNIPA and 40% PC showed an enhanced, although limited, ability to sustain the release. This effect was notably promoted after compression (which provides slowly eroding tablets) or coating of individualized pellets. A new coating technique consisting in forming the polymer film by photo-polymerization/cross-linking of NIPA monomers on pellets surface, using a photoinitiator and UV-irradiation at 366 nm, was developed. The composition of coating mixture and the time of irradiation were optimized using oscillatory rheometry. Coating did not significantly change the shape, size, or friability of the pellets but remarkably decreased the porosity and sustained drug release for several hours. In situ formation and cross-linking of PNIPA on the pellet appears as a feasible way for controlling drug release.

Modeling film-coat non-uniformity in polymer coated pellets: A stochastic approach

The objective of the present study is to include coating thickness non-uniformity in the development of a drug release model using coated ion-exchange pellets through the use of stochastic approaches. Drug release from ion-exchange resins was described using a Nernst-Plank model. Complexes of a model drug (dextromethorphan) and Dowex 50WX4-200 were prepared using a modified batch method and coated with Kollicoat SR 30D polymer. The deterministic model, validated using experimental drug release profiles for different coating thicknesses at 0%, 10%, 15%, 20% (w/w), was in agreement with the experimental data with a maximum root mean square error (RMSE) of 2.4%. An arbitrary Lagrangian-Eulerian approach was pursued to develop models of spherical pellets with non-uniform coating thicknesses. The Monte Carlo method was used to simulate the effect of the level of coating deformity on the cumulative drug release profile. Considering the co-existence of equal percentages of deformed and undeformed pellets in a batch, the cumulative release profile can vary by approximately +/-6% as a result of coating non-uniformity. The release profile obtained for a model of an arbitrary pellet with an actual non-uniform coating profile was in good agreement with the average release profile for the models of the theoretical randomly deformed pellets. The developed mathematical model is a useful tool to evaluate and predict release profiles of polymer coated ion-exchange resin complexes.

Electronic speckle pattern interferometry: A novel non-invasive tool for studying drug transport rate through free films

In this work, Electronic Speckle Pattern Interferometry (ESPI) is presented as a non-invasive tool to study drug transport in controlled release systems. ESPI is shown to be a feasible tool to measure drug film permeability via comparison with an ordinary diaphragm cell. A specially designed cuvette was used in the release study: the polymeric film separated the donor and the receiving chambers of the cuvette to create a diffusion cell with no mixing in the two chambers. Thus, the cuvette mimicked a coated system immersed in a stagnant bulk liquid. Concentration profile data were obtained for the two compartments. Using these data, it was possible to visually discriminate between a film subject only to diffusion and a film subject to diffusion as well as osmotic effects. Moreover, using the concentration profile data collected at different time intervals, it was possible to follow the film properties in terms of drug permeability, thus studying how drug permeability depended on drug concentration. Compared to other measuring techniques, ESPI offers the advantages that no invasive measurements are needed, and that no sampling and calibration are required. Furthermore, the permeability can be measured with no influence of mass transfer in the boundary layers.

A multi-scale stochastic drug release model for polymer-coated targeted drug delivery systems

A multi-scale mathematical model for drug release of oral targeted drug delivery systems was developed and applied to a commercially available delayed release tablet (Asacol) that delivers 5-aminosalicyclic acid (5-ASA) to the colon. Underlying physical and biochemical principles governing the involved processes (diffusion and dissolution) were employed to develop the mathematical description. Finite element formulation was used to numerically solve the model equations. Molecular dynamics (MD) simulations were used to predict macro-scale transport properties of the drug and the biologic fluid. The effect of pH variability in the gastrointestinal tract environment on the dissolution of the polymeric enteric coating was investigated using the Monte Carlo method. The direct coupling method employed (MD) predicted a sufficiently accurate diffusion coefficient (5.7x10(-6) cm2 s-1) of the drug molecules in reasonable (3 h) computation times. The model was validated using experimental data from in vitro dissolution experiments and provided accurate prediction of the drug release from the delivery system (root mean square error of 5%). The amount of drug entering the systemic circulation, computed from the predicted drug release in varying pH environments in the small bowel, was 15-24%. This range was in good agreement with clinical in vivo data (13-36%) obtained from literature. This research shows that in silico experiments using mechanistic models and stochastic approaches can be used for drug design and optimization and as a decision making tool for physicians.