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

Impact of solvent dry down, phase change, vehicle pH and slowly reversible keratin binding on skin penetration of cosmetic relevant compounds: II. Solids

We extended a mechanistic, physics-based framework of the dry down process, previously developed for liquids and electrolytes, to solids and coded it into the latest UB/UC/P&G skin permeation model, herein renamed DigiSkin. The framework accounts for the phase change of the permeant from dissolved in a solvent (liquid) to precipitated on the skin surface (solid). The evaporation rate for the solid is reduced due to lower vapor pressure for the solid state versus subcooled liquid. These vapor pressures may differ by two orders of magnitude. The solid may gradually redissolve and penetrate the skin. The framework was tested by simulating the in vitro human skin permeation of the 38 cosmetically relevant solid compounds reported by Hewitt et al., J. Appl. Toxicol. 2019, 1-13. The more detailed handling of the evaporation process greatly improved DigiSkin evaporation predictions (r2 = 0.89). Further, we developed a model reliability prediction score classification using diverse protein reactivity data and identified that 15 of 38 compounds are out of model scope. Dermal delivery predictions for the remaining chemicals have excellent agreement with experimental data. The analysis highlighted the sensitivity of water solubility and equilibrium vapor pressure values on the DigiSkin predictions outcomes influencing agreement with the experimental observations.

Modeling the percutaneous absorption of solvent-deposited solids over a wide dose range: II. Weak electrolytes

Dermal absorption of weak electrolytes applied to skin from pharmaceutical and cosmetic compositions is an important consideration for both their efficacy and skin safety. We developed a mechanistic, physics-based framework that simulates this process for leave on applications following solvent deposition. We incorporated this framework into our finite dose computational skin permeation model previously tested with nonelectrolytes to generate quantitative predictions for weak electrolytes. To test the model, we analyzed experimental data from an in vitro human skin permeation study of a weak acid (benzoic acid) and a weak base (propranolol) and their sodium and hydrochloride salts from simple, ethanol/water vehicles as a function of dose and ionization state. Key factors controlling absorption are the pH and buffer capacity of the dose solution, the dissolution rate of precipitated solids into a lipid boundary layer and the rate of conversion of the deposited solid to its conjugate form as the nonionized component permeates and (sometimes) evaporates from the skin surface. The resulting framework not only describes the current test data but has the potential to predict the absorption of other weak electrolytes following topical application.

Enhancement of Skin Permeability Prediction through PBPK Modeling, Bayesian Inference, and Experiment Design

Physiologically based pharmacokinetic (PBPK) models of skin absorption are a powerful resource for estimating drug delivery and chemical risk of dermatological products. This paper presents a PBPK workflow for the quantification of the mechanistic determinants of skin permeability and the use of these quantities in the prediction of skin absorption in novel contexts. A state-of-the-art mechanistic model of dermal absorption was programmed into an open-source modeling framework. A sensitivity analysis was performed to identify the uncertain compound-specific, individual-specific, and site-specific model parameters that impact permeability. A Bayesian Markov Chain Monte Carlo algorithm was employed to derive distributions of these parameters given in vitro experimental permeability measurements. Extrapolations to novel contexts were generated by simulating the model following its update with samples drawn from the learned distributions as well as parameters that represent the intended scenario. This algorithm was applied multiple times, each using a unique set of permeability measurements sourced under experimental contexts that differ in terms of the compound, vehicle pH, skin sample anatomical site, and the number of compounds under which each subject's skin samples were tested. Among the data sets used in this study, the highest accuracy and precision in the extrapolated permeability was achieved in those that include measurements conducted under multiple vehicle pH levels and in which individual subjects' skin samples are tested under multiple compounds. This work thus identifies factors for consideration in the design of experiments for the purpose of training dermal models to robustly estimate drug delivery and chemical risk.

Impact of solvent dry down, vehicle pH and slowly reversible keratin binding on skin penetration of cosmetic relevant compounds: I. Liquids

To measure progress and evaluate performance of the newest UB/UC/P&G skin penetration model we simulated an 18-compound subset of finite dose in vitro human skin permeation data taken from a solvent-deposition study of cosmetic-relevant compounds (Hewitt et al., J. Appl. Toxicol. 2019, 1-13). The recent model extension involved slowly reversible binding of solutes to stratum corneum keratins. The selected subset was compounds that are liquid at skin temperature. This set was chosen to distinguish between slow binding and slow dissolution effects that impact solid phase compounds. To adequately simulate the physical experiments there was a need to adjust the evaporation mass transfer coefficient to better represent the diffusion cell system employed in the study. After this adjustment the model successfully predicted both dermal delivery and skin surface distribution of 12 of the 18 compounds. Exceptions involved compounds that were cysteine-reactive, highly water-soluble or highly ionized in the dose solution. Slow binding to keratin, as presently parameterized, was shown to significantly modify the stratum corneum kinetics and diffusion lag times, but not the ultimate disposition, of the more lipophilic compounds in the dataset. Recommendations for further improvement of both modeling methods and experimental design are offered.

