Explaining skin permeation of 2-butoxyethanol from neat and aqueous solutions

Immediate dry decontamination using efficient absorbent materials is beneficial following skin exposure to low-volatile toxic chemicals

In a chemical mass casualty incident requiring skin decontamination, dry removal using absorbent materials may be beneficial to enable immediate decontamination. The efficacy of absorbent materials has therefore been evaluated, alone or procedures including both dry and wet decontamination, following skin exposure to two low volatile toxic chemicals using an in vitro human skin penetration model. Additionally, removal using active carbon wipes was evaluated with or without the Dahlgren Decon solution. All dry decontamination procedures resulted in a significantly decreased skin penetration rate of the industrial chemical 2-butoxyethanol compared to the control without decontamination. Wet decontamination following dry absorption significantly improved the efficacy compared to dry removal alone. Dry decontamination post-exposure to the chemical warfare nerve agent VX showed no decontamination efficacy. However, dry and wet decontamination resulted in a decreased agent skin penetration rate during the last hour of the experiment. At -15°C, significantly reduced VX skin penetration rates were demonstrated for both dry decontamination alone and the dry and wet decontamination procedure. The Dahlgren Decon solution significantly reduced the amount of VX penetrating the skin, but the active carbon wipe alone did not impact the skin penetration rate. In conclusion, absorbent materials are beneficial for the removal of low-volatile chemicals from the skin, but the degree of efficacy varies between chemicals. Despite the variability, immediate dry decontamination using available absorbent materials prior to wet decontamination is recommended as a general procedure for skin decontamination. The procedure should also be prioritized in cold-weather conditions to prevent patient hypothermia.

Predicting Human Dermal Drug Concentrations Using PBPK Modeling and Simulation: Clobetasol Propionate Case Study

Quantitative in silico tools may be leveraged to mechanistically predict the dermato-pharmacokinetics of compounds delivered from topical and transdermal formulations by integrating systems of rate equations that describe permeation through the formulation and layers of skin and pilo-sebaceous unit, and exchange with systemic circulation via local blood flow. Delivery of clobetasol-17 propionate (CP) from DermovateTM cream was simulated using the Transdermal Compartmental Absorption & Transit (TCATTM) Model in GastroPlus®. The cream was treated as an oil-in-water emulsion, with model input parameters estimated from publicly available information and quantitative structure-permeation relationships. From the ranges of values available for model input parameters, a set of parameters was selected by comparing model outputs to CP dermis concentration-time profiles measured by dermal open-flow microperfusion (Bodenlenz et al. Pharm Res. 33(9):2229-38, 2016). Predictions of unbound dermis CP concentrations were reasonably accurate with respect to time and skin depth. Parameter sensitivity analyses revealed considerable dependence of dermis CP concentration profiles on drug solubility in the emulsion, relatively less dependence on dispersed phase volume fraction and CP effective diffusivity in the continuous phase of the emulsion, and negligible dependence on dispersed phase droplet size. Effects of evaporative water loss from the cream and corticosteroid-induced vasoconstriction were also assessed. This work illustrates the applicability of computational modeling to predict sensitivity of dermato-pharmacokinetics to changes in thermodynamic and transport properties of a compound in a topical formulation, particularly in relation to rate-limiting steps in skin permeation. Where these properties can be related to formulation composition and processing, such a computational approach may support the design of topically applied formulations.

