A microscopic multiphase diffusion model of viable epidermis permeability

Compartmental modeling of skin absorption and desorption kinetics: Donor solvent evaporation, variable diffusion/partition coefficients, and slow equilibration process within stratum corneum

This work expands the recently developed compartmental model for skin transport to model variable diffusion and/or partition coefficients, and the presence of slow equilibration/slow binding kinetics within stratum corneum. The model was validated by comparing it with the diffusion model which was solved numerically using the finite element method. It was found that the new compartmental model predictions agreed well with that of the diffusion model, providing a sufficient number of compartments was used. The compartmental model was applied to two previously published experimental data sets: water penetration and desorption data and the finite dose dermal penetration of testosterone. Significant improvement of the fitting quality for all these data sets was achieved using the compartmental model.

Topical drug delivery: History, percutaneous absorption, and product development

Topical products, widely used to manage skin conditions, have evolved from simple potions to sophisticated delivery systems. Their development has been facilitated by advances in percutaneous absorption and product design based on an increasingly mechanistic understanding of drug-product-skin interactions, associated experiments, and a quality-by-design framework. Topical drug delivery involves drug transport from a product on the skin to a local target site and then clearance by diffusion, metabolism, and the dermal circulation to the rest of the body and deeper tissues. Insights have been provided by Quantitative Structure Permeability Relationships (QSPR), molecular dynamics simulations, and dermal Physiologically Based PharmacoKinetics (PBPK). Currently, generic product equivalents of reference-listed products dominate the topical delivery market. There is an increasing regulatory interest in understanding topical product delivery behavior under 'in use' conditions and predicting in vivo response for population variations in skin barrier function and response using in silico and in vitro findings.

Computer Simulation of Skin Permeability of Hydrophobic and Hydrophilic Chemicals - Influence of Follicular Pathway

A recently published mechanistic skin permeability model (Kasting et al., 2019. J Pharm Sci 108:337-349) that included a follicular diffusion pathway has been extended to describe transient diffusion and finite dose applications. The model follows the disposition of two components, solute and solvent, so that solvent deposition processes can be explicitly represented. Experimentally-calibrated permeability characteristics of the follicular pathway leading to the permeation of highly hydrophilic permeants are further refined. Details of the refinements and a comparison with the earlier model using two large experimental datasets are presented. An example calculation shows the marked difference between the time scales for achievement of near steady-state diffusion for large hydrophilic and lipophilic compounds, with the former being more than 100-fold faster than the latter. However, the true steady state for the hydrophilic compound is not reached until much later due to the very slow filling of the corneocyte phase.

The strategies for the modelling of the passive mass transport through porous membranes: Applicability to transdermal delivery systems

The paper concerns the modelling of the passive solute transport through porous membranes. A general scheme for the mass transport has been developed upon the mixed diffusion-advection-reaction model. The passive advection has been introduced as a certain simplification of the Navier-Stokes problem, involving a pressure gradient-induced creeping flow of an incompressible Newtonian fluid. Nine scenarios for the drug transport process have been tested versus two experimental datasets acquired earlier (photoacoustic depth-profiling and contact angle surface wettability techniques) for the characterization of bulk and interfacial processes in a model pharmaceutical system: the synthetic dodecanol-collodion porous membrane in contact with a photodegradable pigment dithranol. The scenarios considered include three mass transport models (the diffusion-advection-reaction, diffusion-advection and diffusion-reaction models) under three distinct types of the lower (the donor/acceptor interface) boundary conditions: the Dirichlet-type instantaneous source, the Dirichlet-type interface relaxation, and the Neumann-type concentration gradient. The results obtained indicate a considerable agreement between the experimental data and predictions of the diffusion-reaction and the general models for long times, however, some deviations were exhibited at the initial stages of the permeation process. It is considered, that the discrepancies originate from a specific penetrant behaviour at the interfaces, which violates boundary transfer schemes classically employed for the mass transport phenomena quantification. Moreover, an additional mixing process taking place close to the interface related to the liquid flow driven by the surface tension gradients (so-called classic and thermal Marangoni effect) could play a still underestimated role in the trans-interfacial mass transport.

Modeling drug transport within the viable skin - a review

INTRODUCTION

In the past, mathematical modeling of the transport of transdermal drugs has been primarily focused on the stratum corneum. However, the development of pharmaceutical technologies, such as chemical enhancers, iontophoresis, and microneedles, has led to two outcomes; an increase in permeability in the stratum corneum or the ability to negate the layer entirely. As a result, these outcomes have made the transport of a solute in the viable skin far more critical when studying transdermal drug delivery.

