A Refined Thin-Film Model for Drug Dissolution Considering Radial Diffusion - Simulating Powder Dissolution.

PURPOSE: We aim to present a refined thin-film model describing the drug particle dissolution considering radial diffusion in spherical boundary layer, and to demonstrate the ability of the model to describe the dissolution behavior of bulk drug powders. METHODS: The dissolution model introduced in this study was refined from a radial diffusion-based model previously published by our laboratory (So et al. in Pharm Res. 39:907-17, 2022). The refined model was created to simulate the dissolution of bulk powders, and to account for the evolution of particle size and diffusion layer thickness during dissolution. In vitro dissolution testing, using fractionated hydrochlorothiazide powders, was employed to assess the performance of the model. RESULTS: Overall, there was a good agreement between the experimental dissolution data and the predicted dissolution profiles using the proposed model across all size fractions of hydrochlorothiazide. The model over-predicted the dissolution rate when the particles became smaller. Notably, the classic Nernst-Brunner formalism led to an under-estimation of the dissolution rate. Additionally, calculation based on the equivalent particle size derived from the specific surface area substantially over-predicted the dissolution rate. CONCLUSION: The study demonstrated the potential of the radial diffusion-based model to describe dissolution of drug powders. In contrast, the classic Nernst-Brunner equation could under-estimate drug dissolution rate, largely due to the underlying assumption of translational diffusion. Moreover, the study indicated that not all surfaces on a drug particle contribute to dissolution. Therefore, relying on the experimentally-determined specific surface area for predicting drug dissolution is not advisable.

Modeling Drug Dissolution in 3-Dimensional Space.

PURPOSE: The purpose of the study is to present a mathematical model capable of describing drug particle dissolution in 3-dimensional (3D) space, and to provide experimental model verification. Through this study, we also aim to elaborate limitations of the classic, 1D-based Nernst-Brunner formalism in dissolution modeling. METHODS: The 3D dissolution model was derived by treating the dissolution of a spherical particle as a diffusion-driven process, and by solving Fick's 2nd law of diffusion in spherical coordinates using numerical methods. The resulting model was experimentally verified through analyzing the dissolution behavior of single succinic acid particles in un-stirred water droplet under polarized light microscopy, in combination with image segmentation techniques. RESULTS: A set of working equations was developed to describe drug particle dissolution in 3D space. The predicted dissolution time and profile are in good agreement with the experimental results. The model clearly shows that the concentration gradient within the diffusion layer, in realistic 3D condition, must not be a constant value as implicated in the Nernst-Brunner formalism. The actual concentration profile is a hyperbola, and the concentration gradient at the surface of the particle can be significantly higher than the classic 1D-based dissolution model. CONCLUSION: The study demonstrates that the classic, 1D-based dissolution models may lead to significant under-estimation of drug dissolution rates. In contrast, modeling dissolution in 3D space yields more reliable results. This study merits further development of comprehensive 3D drug dissolution models, by considering polydispersed particle ensemble and imposing the changes of diffusion layer thickness during dissolution.

Cefdinir nanosuspension for improved oral bioavailability by media milling technique: formulation, characterization and in vitro-in vivo evaluations.

Cefdinir (Cef) is an orally active Biopharmaceutics Classification System (BCS) class IV drug with incomplete absorption and low bioavailability (16-21%). The aim of this investigation was to develop nanosuspensions (NS) of Cef to improve its oral bioavailability. Cef NS were prepared by the media milling technique using zirconium oxide beads as the milling media. Cef NS were characterized by particle size, Scanning Electron Microscopy, Differential Scanning Calorimetry, X-Ray Diffraction pattern and evaluated for saturation solubility, in vitro release studies, ex vivo permeability studies and in vivo bioavailability studies. The particle size and zeta potential were found to be 224.2 ± 2.7 nm and -15.7 ± 1.9 mV, respectively. Saturation solubility of NS was found to be 1985.3 ± 10.2 µg/ml which was 5.64 times higher than pure drug (352.2 ± 6.5 µg/ml). The DSC thermograms and XRD patterns indicated that there was no interaction between drug and excipients and that the crystallinity of Cef remained unchanged after media milling process. Results of in vitro release studies and ex vivo permeation studies showed improved drug release of 88.2 1 ± 2.90 and 83.11 ± 2.14%, respectively, from NS after 24 h as compared to drug release of 54.09 ± 2.54 and 48.2 1 ± 1.27%, respectively, from the marketed suspension (Adcef). In vivo studies in rats demonstrated a 3-fold increase in oral bioavailability from the NS in comparison to marketed suspension. The results of this investigation conclusively show that the developed nanosuspension of Cef exhibited improved solubility, dissolution and permeation which led to a significant enhancement in its oral bioavailability.

