Pacific Northwest National Laboratories, PO Box 999, Richland, WA 99352
ABSTRACT
The article highlighted in this issue is "Comparative in Vitro-in Vivo Percutaneous Absorption of the Pesticide Propoxur" by Johannes J. M. van de Sandt, Wim J. A. Meuling, Graham R. Elliott, Nicole H. P. Cnubben, and Betty C. Hakkert (pp 1522).
The recent interest in aggregate exposure assessment has highlighted the potential importance of dermal absorption to total systemic exposure. The extent and rate at which penetration through the skin occurs depends upon a large number of biological and environmental variables. The skin can function as a barrier, has some metabolic activity, contains multiple appendages and cell types, and is the largest organ of the body. Skin absorption is dictated by skin physiology, site of exposure, and exposure area, and it is species-specific. The extent of absorption through skin also is site-specific (Wester et al., 1984). In addition, exposure variables such as concentration, media (e.g., soil vs. water), temperature, and occlusion, which prevents volatilization or loss of the compound and may increase the hydration of the skin, compound the problem of assessing the amount of chemical absorbed. Taken together, these characteristics impose many challenges to estimating the contribution of dermal absorption to systemic dose.
A plethora of data recently coming to the forefront shows that exposures to xenobiotics are not limited to oral consumption or inhalation. Estimates for the contribution of the dermal absorption of 2-butoxyethanol vary from 127% of the total absorption from dual percutaneous and inhalation exposures (Corley et al., 1997). McKone (1993) used mathematical relationships to estimate that the dermal route accounts for more than half as much uptake of chloroform as from inhalation during a typical shower in chlorinated water. Thus, the dermal route of exposure may contribute significantly to the total body burden.
An additional challenge in the interpretation of dermal dose relates to the way absorption is assessed. A general approach used to calculate absorption has been as a percentage of the total applied dose, but this is highly dependent upon exposure conditions. Obviously, percentage of an infinite source would be meaningless. On the other hand, percentage of a limited source depends on the concentration and length of exposure and necessitates that the exposure dose remain constant over the length of exposures or is accounted for using a mathematical transformation. In order to express absorption as a percentage, the exposure conditions should mimic the real-world scenario as closely as possible. In addition, if comparisons are to be made between exposure systems and species, the initial exposure conditions must be as identical as possible. The study by van de Sandt et al. in this issue of Toxicological Sciences compares similar conditions of exposure concentration and exposed area in several in vitro and in vivo systems to assess the dermal absorption of the pesticide propoxur. By using similar exposure conditions in multiple in vivo and in vitro systems, van de Sandt et al. were able to compare these systems with regard to flux, percentage absorbed and potential absorbed dose. The potential absorbed dose has been used to assess the amount absorbed by comparing the exposure dose to the total amount recovered; this is useful when all of the chemical can be accounted for under experimental conditions. The use of the potential absorbed dose for both in vivo and in vitro calculations has the additional advantage that it includes the amount of chemical within the skin at the end of exposure. Lipophilic compounds can be retained in the skin for hours, and even days, post exposure and slowly seep into the general circulation (or in vitro into the receptor fluid). Thus, systemic absorption may not readily be accounted for by the chemical's concentration in blood or excreted products.
The flux of propoxur through the skin was accounted for in all of the systems employed by van de Sandt and colleagues. Flux is calculated using Fick's first law of diffusion to relate the flux rate (J in mg/cm2/hr) to the permeability, concentration, area of exposed surface, and length of exposure. An additional important parameter that needs to be utilized is the chemical's permeability coefficient (Kp). The permeability coefficient (in cm2/hr) should be consistent regardless of exposure concentration (provided that the concentration is infinite) and surface area for any given exposure site and chemical, but it can vary between exposure sites. Unless a mathematical model is used, the calculation of flux or permeability coefficient must be assessed at steady state (Poet et al., 2000).
Recently, mathematical models have been developed to attempt to describe percutaneous absorption kinetics. Like any biokinetic model, these models must be descriptive enough to obtain reasonable parameters while still being simple enough to have utility. A model that is too involved results in a number of parameters that must be estimated, and a model that is too simple may not reflect the data. Percutaneous absorption models are generally diffusion based (McKone, 1993) or compartmental in nature (McDougal et al., 1990
). Regardless of the form, Fick's first law of diffusion is at the center of such models.
In almost every case, absorption through rodent skin is more than 3-fold higher than through human skin and the increased absorption through rodent skin does not show a consistent pattern between compounds. The higher absorption in rodent skin may be due to differences in skin appendages (e.g., hair follicles), different morphology of the individual skin layers, or immunological or metabolic factors (U.S. Environmental Protection Agency, 1992). The study by van de Sandt et al. demonstrates that the absorption of propoxur exhibits the typical increased absorption by rodents over humans. An additional difference between human and rodent dermal absorption that was investigated under their experimental design is the difference in the lag phase before the appearance of chemical in the blood; human absorption is delayed, whereas chemical absorption through the skin of rodents occurs with no apparent delay.
Since dermal absorption analysis with animal models consistently over-estimates human absorption, attempts have been made to establish guidelines for in vitro dermal absorption studies (reviewed in Bronaugh, 1998). In vitro human systems may provide a better quantitative measure of chemical absorption than animal models. Many of the same confounding factors in vivo affect the in vitro systems, for example, exposure concentration, duration, site from which the skin is obtained, surface area, and occlusion. Once again, the best situation is one in which the real-world exposure scenario is mimicked as closely as possible.
In vitro percutaneous absorption systems have definite utility in screening, allow for manipulations not possible in vivo, and may be more useful than animal models that overvalue dermal absorption. In vitro estimations must be carried out in conditions where the donor concentration and receptor characteristics are constant over time. As is true of in vivo absorption, the percentage of chemical absorbed in vitro depends on the amount in the exposure matrix. As described by van de Sandt et al., most in vitro-in vivo comparison studies are carried out with different exposure conditions. The attempts of the authors to control the exposure conditions are important steps toward comprehensive in vitro to in vivo and species to species comparisons.
The thickness of the skin used in vitro affects the amount absorbed. Many studies suggest the use of partial thickness skin since full thickness skin may serve as a reservoir (Bronaugh, 1998); van de Sandt et al. employed both full thickness, viable skin and epidermal membranes. The use of full thickness skin reduced the difference between species in the absorption parameters, possibly due to biotransformation which occurred only in full thickness membranes.
Overestimating or underestimating dermal absorption will have a strong impact on toxicity assessments. The study by van de Sandt et al. reveals a qualitative comparison between in vivo and in vitro systems where the rat viable skin, epidermal skin, and in vivo exposures all show a greater absorption of the pesticide propoxur than comparable human exposure systems. Studies comparable to that of van de Sandt et al. will be useful adjuncts to describing the relationships between animal and in vitro systems used in percutaneous absorption assessments. Relevant model systems to assist risk assessment are critical since skin exposure can potentially represent a significant route of exposure.
REFERENCES
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