Confocal Raman Spectroscopic Characterization of Dermatopharmacokinetics Ex Vivo

Confocal Raman spectroscopy is being assessed as a tool with which to quantify the rate and extent of drug uptake to and its clearance from target sites of action within the viable epidermis below the skin's stratum corneum (SC) barrier. The objective of this research was to confirm that Raman can interrogate drug disposition within the living layers of the skin (where many topical drugs elicit their pharmacological effects) and to identify procedures by which Raman signal attenuation with increasing skin depth may be corrected and normalized so that metrics descriptive of topical bioavailability may be identified. It was first shown in experiments on skin cross-sections parallel to the skin surface that the amide I signal, originating primarily from keratin, was quite constant with depth into the skin and could be used to correct for signal attenuation when confocal Raman data were acquired in a "top-down" fashion. Then, using 4-cyanophenol (CP) as a model skin penetrant with a strong Raman-active C≡N functionality, a series of uptake and clearance experiments, performed as a function of time, demonstrated clearly that normalized spectroscopic data were able to detect the penetrant to at least 40-80 μm into the skin and to distinguish the disposition of CP from different vehicles. Metrics related to local bioavailability (and potentially bioequivalence) included areas under the normalized C≡N signal versus depth profiles and elimination rate constants deduced post-removal of the formulations. Finally, Raman measurements were made with an approved dermatological drug, crisaborole, for which delivery from a fully saturated formulation into the skin layers just below the SC was detectable.

Quantification of Chemical Uptake into the Skin by Vibrational Spectroscopies and Stratum Corneum Sampling

Evaluation of the bioavailability of drugs intended to act within the skin following the application of complex topical products requires the application of multiple experimental tools, which must be quantitative, validated, and, ideally and ultimately, sufficiently minimally invasive to permit use in vivo. The objective here is to show that both infrared (IR) and Raman spectroscopies can assess the uptake of a chemical into the stratum corneum (SC) that correlates directly with its quantification by the adhesive tape-stripping method. Experiments were performed ex vivo using excised porcine skin and measured chemical disposition in the SC as functions of application time and formulation composition. The quantity of chemicals in the SC removed on each tape-strip was determined from the individually measured IR and Raman signal intensities of a specific molecular vibration at a frequency where the skin is spectroscopically silent and by a subsequent conventional extraction and chromatographic analysis. Correlations between the spectroscopic results and the chemical quantification on the tape-strips were good, and the effects of longer application times and the use of different vehicles were clearly delineated by the different measurement techniques. Based on this initial investigation, it is now possible to explore the extent to which the spectroscopic approach (and Raman in particular) may be used to interrogate chemical disposition deeper in the skin and beyond the SC.

Exposure to oxybenzone from sunscreens: daily transdermal uptake estimation

BACKGROUND

Fugacity, the driving force for transdermal uptake of chemicals, can be difficult to predict based only on the composition of complex, non-ideal mixtures such as personal care products.

OBJECTIVE

Compare the predicted transdermal uptake of benzophenone-3 (BP-3) from sunscreen lotions, based on direct measurements of BP-3 fugacity in those products, to results of human subject experiments.

METHODS

We measured fugacity relative to pure BP-3, for commercial sunscreens and laboratory mixtures, using a previously developed/solid-phase microextraction (SPME) method. The measured fugacity was combined with a transdermal uptake model to simulate urinary excretion rates of BP-3 resulting from sunscreen use. The model simulations were based on the reported conditions of four previously published human subject studies, accounting for area applied, time applied, showering and other factors.

RESULTS

The fugacities of commercial lotions containing 3-6% w/w BP-3 were ~20% of the supercooled liquid vapor pressure. Simulated dermal uptake, based on these fugacities, are within a factor of 3 of the mean results reported from two human-subject studies. However, the model significantly underpredicts total excreted mass from two other human-subject studies. This discrepancy may be due to limitations in model inputs, such as fugacity of BP-3 in lotions used in those studies.

SIGNIFICANCE

The results suggest that combining measured fugacity with such a model may provide order-of-magnitude accurate predictions of transdermal uptake of BP-3 from daily application of sunscreen products.

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.

The wash-in effect and its significance for mass casualty decontamination

Decontamination of skin by washing may increase dermal absorption, a phenomenon known as the wash-in effect. The wash-in effect is frequently discussed in studies investigating casualty decontamination where potentially life-saving interventions may enhance the dermal penetration of toxic chemicals, leading to an increase in incidence of morbidity and rates of mortality. However, the wash-in effect is seldom investigated within the context of mass casualty decontamination and real-life consequences are therefore poorly understood. This paper reviews the existing literature on the wash-in effect to highlight the proposed mechanisms for enhanced absorption and evaluate the wash-in effect within the context of mass casualty chemical decontamination.