Exploring structure/property relationships to health and environmental hazards of polymeric polyisocyanate prepolymer substances-2. Dermal sensitization potential in the mouse local lymph node assay

The sensitization potencies of twenty custom-designed monomer-depleted polymeric polyisocyanate prepolymer substances and their associated toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI) monomer precursors were investigated by means of the mouse Local Lymph Node Assay (LLNA). These polymeric prepolymers were designed to represent the structural features and physical-chemical properties exhibited by a broad range of commercial polymeric polyisocyanate prepolymers that are produced from the reaction of aromatic and aliphatic diisocyanate monomers with aliphatic polyether and polyester polyols. The normalization of LLNA responses to the applied (15-45-135 mM) concentrations showed that the skin sensitization potency of polymeric polyisocyanate prepolymers is at least 300 times less than that of the diisocyanate monomers from which they are derived. The sensitization potency of the prepolymers was shown to be mainly governed by their hydrophobicity (as expressed by the calculated octanol-water partition coefficient, log K_ow) and surfactant properties. Neither hydrophilic (log K_ow <0) nor very hydrophobic (log K_ow >25) prepolymers stimulated lymphocyte proliferation beyond that of the dosing vehicle control. The findings of this investigation challenge the generally held assumption that all isocyanate (-N=C=O) bearing substances are potential skin (and respiratory) sensitizers. Further, these findings can guide the future development of isocyanate chemistries and associated polyurethane applications toward reduced exposure and health hazard potentials.

A Mechanistic Bayesian Inferential Workflow for Estimation of In Vivo Skin Permeation from In Vitro Measurements

Computational models can play an integral role in the chemical risk assessment of dermatological products. However, a limitation on the ability of mathematical models to extrapolate from in vitro measurements to in human predictions arises from context-dependence: modeling assumptions made in one setting may not carry over to another scenario. Mechanistic models of dermal absorption relate the skin penetration kinetics of permeants to their partitioning and diffusion across elementary sub-compartments of the skin. This endows them with a flexibility through which specific model components can be adjusted to better reflect dermal absorption in contexts that differ from the in vitro setting, while keeping fixed any context-invariant parameters that remain unchanged in the two scenarios. This paper presents a workflow for predicting in vivo dermal absorption by integrating a mechanistic model of skin penetration with in vitro permeation test (IVPT) measurements. A Bayesian approach is adopted to infer a joint posterior distribution of context-invariant model parameters. By populating the model with samples of context-invariant parameters from this distribution and adjusting context-dependent parameters to suit the in vivo setting, simulations of the model yield estimates of the likely range of in vivo dermal absorption given the IVPT data. This workflow is applied to five compounds previously tested in vivo. In each case, the range of in vivo predictions encompassed the range observed experimentally. These studies demonstrate that the proposed workflow enables the derivation of mechanistically derived upper bounds on dermal absorption for the purposes of chemical risk assessment.

Modeling the Percutaneous Absorption of Solvent-deposited Solids Over a Wide Dose Range

The transient absorption of two skin care agents, niacinamide (nicotinamide, NA) and methyl nicotinate (MN), solvent-deposited on ex vivo human skin mounted in Franz diffusion cells has been analyzed according to a new variation on a recently published mechanistic skin permeability model (Yu et al. 2020. J Pharm Sci 110:2149-56). The model follows the absorption and evaporation of two components, solute and solvent, and it includes both a follicular transport component and a dissolution rate limitation for high melting, hydrophilic solids deposited on the skin. Explicit algorithms for improving the simulation of transient diffusion of solvent-deposited solids are introduced. The simulations can account for the ex vivo skin permeation time course of both NA and MN over a dose range exceeding 4.5 orders of magnitude. The model allows one to describe on a mechanistic basis why the percutaneous absorption rate of NA is approximately 60-fold lower than that of its lower melting, more lipophilic analog, MN. It furthermore suggests that MN perturbs stratum corneum barrier lipids and increases their permeability while NA does not, presenting a challenge to molecular modelers engaged in simulating biological lipid barriers.

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.