Impact of Insoluble Separation Layer Mechanical Properties on Disintegration and Dissolution Kinetics of Multilayer Tablets.

Dissolution and disintegration of solid dosage forms such as multiple-layer tablet with different active ingredients depend on formulation and properties used in the formulations, and it may sometimes result in counterintuitive release kinetics. In this manuscript, we investigate the behavior of combined acetylsalicylic acid and mefenamic acid bi- and triple-layer formulations. We show that the simulation model with a cellular automata predicted the impact of the inert layer between the different active ingredients on each drug release and provide a good agreement with the experimental results. Also, it is shown that the analysis based on the Noyes-Whitney equation in combination with a cellular automata-supported dissolution and disintegration numerical solutions explain the nature of the unexpected effects. We conclude that the proposed simulation approach is valuable to predict the influence of material attributes and process parameters on drug release from multicomponent and multiple-layer pharmaceutical tablets and to help us develop the drug product formulation.

Sink conditions do not guarantee the absence of saturation effects.

Limited drug solubility effects can play a major role for the control of drug release from a variety of drug delivery systems, e.g. tablets, pellets, implants and microparticles. Importantly, such saturation effects can occur inside and/or outside the dosage form. This is true for drug release occurring in vitro and in vivo. In vivo, released drug might be rapidly transported away from the site of administration, e.g. due to absorption into the blood stream. In vitro, many frequently used experimental set-ups are "closed systems" and eventually drug saturation effects in the surrounding release medium might artificially occur, "falsifying" the resulting release kinetics. To avoid such errors, often "sink conditions" are provided: Selecting appropriate release medium volumes, renewal rates and/or "open systems", it is assured that the maximum concentration in the release medium does not exceed about 20% of the drug solubility. However, this does not mean that drug saturation effects within the dosage form are also avoided. It should clearly be distinguished between potential limited drug solubility effects inside versus outside the drug delivery system. This articles aims at: (i) giving a brief overview on the underlying physico-chemical phenomena involved in drug dissolution and drug release, (ii) clarifying some key terms, and (iii) presenting several examples of dosage forms in which drug saturation effects within the system are of importance, even when providing sink conditions in the surrounding bulk fluid. Interestingly, this can also include highly hydrated delivery systems containing freely water-soluble drugs.

Carrier characteristics influence the kinetics of passive drug loading into lipid nanoemulsions.

Passive loading as a novel screening approach is a material-saving tool for the efficient selection of a suitable colloidal lipid carrier system for poorly water soluble drug candidates. This method comprises incubation of preformed carrier systems with drug powder and subsequent determination of the resulting drug load of the carrier particles after removal of excess drug. For reliable routine use and to obtain meaningful loading results, information on the kinetics of the process is required. Passive loading proceeds via a dissolution-diffusion-based mechanism, where drug surface area and drug water solubility are key parameters for fast passive loading. While the influence of the drug characteristics is mostly understood, the influence of the carrier characteristics remains unknown. The aim of this study was to examine how the lipid nanocarriers' characteristics, i.e. the type of lipid, the lipid content and the particle size, influence the kinetics of passive loading. Fenofibrate was used as model drug and the loading progress was analyzed by UV spectroscopy. The saturation solubility in the nanocarrier particles, i.e. the lipid type, did not influence the passive loading rate constant. Low lipid content in the nanocarrier and a small nanocarrier particle size both increased passive loading speed. Both variations increase the diffusivity of the nanocarrier particles, which is the primary cause for fast loading at these conditions: The quicker the carrier particles diffuse, the higher is the speed of passive loading. The influence of the diffusivity of the lipid nanocarriers and the effect of drug dissolution rate were included in an overall mechanistic model developed for similar processes (A. Balakrishnan, B.D. Rege, G.L. Amidon, J.E. Polli, Surfactant-mediated dissolution: contributions of solubility enhancement and relatively low micelle diffusivity, J. Pharm. Sci. 93 (2004) 2064-2075). The resulting mechanistic model gave a good estimate of the speed of passive loading in nanoemulsions. Whilst the drug's characteristics - apart from drug surface area - are basically fixed, the lipid nanocarriers can be customized to improve passive loading speed, e.g. by using small nanocarrier particles. The knowledge of the loading mechanism now allows the use of passive loading for the straightforward, material-saving selection of suitable lipid drug nanocarriers.