Occupational exposure assessment with solid substances: choosing a vehicle for in vitro percutaneous absorption experiments

Percutaneous occupational exposure to industrial toxicants can be assessed in vitro on excised human or animal skins. Numerous factors can significantly influence skin permeation of chemicals and the flux determination. Among them, the vehicle used to solubilize the solid substances is a tricky key step. A "realistic surrogate" that closely matches the exposure scenario is recommended in first intention. When direct transposition of occupational exposure conditions to in vitro experiments is impossible, it is recommended that the vehicle used does not affect the skin barrier (in particular in terms of structural integrity, composition, or enzymatic activity). Indeed, any such effect could alter the percutaneous absorption of substances in a number of ways, as we will see. Potential effects are described for five monophasic vehicles, including the three most frequently used: water, ethanol, acetone; and two that are more rarely used, but are realistic: artificial sebum and artificial sweat. Finally, we discuss a number of criteria to be verified and the associated tests that should be performed when choosing the most appropriate vehicle, keeping in mind that, in the context of occupational exposure, the scientific quality of the percutaneous absorption data provided, and how they are interpreted, may have long-range consequences. From the narrative review presented, we also identify and discuss important factors to consider in future updates of the OECD guidelines for in vitro skin absorption experiments.

Skin Absorption of Bisphenol A and Its Alternatives in Thermal Paper

OBJECTIVES

Bisphenol A (BPA) is the most used colour developer in thermal paper for cashiers receipts, labels, and tickets. BPA can migrate onto the skin and be absorbed when handling these papers. BPA is a known endocrine disruptor and is therefore being replaced in thermal paper by some alternatives such as Bisphenol S (BPS), D-8, and Pergafast 201® (PF201). To our knowledge, no studies have characterized skin permeation of these BPA alternatives.

METHODS

We measured/characterized skin absorption for BPA, BPS, D-8, and PF201 through ex vivo human skin using flow-through diffusion cells according to OECD guideline 428. Skin samples were 7-12 per test substance from three different skin donors. Skin metabolism was studied for BPA. Dermal absorption was expressed as the amount of the BPA alternatives in the receptor fluid over applied dose in percent (%).

RESULTS

The absorbed dose after 24 h of exposure was 25% for BPA, 17% for D-8, 0.4% for BPS, and <LLOQ for PF201. The amount of BPA-glucuronide in the receptor fluid after 24 h was under the limit of quantification (LLOQ = 0.2 µg l-1). Despite the 10-fold lower concentration of the aq solution applied on the skin, D-8's permeation rate JMAX was 5-fold higher than the one for BPS (0.032 versus 0.006 µg cm-2 h-1). Neither D-8 nor BPS permeated readily through the skin (tlag = 3.9 h for D-8, 6.4 h for BPS). None of PF201's skin permeation kinetic parameters could be determined because this BPA analogue was not quantifiable in the receptor fluid in our test conditions.

CONCLUSIONS

Skin absorption was in decreasing order: BPA > D-8 >> BPS > PF201. These results are in agreement with their log Kow and molecular weights. We provided here the necessary data to estimate the extent of skin absorption of BPA analogues, which is a necessary step in risk assessment, and ultimately evaluate public health risks posed by D-8, BPS, and PF201.