AREAS COVERED

The review will explicitly show the various attempts to model drug transport within the viable skin. Furthermore, a brief review will be conducted on the different models that explain stratum corneum transport, microneedle dynamics and estimation of the diffusion coefficient.

EXPERT OPINION

Future development of mathematical models requires the focus to be changed from traditional diffusion-based tissue models to more sophisticated three-dimensional models that incorporate the physiology of the skin.

Finite Element Analysis for Predicting Skin Pharmacokinetics of Nano Transdermal Drug Delivery System Based on the Multilayer Geometry Model

Background

Skin pharmacokinetics is an indispensable indication for studying the drug fate after administration of transdermal drug delivery systems (TDDS). However, the heterogeneity and complex skin structured with stratum corneum, viable epidermis, dermis, and subcutaneous tissue inevitably leads the drug diffusion coefficient (Kp) to vary depending on the skin depth, which seriously limits the development of TDDS pharmacokinetics in full thickness skin.

Methods

A multilayer geometry skin model was established and the Kp of drug in SC, viable epidermis, and dermis was obtained using the technologies of molecular dynamics simulation, in vitro permeation experiments, and in vivo microdialysis, respectively. Besides, finite element analysis (FEA) based on drug Kps in different skin layers was applied to simulate the paeonol nanoemulsion (PAE-NEs) percutaneous dynamic penetration process in two and three dimensions. In addition, PAE-NEs skin pharmacokinetics profile obtained by the simulation was verified by in vivo experiment.

Results

Coarse-grained modeling of molecular dynamic simulation was successfully established and the Kp of PAE in SC was 2.00×10⁻⁶ cm²/h. The Kp of PAE-NE in viable epidermis and in dermis detected using penetration test and microdialysis probe technology, was 1.58×10⁻⁵ cm²/h and 3.20×10⁻⁵ cm²/h, respectively. In addition, the results of verification indicated that PAE-NEs skin pharmacokinetics profile obtained by the simulation was consistent with that by in vivo experiment.

Discussion

This study demonstrated that the FEA combined with the established multilayer geometry skin model could accurately predict the skin pharmacokinetics of TDDS.

Establishment of an in vitro model of cultured viable human, porcine and canine skin and comparison of different media supplements

Transdermal drug delivery provides several advantages over conventional drug administration, such as the avoidance of first-pass metabolism and better patient compliance. In vitro research can abbreviate and facilitate the pharmaceutical development considerably compared to in vivo research as drug screening and clinical studies can be reduced. These advantages led to the development of corresponding skin models. Viable skin models are more useful than non-viable ones, due to the influence of skin metabolism on the results. While most in vitro studies concentrate on evaluating human-based models, the current study is designed for the investigation of both human and animal diseases. So far, there is little information available in the literature about viable animal skin cultures which are in fact intended for application in the veterinary and not the human field. Hence, the current study aims to fill the gap. For the in vitro viable skin model, specimens of human, porcine and canine skin were cultured over two weeks under serum-free conditions. To evaluate the influence of medium supplementation on skin viability, two different supplement mixtures were compared with basic medium. The skin specimens were maintained at a viability-level >50% until the end of the study. From the tested supplements, the addition of bovine pituitary extract and epidermal growth factor increased skin viability whereas hydrocortisone and insulin induced a decrease. This in vitro viable skin model may be a useful tool for the investigation of skin diseases, especially for the veterinary field.

Individually coated microneedles for co-delivery of multiple compounds with different properties

Microneedle (MN) patches provide a simple method for delivery of drugs that might otherwise require hypodermic injection. Conventional MN patch fabrication methods typically can load only one or possibly multiple miscible agents with the same formulation on all MNs, which limits the combination and spatial distribution of drugs and formulations having different properties (such as solubility) in a single patch. In this study, we coated MNs individually instead of coating all MNs from the same formulation, making possible a patch where each individual MN is coated with different formulations and drugs. In this way, individually coated MN patches co-delivered multiple agents with different physicochemical characteristics (immiscible molecules, proteins, and nanoparticles) and in different spatial patterns in the skin. MN loading was adjusted by modifying the number of coating layers, and co-delivery of multiple agents was demonstrated in the porcine skin. We conclude that individually coating MNs enables co-delivery of multiple different compounds and formulations with needle-by-needle spatial control in the skin.

