Experimental Techniques Used to Assess Absorption

Whole-animal studies assess the percent of the applied dose absorbed into the body using classic techniques of bioavailability, where absorbed chemical is measured in the blood, urine, feces, and tissues with mass balance techniques. Recently, methods have been developed to assess absorption by measuring the amount of chemical in the stratum comeum because it is the driving force for diffusion. Cellophane tape strips are collected 30 minutes after chemical exposure and the amount of drug assayed in these tape strips correlates to the amount systemically absorbed. If the focus of the research is to determine the amount of chemical that has penetrated into skin, core biopsies may be collected and serially sectioned, and a profile of the chemical as a function of skin depth may be obtained. 

The next level of in vitro systems employed is the use of isolated perfused skin flap preparations that are surgically prepared vascularized skin flaps harvested from pigs and then transferred to an isolated organ perfusion chamber. This model allows absorption to be assessed in skin that is viable and anatomically intact and that has a functional microcirculation. Studies conducted to assess the percutaneous absorption of drugs and pesticides in this model compared to humans show a high correlation. Validation of these in vitro methods is a prerequisite for regulatory acceptance. 

If a cytotoxic chemical is capable of being absorbed across the stratum corneum barrier, it has the potential to cause toxicity to the skin. Chemical-induced damage 

1 Mechanisms of Keratinocyte Mediation of Skin Irritation/lnflammation 

The main sources of TNFa in the skin are mast cells, resident keratinocytes and Langerhans cells. Mature mast cells contain significant quantities of preformed TNFa in their granules that are released in an IgE-dependent manner. Investigators studying the inflammatory response in mouse skin revealed by Western blot analysis that exposure to acetone and tape stripping increased TNFa expression by 72% at 2.5 hr after treatment. The mRNA levels were highest at 1 hr after acetone, and then they decreased to control levels by 8 hr. These studies indicate an immediate response of TNFa to irritants that perturb the epidermal barrier. 

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