James D. Brooks

Prediction of dermal absorption from complex chemical mixtures: incorporation of vehicle effects and interactions into a QSPR framework †

Significant progress has been made on predicting dermal absorption/penetration of topically applied compounds by developing QSPR models based on linear free energy relations (LFER). However, all of these efforts have employed compounds applied to the skin in aqueous or single solvent systems, a dosing scenario that does not mimic occupational, environmental or pharmaceutical exposure. We have explored using hybrid QSPR equations describing individual compound penetration based on the molecular descriptors for the compound modified by a mixture factor (MF) which accounts for the physicochemical properties of the vehicle/mixture components. The MF is calculated based on percentage composition of the vehicle/mixture components and physical chemical properties selected using principal components analysis. This model has been applied to 12 different compounds in 24 mixtures for a total of 288 treatment combinations obtained from flow-through porcine skin diffusion cells and in an additional dataset of 10 of the same compounds in five mixtures for a total of 50 treatment combinations in the ex vivo isolated perfused porcine skin flap. The use of the MF in combination with a classic LFER based on penetrant properties significantly improved the ability to predict dermal absorption of compounds dosed in complex chemical mixtures. †Presented at the 12th International Workshop on Quantitative Structure--Activity Relationships in Environmental Toxicology (QSAR2006), 8--12 May 2006, Lyon, France.

Predicting skin permeability from complex vehicles

It is now widely accepted that vehicle and formulation components influence the rate and extent of passive chemical absorption through skin. Significant progress, over the last decades, has been made in predicting dermal absorption from a single vehicle; however the effect of a complex, realistic mixture has not received its due attention. Recent studies have aimed to bridge this gap by extending the use of quantitative structure-permeation relationship (QSPR) models based on linear free energy relationships (LFER) to predict dermal absorption from complex mixtures with the inclusion of significant molecular descriptors such as a mixture factor that accounts for the physicochemical properties of the vehicle/mixture components. These models have been compiled and statistically validated using the data generated from in vitro or ex vivo experimental techniques. This review highlights the progress made in predicting skin permeability from complex vehicles.

Predicting Skin Permeability from Complex Chemical Mixtures: Dependency of Quantitative Structure Permeation Relationships on Biology of Skin Model Used

Dermal absorption of topically applied chemicals usually occurs from complex chemical mixtures; yet, most attempts to quantitate dermal permeability use data collected from single chemical exposure in aqueous solutions. The focus of this research was to develop quantitative structure permeation relationships (QSPR) for predicting chemical absorption from mixtures through skin using two levels of in vitro porcine skin biological systems. A total of 16 diverse chemicals were applied in 384 treatment mixture combinations in flow-through diffusion cells and 20 chemicals in 119 treatment combinations in isolated perfused porcine skin. Penetrating chemical flux into perfusate from diffusion cells was analyzed to estimate a normalized dermal absorptive flux, operationally an apparent permeability coefficient, and total perfusate area under the curve from perfused skin studies. These data were then fit to a modified dermal QSPR model of Abraham and Martin including a sixth term to account for mixture interactions based on physical chemical properties of the mixture components. Goodness of fit was assessed using correlation coefficients (r²), internal and external validation metrics (q²L00, q²L25%, q²EXT), and applicable chemical domain determinations. The best QSPR equations selected for each experimental biological system had r² values of 0.69-0.73, improving fits over the base equation without the mixture effects. Different mixture factors were needed for each model system. Significantly, the model of Abraham and Martin could also be reduced to four terms in each system; however, different terms could be deleted for each of the two biological systems. These findings suggest that a QSPR model for estimating percutaneous absorption as a function of chemical mixture composition is possible and that the nature of the QSPR model selected is dependent upon the biological level of the in vitro test system used, both findings having significant implications when dermal absorption data are used for in vivo risk assessments.

Validated models for predicting skin penetration from different vehicles

The permeability of a penetrant though skin is controlled by the properties of the penetrants and the mixture components, which in turn relates to the molecular structures. Despite the well-investigated models for compound permeation through skin, the effect of vehicles and mixture components has not received much attention. The aim of this Quantitative Structure Activity Relationship (QSAR) study was to develop a statistically validated model for the prediction of skin permeability coefficients of compounds dissolved in different vehicles. Furthermore, the model can help with the elucidation of the mechanisms involved in the permeation process. With this goal in mind, the skin permeability of four different penetrants each blended in 24 different solvent mixtures were determined from diffusion cell studies using porcine skin. The resulting 96 kp values were combined with a previous dataset of 288 kp data for QSAR analysis. Stepwise regression analysis was used for the selection of the most significant molecular descriptors and development of several regression models. The selected QSAR employed two penetrant descriptors of Wiener topological index and total lipole moment, boiling point of the solvent and the difference between the melting point of the penetrant and the melting point of the solvent. The QSAR was validated internally, using a leave-many-out procedure, giving a mean absolute error of 0.454 for the logkp value of the test set.