Iontophoretic transdermal drug delivery: a multi-layered approach

We present a multi-layer mathematical model to describe the transdermal drug release from an iontophoretic system. The Nernst-Planck equation describes the basic convection-diffusion process, with the electric potential obtained by solving the Laplace's equation. These equations are complemented with suitable interface and boundary conditions in a multi-domain. The stability of the mathematical problem is discussed in different scenarios and a finite-difference method is used to solve the coupled system. Numerical experiments are included to illustrate the drug dynamics under different conditions.

Surging footprints of mathematical modeling for prediction of transdermal permeability

In vivo skin permeation studies are considered gold standard but are difficult to perform and evaluate due to ethical issues and complexity of process involved. In recent past, a useful tool has been developed by combining the computational modeling and experimental data for expounding biological complexity. Modeling of percutaneous permeation studies provides an ethical and viable alternative to laboratory experimentation. Scientists are exploring complex models in magnificent details with advancement in computational power and technology. Mathematical models of skin permeability are highly relevant with respect to transdermal drug delivery, assessment of dermal exposure to industrial and environmental hazards as well as in developing fundamental understanding of biotransport processes. Present review focuses on various mathematical models developed till now for the transdermal drug delivery along with their applications.

Effect of stratum corneum heterogeneity, anisotropy, asymmetry and follicular pathway on transdermal penetration

The impact of the complex structure of the stratum corneum on transdermal penetration is not yet fully described by existing models. A quantitative and thorough study of skin permeation is essential for chemical exposure assessment and transdermal delivery of drugs. The objective of this study is to analyze the effects of heterogeneity, anisotropy, asymmetry, follicular diffusion, and location of the main barrier of diffusion on percutaneous permeation. In the current study, the solution of the transient diffusion through a two-dimensional-anisotropic brick-and-mortar geometry of the stratum corneum is obtained using the commercial finite element program COMSOL Multiphysics. First, analytical solutions of an equivalent multilayer geometry are used to determine whether the lipids or corneocytes constitute the main permeation barrier. Also these analytical solutions are applied for validations of the finite element solutions. Three illustrative compounds are analyzed in these sections: diethyl phthalate, caffeine and nicotine. Then, asymmetry with depth and follicular diffusion are studied using caffeine as an illustrative compound. The following findings are drawn from this study: the main permeation barrier is located in the lipid layers; the flux and lag time of diffusion through a brick-and-mortar geometry are almost identical to the values corresponding to a multilayer geometry; the flux and lag time are affected when the lipid transbilayer diffusivity or the partition coefficients vary with depth, but are not affected by depth-dependent corneocyte diffusivity; and the follicular contribution has significance for low transbilayer lipid diffusivity, especially when flux between the follicle and the surrounding stratum corneum is involved. This study demonstrates that the diffusion is primarily transcellular and the main barrier is located in the lipid layers.

Predicting dermal absorption of gas-phase chemicals: transient model development, evaluation, and application

A transient model is developed to predict dermal absorption of gas-phase chemicals via direct air-to-skin-to-blood transport under non-steady-state conditions. It differs from published models in that it considers convective mass-transfer resistance in the boundary layer of air adjacent to the skin. Results calculated with this transient model are in good agreement with the limited experimental results that are available for comparison. The sensitivity of the modeled estimates to key parameters is examined. The model is then used to estimate air-to-skin-to-blood absorption of six phthalate esters for scenarios in which (A) a previously unexposed occupant encounters gas-phase phthalates in three different environments over a single 24-h period; (B) the same as 'A', but the pattern is repeated for seven consecutive days. In the 24-h scenario, the transient model predicts more phthalate absorbed into skin and less absorbed into blood than would a steady-state model. In the 7-day scenario, results calculated by the transient and steady-state models converge over a time period that varies between 3 and 4 days for all but the largest phthalate (DEHP). Dermal intake is comparable to or larger than inhalation intake for DEP, DiBP, DnBP, and BBzP in Scenario 'A' and for all six phthalates in Scenario 'B'.

PRACTICAL IMPLICATIONS

Dermal absorption from air has often been overlooked in exposure assessments. However, our transient model suggests that dermal intake of certain gas-phase phthalate esters is comparable to, or larger than, inhalation intake under commonly occurring indoor conditions. This may also be the case for other organic chemicals that have physicochemical properties that favor dermal absorption directly from air. Consequently, this pathway should be included in aggregate exposure and risk assessments. Furthermore, under conditions where the exposure concentrations are changing or there is insufficient time to achieve steady-state, the transient model presented in this study is more appropriate for estimating dermal absorption than is a steady-state model.