Parameters influencing the course of passive drug loading into lipid nanoemulsions.

Passive drug loading can be used to effectively identify suitable colloidal lipid carrier systems for poorly water-soluble drugs. This method comprises incubation of preformed carrier systems with drug powder and subsequent determination of the resulting drug load of the carrier particles. Until now, the passive loading mechanism is unknown, which complicates reliable routine use. In this work, the influence of drug characteristics on the course of passive loading was investigated systematically varying drug surface area and drug solubility. Fenofibrate and flufenamic acid were used as model drugs; the carrier system was a trimyristin nanodispersion. Loading progress was analyzed by UV spectroscopy or by a novel method based on differential scanning calorimetry. While increasing drug solubility by micelle incorporation did not speed up passive loading, a large drug surface area and high water solubility were key parameters for fast loading. Since both factors are crucial in drug dissolution as described by the Noyes-Whitney equation, these findings point to a dissolution-diffusion-based passive loading mechanism. Accordingly, passive loading also occurred when drug and carrier particles were separated by a dialysis membrane. Knowledge of the loading mechanism allows optimizing the conditions for future passive loading studies and assessing the limitations of the method.

Modeling of Disintegration and Dissolution Behavior of Mefenamic Acid Formulation Using Numeric Solution of Noyes-Whitney Equation with Cellular Automata on Microtomographic and Algorithmically Generated Surfaces.

Manufacturing parameters may have a strong impact on the dissolution and disintegration of solid dosage forms. In line with process analytical technology (PAT) and quality by design approaches, computer-based technologies can be used to design, control, and improve the quality of pharmaceutical compacts and their performance. In view of shortcomings of computationally intensive finite-element or discrete-element methods, we propose a modeling and simulation approach based on numerical solutions of the Noyes-Whitney equation in combination with a cellular automata-supported disintegration model. The results from in vitro release studies of mefenamic acid formulations were compared to calculated release patterns. In silico simulations with our disintegration model showed a high similarity of release profile as compared to the experimental evaluation. Furthermore, algorithmically created virtual tablet structures were in good agreement with microtomography experiments. We conclude that the proposed computational model is a valuable tool to predict the influence of material attributes and process parameters on drug release from tablets.

Mathematical modeling of drug dissolution.

The dissolution of a drug administered in the solid state is a pre-requisite for efficient subsequent transport within the human body. This is because only dissolved drug molecules/ions/atoms are able to diffuse, e.g. through living tissue. Thus, generally major barriers, including the mucosa of the gastro intestinal tract, can only be crossed after dissolution. Consequently, the process of dissolution is of fundamental importance for the bioavailability and, hence, therapeutic efficacy of various pharmaco-treatments. Poor aqueous solubility and/or very low dissolution rates potentially lead to insufficient availability at the site of action and, hence, failure of the treatment in vivo, despite a potentially ideal chemical structure of the drug to interact with its target site. Different physical phenomena are involved in the process of drug dissolution in an aqueous body fluid, namely the wetting of the particle's surface, breakdown of solid state bonds, solvation, diffusion through the liquid unstirred boundary layer surrounding the particle as well as convection in the surrounding bulk fluid. Appropriate mathematical equations can be used to quantify these mass transport steps, and more or less complex theories can be developed to describe the resulting drug dissolution kinetics. This article gives an overview on the current state of the art of modeling drug dissolution and points out the assumptions the different theories are based on. Various practical examples are given in order to illustrate the benefits of such models. This review is not restricted to mathematical theories considering drugs exhibiting poor aqueous solubility and/or low dissolution rates, but also addresses models quantifying drug release from controlled release dosage forms, in which the process of drug dissolution plays a major role.