Membrane transport of naphthalene and dodecane in jet fuel mixtures

Jet fuels are formulated with numerous aliphatic and aromatic components that are thought to cause dermal irritation in air force personnel. However, diffusion of these components in such a complex mixture is not well understood. The purpose of this study is to evaluate the physicochemical properties of these mixtures in the context of how they influence partitioning, diffusion, and absorption of aromatic (14C-naphthalene) and aliphatic (14C-dodecane) markers in porcine skin and silastic membranes in vitro. In these 5-h flowthrough diffusion studies, Jet-A, JP-8, and JP-8(100) fuels, and weathered JP-8 (JP-8 (Puddle)) were tested. In both membrane systems and across all jet fuels tested, naphthalene absorption (1.29-1.84% dose) was significantly greater than dodecane absorption (0.14-0.28% dose). However, significantly more dodecane than naphthalene was observed in the stratum corneum (SC; 4.23-7.23% dose vs. 1.88-4.08% dose) and silastic membranes (59.2-81.7% dose vs. 30.5-36.7% dose). Naphthalene was least likely to be retained on the skin surface compared to dodecane, while this trend was reversed in silastic membranes. In porcine skin. weathered JP-8 significantly increased dodecane absorption, permeability (0.19×10−4 cm/h), and diffusivity, and also naphthalene deposition in the SC compared to other jet fuels. In contrast, weathered JP-8 appears to decrease naphthalene flux (1.56 μg/cm2/h) and permeability (1.14×10−4 cm/h) in skin. There were no differences among the three jet fuels in terms of their ability to influence naphthalene or dodecane disposition in skin and, generally, no significant differences among the four jet fuel mixtures were observed in silastic membranes. In conclusion, these transport studies suggest that absorption and deposition of naphthalene and dodecane are different when dosed in various jet fuel mixtures, and disposition in weathered jet fuel is significantly different from that in commercial and military fuels. These interactions may not only be related to the unique chemistry of these components, but also specific membrane interactions in the SC and viable epidermis.

Percutaneous Absorption of Topical N , N -Diethyl- m -Toluamide (Deet): Effects of Exposure Variables and Coadministered Toxicants

Exposure to N,N-diethyl-m-toluamide (DEET) commonly occurs in the general population and has been implicated as a contributory factor to the Gulf War Illness. The focus of the present studies was to determine the effect of coexposure factors, potentially encountered in a military environment, that could modulate transdermal flux of topically applied DEET. Factors investigated were vehicle, dose, coexposure to permethrin, low-level sulfur mustard, occlusion, and simultaneous systemic exposure to pyridostigmine bromide and the nerve agent simulant diisopropylfluorophosphate (DFP). Studies were conducted using the isolated perfused porcine skin flap (IPPSF), with a few mechanistically oriented studies conducted using in vitro porcine skin and silastic membrane diffusion cells. DEET was quantitated using high-performance liquid chromatography. The vehicle-control transdermal DEET flux in the IPPSF was approximately 2 w g/cm 2 /h for both 7.5 and 75% DEET concentrations, a value similar to that reported in humans. DEET absorption was enhanced by coinfusion of pyridostigmine bromide and DFP, by the presence of sulfur mustard, or by dosing under complete occlusion. The greatest increase in baseline flux was fivefold. In vitro diffusion cell studies indicated that silastic membranes had two orders of magnitude greater permeability than porcine skin, and showed vehicle effects on flux that were not detected in the IPPSF. These results suggest that coexposure to a number of chemicals that potentially could be encountered in a military environment may modulate the percutaneous absorption of topically applied DEET beyond that seen for normal vehicles at typically applied concentrations.

Modelling the effect of mixture components on permeation through skin

A vehicle influences the concentration of penetrant within the membrane, affecting its diffusivity in the skin and rate of transport. Despite the huge amount of effort made for the understanding and modelling of the skin absorption of chemicals, a reliable estimation of the skin penetration potential from formulations remains a challenging objective. In this investigation, quantitative structure-activity relationship (QSAR) was employed to relate the skin permeation of compounds to the chemical properties of the mixture ingredients and the molecular structures of the penetrants. The skin permeability dataset consisted of permeability coefficients of 12 different penetrants each blended in 24 different solvent mixtures measured from finite-dose diffusion cell studies using porcine skin. Stepwise regression analysis resulted in a QSAR employing two penetrant descriptors and one solvent property. The penetrant descriptors were octanol/water partition coefficient, logP and the ninth order path molecular connectivity index, and the solvent property was the difference between boiling and melting points. The negative relationship between skin permeability coefficient and logP was attributed to the fact that most of the drugs in this particular dataset are extremely lipophilic in comparison with the compounds in the common skin permeability datasets used in QSAR. The findings show that compounds formulated in vehicles with small boiling and melting point gaps will be expected to have higher permeation through skin. The QSAR was validated internally, using a leave-many-out procedure, giving a mean absolute error of 0.396. The chemical space of the dataset was compared with that of the known skin permeability datasets and gaps were identified for future skin permeability measurements.