Influence of experimental parameters on in vitro human skin permeation of Bisphenol A

Bisphenol A (BPA) in vitro skin permeation studies have shown inconsistent results, which could be due to experimental conditions. We studied the impact of in vitro parameters on BPA skin permeation using flow-through diffusion cells with ex-vivo human skin (12 donors, 3-12 replicates). We varied skin status (viable or frozen skin) and thickness (200, 400, 800 μm), BPA concentrations (18, 250 mg/l) and vehicle volumes (10, 100 and 1000 μl/cm²). These conditions led to a wide range of BPA absorption (2%-24% after 24 h exposure), peak permeation rates (J = 0.02-1.31 μg/cm²/h), and permeability coefficients (K_p = 1.6-5.2 × 10^-3 cm/h). This is the first time steady state conditions were reached for BPA aqueous solutions in vitro (1000 μl/cm² applied at concentration 250 mg/l). A reduction of the skin thickness from 800 and 400 μm to 200 μm led to a 3-fold increase of J (P < 0.05). A reduction of the vehicle volume from 1000 to 100 led to a 2-fold decrease in J (P > 0.05). Previously frozen skin led to a 3-fold increase in J compared to viable skin (P < 0.001). We found that results from published studies were consistent when adjusting J according to experimental parameters. We propose appropriate J values for different exposure scenarios to calculate BPA internal exposures for use in risk assessment.

Reflections on the OECD guidelines for in vitro skin absorption studies

At the 8th conference of Occupational and Environmental Exposure of the Skin to Chemicals (OEESC) (16-18 September 2019) in Dublin, Ireland, several researchers performing skin permeation assays convened to discuss in vitro skin permeability experiments. We, along with other colleagues, all of us hands-on skin permeation researchers, present here the results from our discussions on the available OECD guidelines. The discussions were especially focused on three OECD skin absorption documents, including a recent revision of one: i) OECD Guidance Document 28 (GD28) for the conduct of skin absorption studies (OECD, 2004), ii) Test Guideline 428 (TGD428) for measuring skin absorption of chemical in vitro (OECD, 2004), and iii) OECD Guidance Notes 156 (GN156) on dermal absorption issued in 2011 (OECD, 2011). GN156 (OECD, 2019) is currently under review but not finalized. A mutual concern was that these guidance documents do not comprehensively address methodological issues or the performance of the test, which might be partially due to the years needed to finalize and update OECD documents with new skin research evidence. Here, we summarize the numerous factors that can influence skin permeation and its measurement, and where guidance on several of these are omitted and often not discussed in published articles. We propose several improvements of these guidelines, which would contribute in harmonizing future in vitro skin permeation experiments.

Dissipative Particle Dynamics Investigation of the Transport of Salicylic Acid through a Simulated In Vitro Skin Permeation Model

Permeation models are often used to determine diffusion properties of a drug through a membrane as it is released from a delivery system. In order to circumvent problematic in vivo studies, diffusion studies can be performed in vitro, using (semi-)synthetic membranes. In this study salicylic acid permeation was studied, employing a nitrocellulose membrane. Both saturated and unsaturated salicylic acid solutions were studied. Additionally, the transport of salicylic acid through the nitrocellulose membrane was simulated by computational modelling. Experimental observations could be explained by the transport mechanism that was revealed by dissipative particle dynamics (DPD) simulations. The DPD model was developed with the aid of atomistic scale molecular dynamics (AA-MD). The choice of a suitable model membrane can therefore, be predicted by AA-MD and DPD simulations. Additionally, the difference in the magnitude of release from saturated and unsaturated salicylic acid and solutions could also be observed with DPD. Moreover, computational studies can reveal hidden variables such as membrane-permeant interaction that cannot be measured experimentally. A recommendation is made for the development of future model permeation membranes is to incorporate computational modelling to aid the choice of model.