Comparative in vitro percutaneous absorption of nonylphenol and nonylphenol ethoxylates (NPE-4 and NPE-9) through human, porcine and rat skin

The purpose of this study was to assess the percutaneous absorption of nonylphenol (NP) and the nonylphenol ethoxylates, NPE-4 and NPE-9, in human, porcine and rat skin. In vitro studies with the NPEs were conducted for 8 h in flowthrough diffusion cells using topical solutions of 0.1, 1.0 and 10% in PEG-400 or 1% in water (NPE-9 only). NP absorption was assessed as a 1% solution in PEG-400. All compounds were 14C ring-labeled and radioactivity in perfusate was monitored over time. Skin deposition was measured at the termination of the experiment. Absorption into perfusate and total penetration (compound absorbed plus compound sequestered in skin) were calculated. Absorption of NPE-4, NPE-9 and NP was similar across all species at less than 1% of the applied dose over 8 h. Penetration was generally below 5% of applied dose, the majority located in the stratum corneum. In all species and for both NPEs, the fraction of dose absorbed was highest for the lowest applied dose. Absorptions expressed as actual mass absorbed over 8 h were similar (approximately 0.3 μg/cm2) across all concentrations. Penetration, but not absorption, was greater from a water vehicle compared to a PEG-400 vehicle, particularly in rat skin. These studies suggest that NP, NPE-4 and NPE-9 were minimally absorbed across skin from all three species. Fractional absorption was concentration-dependent, making the actual absorbed flux constant across all doses.

Experimental factors affecting in vitro absorption of six model compounds across porcine skin

This comparative study evaluated the effect of several experimental variables on the absorption of six model [(14)C]-labeled compounds (caffeine, cortisone, diclofenac sodium, mannitol, salicylic acid, and testosterone) through porcine skin. Using static and flow-through diffusion cells, finite or infinite, saturated or unsaturated doses were applied in one of three vehicles: propylene glycol, water, and ethanol following a full factorial experimental design. The flux of each compound into the receptor phase, with or without bovine serum albumin (BSA), was monitored over 24 h. Levels of radioactivity were also determined in the stratum corneum by tape stripping and in the remaining skin. Apparent permeability coefficients (Kp) and dose absorbed were calculated and compared. The overall results emphasize the importance of experimental design and confirm previous findings that identified dose volume, saturation level and vehicle as the main sources of variation in the in vitro assessment of dermal absorption, whilst diffusion cell model and the presence/absence of BSA in the receptor phase had minimal effect. Although the acquired data do not directly reveal new mechanistic information on dermal absorption, the unique and complete study design has provided a suitable data source for the development of dermal absorption prediction models.

Physicochemical determinants of linear alkylbenzene sulfonate (LAS) disposition in skin exposed to aqueous cutting fluid mixtures.

Linear alkylbenzene sulfonate (LAS) is added to cutting fluid formulations to enhance the performance of metal machining operations, but this surfactant can cause contact dermatitis in workers involved in these operations. The purpose of this study was to determine how cutting fluid additives influence dermal disposition of 14C-LAS in mineral oil-or polyethylene glycol 200 (PEG)-based mixtures when topically applied to silastic membranes and porcine skin in an in vitroflow-through diffusion cell system. 14C-LAS mixtures were formulated with three commonly used cutting fluid additives; 0 or 2% triazine (TRI), 0 or 5% triethanolamine (TEA), and 0 or 5% sulfurized ricinoleic acid (SRA). LAS absorption was limited to less than a 0.5% dose and the additives in various combinations influenced the physicochemical characteristics of the dosing mixture. LAS was more likely to partition into the stratum corneum (SC) in mineral oil mixtures, and LAS absorption was significantly greater in the complete mixture. TRI enhanced LAS transport, and the presence of SRA decreased LAS critical micelle concentration (CMC) which reduced LAS monomers available for transport. TEA increased mixture viscosity, and this may have negated the apparent enhancing properties of TRI in several mixtures. In summary, physicochemical interactions in these mixtures influenced availability of LAS for absorption and distribution in skin, and could ultimately influence toxicological responses in skin.