Percutaneous absorption of thirty-eight organic solvents in vitro using pig skin

Percutaneous absorption is highly variable between chemicals but also within chemicals depending on exposure conditions and experimental set up. We tested a larger number of organic solvents with the same experimental set up, using skin from new-born piglets and static diffusion cells. Thirty-six common organic solvents were studied neat (and 31 of them also in water dilution): acetone, acetonitrile, n-butanol 2-butanone 2-butoxyethanol, 1-butoxy-2-propanol, n-butyl acetate, butyl acrylate, cyclohexane, cyclohexanone, 1,2-dichloroethane, dichloromethane, ethanol, 2-ethoxyethanol, ethyl acetate, ethyl acrylate, ethylbenzene, furfuryl alcohol, n-hexane, 2-hexanone, 2-isopropoxyethanol, methanol, 1-methoxy-2-propanol, methyl acrylate, 3-methyl-1-butanol, methyl tertiary butyl ether, 4-metyl-2-pentanol, methyl methacrylate, 2-propanol, 2-propen-1-ol, 2-propoxyethanol, 1-propoxy-2-propanol, styrene, trichloromethane, toluene and m-xylene. In addition, a mixture of 2-methylbutyl acetate and n-pentyl acetate was tested. For most of the solvents, little or no percutaneous absorption data have been published. Lag times, steady-state fluxes and apparent permeability coefficients were obtained from the time courses of solvent appearance in the receptor medium, as measured by gas chromatography. The use of the same methodology and kind of skin resulted in small variability within experiments, underlining the need for consistent methodology for useful results for developing predictive models. Furthermore, a comparison of the neat and diluted data shows that water dilution affects all these variables and that the direction and magnitude of the effects vary between chemicals. This comparison strongly supports that prediction of percutaneous absorption of neat and water diluted chemicals requires different models.

Evaluation on the reliability of the permeability coefficient (Kp) to assess the percutaneous penetration property of chemicals on the basis of Flynn's dataset

PURPOSE

The permeability coefficient (K_p) is often used for prediction of the dermal penetration of chemicals. Mathematical models have mostly been derived on K_p data basis. However, confusing K_p values are reported, questioning the general reliability of this parameter. In this study, we tested the plausibility of K_p values expressing the dermal penetration velocity (cm h^-1) of chemicals on a larger dataset from literature.

METHODS

K_p was applied for the calculation of the time for penetration through skin membranes of defined thickness (t_CrossSkin). K_p values were obtained from Flynn's dataset (1990), containing data determined mostly under similar experimental conditions using diffusion cells. Further skin penetration parameters, e.g., times at which the chemicals were firstly measured in the receptor phase, lag times, steady-state times, and exposure duration, where available, were related to K_p values. The data congruence was tested comparing K_p values from Flynn's dataset with those reported in the EDETOX database. Variables, which could bias the results, such as different experimental protocols and research groups were also considered.

RESULTS

K_p data for 94 chemicals matched the inclusion criteria were evaluated. According to the K_p values, 21 (22%) compounds would require longer than 100 h, and 20 (21%) further compounds longer than 10 h of exposure to penetrate skin membranes of ~ 0.01-2.5 mm thickness. Obviously, erroneous K_p were found in studies of almost all research groups in Flynn's database, indicating that neither the observer nor the experimental conditions alone biased the values.

CONCLUSIONS

Our evaluation demonstrates high implausibility of K_p values to represent the dermal penetration velocity and supports general invalidity of the parameter for implementation in studies using skin membranes. The K_p should not be used to characterize the percutaneous penetration of chemicals or in risk assessment without verification.

Mechanistic Evaluation of Hydration Effects on the Human Epidermal Permeation of Salicylate Esters

We sought to understand when and how hydration enhances the percutaneous absorption of salicylate esters. Human epidermal membrane fluxes and stratum corneum solubilities of neat and diluted solutions of three esters were determined under hydrated and dehydrated conditions. Hydration doubled the human epidermal flux seen for methyl and ethyl salicylate under dehydrated conditions and increased the flux of neat glycol salicylate 10-fold. Mechanistic analyses showed that this hydration-induced enhancement arises mainly from an increase in the stratum corneum diffusivity of the three esters. Further, we showed that unlike methyl and ethyl salicylate, glycol salicylate is hygroscopic and the ∼10-fold hydration-induced flux enhancement seen with neat glycol salicylate may be due to its ability to hydrate the stratum corneum to a greater extent. The hydration-induced enhancements in in vitro epidermal flux seen here for glycol and ethyl salicylate were similar to those reported for their percutaneous absorption rates in a comparable in vivo study, whilst somewhat higher enhancement was seen for methyl salicylate in vivo. This may be explained by a physiologically induced self enhancement of neat methyl salicylate absorption in vivo which is not applicable in vitro.

Concentration dependency in nicotine skin penetration flux from aqueous solutions reflects vehicle induced changes in nicotine stratum corneum retention

PURPOSE

This study sought to understand the mechanism by which the steady state flux of nicotine across the human skin from aqueous solutions is markedly decreased at higher nicotine concentrations.

METHODS

Nicotine's steady state flux through human epidermis and its amount in the stratum corneum for a range of aqueous nicotine solutions was determined using Franz diffusion cells, with the nicotine analysed by high performance liquid chromatography (HPLC). Nicotine's thermodynamic activity in the various solutions was estimated from its partial vapour pressure and stratum corneum hydration was determined using a corneometer. The amount of nicotine retained in the stratum corneum was estimated from the nicotine amount found in individual stratum corneum tape strips and a D-Squame determined weight for each strip.

RESULTS

The observed steady state flux of nicotine across human epidermis was found to show a parabolic dependence on nicotine concentration, with the flux proportional to its thermodynamic activity up to a concentration of 48% w/w. The nicotine retention in the stratum corneum showed a similar dependency on concentration whereas the diffusivity of nicotine in the stratum corneum appeared to be concentration independent. This retention, in turn, could be estimated from the extent of stratum corneum hydration and the nicotine concentration in the applied solution and volume of water in the skin.

CONCLUSIONS

Nonlinear dependency of nicotine skin flux on its concentration results from a dehydration induced decrease in its stratum corneum retention at higher concentration and not dehydration induced changes nicotine diffusivity in the stratum corneum.

Analysis of finite dose dermal absorption data: implications for dermal exposure assessment

A common dermal exposure assessment strategy estimates the systemic uptake of chemical in contact with skin using the fixed fractional absorption approach: the dermal absorbed dose is estimated as the product of exposure and the fraction of applied chemical that is absorbed, assumed constant for a given chemical. Despite the prominence of this approach there is little guidance regarding the evaluation of experiments from which fractional absorption data are measured. An analysis of these experiments is presented herein, and limitations to the fixed fractional absorption approach are discussed. The analysis provides a set of simple algebraic expressions that may be used in the evaluation of finite dose dermal absorption experiments, affording a more data-driven approach to dermal exposure assessment. Case studies are presented that demonstrate the application of these tools to the assessment of dermal absorption data.

Dermal permeation of 2-hydroxypropyl acrylate, a model water-miscible compound: effects of concentration, thermodynamic activity and skin hydration

The goal of these studies was to measure and interpret the skin permeability characteristics of 2-hydroxypropyl acrylate (HPA) as a model compound that is completely miscible with water.

METHODS

In vitro permeation from HPA-H2O binary mixtures through human epidermis and silicone membranes was measured. Thermodynamic activities of HPA and H2O in these mixtures were determined. Permeation was also measured through epidermis and silicone from donor solutions with constant HPA activity but different H2O activities. Water uptake into desiccated human stratum corneum (SC) equilibrated with HPA-H2O mixtures was determined.

RESULTS

Steady-state flux of HPA through silicone was a linear function of HPA activity but not HPA concentration. For epidermis on the other hand, flux increased with HPA activity only for HPA activities ≤ 0.35. At constant HPA activity, flux decreased 4.5-fold as water activity decreased from 1 to 0.8. Incubation of SC with HPA-H2O mixtures resulted in substantial changes in SC water content, dependent on the water activity of the mixture and consistent with measured SC water sorption data.

CONCLUSIONS

These experiments provide unequivocal evidence of a substantial increase in epidermal barrier function resulting from SC dehydration. Dehydration-related alterations in the SC appear responsible for the observed flux characteristics.